MXPA00012452A - Quadriphase spreading codes in code division multiple access communications - Google Patents

Abstract

Optimal code sequences are generated for use in spreading and de-spreading functions in a code division multiple access (CDMA) communications system. In particular, a family of quadriphase spreading codes is employed that provides a maximal number of spreading codes to achieve a high capacity in the CDMA communications system while at the same time having a minimal peak cross-correlation between any two spreading codes within that family to ensure cross-correlation interference is kept at or below acceptable levels. That optimal quadriphase spreading code family is the S(2) family of four phase code sequences of length L=2m-1, where m is an integer greater than or equal to 5. The size of the S(2) family of quaternary spreading codes is (L + 2)(L + 1)2, and the maximum cross-correlation is 1 + 4v(L + 1). The spreading codes are preferably allocated to base stations using specific code subsets of the S(2) family having the same cross-correlation properties of the S(0) and/or S(1) family of codes. Spreading codes are advantageously extended by one or more code symbols as necessary or otherwise desirable. For example, to support variable transmission rate services, it is desirable to employ spreading codes whose length may be expressed as an integer multiple of each spreading factor in the mobile communications system. Since individual spreading codes have a length of 2m - 1, one code symbol is added to the generated spreading code.

Description

QUADRAPHIC DISPERSION CODES IN MULTIPLE ACCESS CODE DIVISION COMMUNICATIONS FIELD OF THE INVENTION The present invention relates to dispersion spectrum communications, and more particularly to the generation of optimal code sequences used to perform dispersion and concentration functions in a code division multiple access communication system. BACKGROUND AND COMPENDIUM OF THE INVENTION A direct sequence dispersion spectrum (DSSS) system is a broadband system in which the entire frequency bandwidth of the system is available to each user at all times. A DSSS system employs a scatter signal that expands or "disperses" the bandwidth of the transmitted signal much more than is required for the transmission of information symbols. The dispersion signal is usually referred to as a code or dispersion or coding sequence. The term dispersion code is generally adopted for this description. Different users in a DSSS system are distinguished using the different dispersion codes. This is the reason why DSSS systems are also known as direct sequence code division multiple access (DS-CDMA) systems. In general, dispersion codes are usually biphasic with elements that they belong to the whole. { +1, -1 } , or polyphasic, with elements that belong to the set of complex numbers that correspond to points equidistant in the unit circle in the complex plane. For example, a quadriple 5 corresponds to four points of unit length from the origin. In general, there is a tradeoff between the increase in the number of dispersion codes and the decrease in ^ - interference. The number of scatter codes that are used to distinguish users of mobile stations, particularly in the uplink direction from a mobile station to a base station should be as large as possible. This is because a greater number of scatter codes offers more radio channels in such a way that more mobile stations can communicate at the same time in the same geographical area. But the increase in capacity in ^ A CDMA system causes an increase in interference which reduces the quality of communication for all users. However, it is desirable that the amount of The correlation between two of the scatter codes is reduced to minimize the interference between the mobile stations that communicate using these codes. More formally, a cross-correlation, periodic, maximum between two scatter codes should be as low as possible. 25 the periodic cross-correlation, which is also known as correlation, is equal to a correlation output considering that the data modulation format does not change during the correlation operation. In practice, successive data modulation symbols have random values instead of periodic values. Accordingly, an odd correlation function best represents the correlation output when an interfering signal data symbol changes during the correlation operation. While both even and odd correlation functions must be evaluated to obtain an interference measurement for two scatter codes assigned to a pair of mobile stations to determine the degree of cross-correlation, an odd cross-retylation is difficult to determine theoretically for a given set of dispersion codes. Therefore, the pair correlation function is used to compare families or different sets of scatter codes to determine an optimal family / set. The present invention offers an optimal set of dispersion codes for use for example in a broadband CDMA mobile radio communication system (WCDMA). Although this set of spreading codes can be employed in synchronized downlink transmissions from the base station, it is particularly useful in the uplink relationship from the mobile station to the base station when the mobile stations different are not mutually synchronized. The optimal dispersion code array offers a large number of codes that also have low cross-correlation between scatter codes for all possible temporary changes between the different mobile stations. In this way, the capacity of the mobile communications system improves significantly while still providing satisfactory radio communications with minimal interference to other mobile stations or from other mobile stations. In a preferred embodiment, the optimal code family is the S (2) family of quadriplegic code sequences of length L = 2m-1, where m is an integer greater than 5 or equal to 5. The codes in the S family (2) are generated by the addition of module 4 of three component sequences including a first quaternary sequence of component aa (n), a second binary sequence of component b (n), and a third binary sequence of component c (n), where the binary sequences of b (n) and c (n) are multiplied by two before adding. The size of the family, that is, the number of quaternary dispersion codes, is (L + 2) (L + l) 2, and the maximum cross-correlation between two of the codes is l + 4"-J (L + l) • The three component sequences can be generated using corresponding linear feedback shift register generators.

The set of (L + 2) (L + 1) 2 of different S (2) sequences is obtained by combining the different component sequences produced by the different initial shift register states: (L + 2) initial states for a sequence a (n) and (L + l) initial states for sequences b (n) and for c (n). As an example, the number of S (2) scatter codes that have a length (L) of 255 chips is 16, 842, 752 with a par, absolute, maximum cross-correlation of 65. More than 16 million scatter codes of uplink provide a considerable system capacity. If it is considered that no more than 256 mobile stations will be served in a single base station * sector, then 65, 792 sets of codes can be reused in the mobile communication system. This large number of code sets offers considerable flexibility when planning the network. Even though the scatter codes from the family s (2) can be randomly selected and assigned to several users in a CDMA mobile communication system, a preferred exemplary embodiment of the present invention allocates the scatter codes in accordance with a method of specific code allocation that achieves more profitable results compared to the random selection of codes. Whereas the system of entire mobile communication uses the S family (2) of codes, specific subsets of scatter codes of the fl) family S (2) codes are assigned to each base station (or base station sector). The subsets of dispersion codes 5 have the same cross-correlation properties of the S (2) and / or S (l) families of codes and provide reduced interference for mobile stations operating in the same base station (or else base station sector) compared to codes randomly selected from the S (2) family of codes. Capacity is an important aspect of a communications system, but services are also very important. There are certain services provided in mobile communications systems, such as WCDMA cellular systems that may require or support more than one data rate. In the case of a variable speed and other services, it is desirable to offer scatter codes whose length can be expressed as an integer multiple of each dispersion factor in the mobile communication system. The dispersion factor corresponds to the number of chips used to disperse a single data symbol. Relatively short dispersion codes whose code period encompasses one or more data symbols are desirable in order to support detection of multiple users, of ' low complexity in radio base stations in accordance with CDMA. One way to implement multiple data rates is the use of these data rates that allow corresponding scattering factors (SF) to be expressed as SF (k) = L / 2lc, where L is the length of each scatter code in the family of codes and k is a positive integer and varies in proportion to the data rate, therefore, the length of the scatter code must be a power of two. By having the dispersion code length that can be expressed as an integer multiple of each possible scattering factor in the system, the overall synchronization in the receiver is significantly alleviated by making it independent of the data rate. In other words, if the dispersion code period contains an integer number of data symbols, data frames and data synchronization in the receiver are automatically derived when the receiver concentration sequence is synchronized with the incoming signal. Otherwise, the data symbol position in relation to the dispersion code period (relatively small) fluctuates with the passage of time, i.e. it is different in consecutive scattering code periods. As a result, it is difficult to assign a unique data synchronization signal to a dispersion code period and for consequently a separate circuit in addition to a code synchronization circuit must be employed to acquire and track the data synchronization. However, the length (L) of the codes in typical dispersion code families is 2m-1, for example the family of dispersion codes S (2) described above. For example, if m = 8, the code length is 255. In order to obtain the advantages of an optimal high capacity in a minimum cross-correlation code interference as well as support of the variable data rate applications, the present invention extends the length of each scatter code by a code symbol so that the dispersion code length is a power of two. In a preferred exemplary embodiment, an additional code symbol is added to the end of each scatter code. More specifically, the extended spread code is obtained by the addition of another code symbol after the L symbols of the original (not extended) code of length L. In an exemplary embodiment, the aggregate code symbol can be set, i.e. , have the same value, for all codes of family dispersion. In other example embodiments, the added code symbol has the same value as the first chip in the original spread code. In the case of quaternary dispersion codes such as those of the S family (2), the additional scatter code symbol can have four possible values, that is, 0, 1, 2, or 3. Preferably, the value of the additional scatter code symbol is selected to optimize the correlation mutual cross between the extended scatter codes. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing as well as other objects, features and advantages of the present invention will be apparent from the following description of the preferred embodiments and will be illustrated in the accompanying drawings in which identical reference numbers refer to the same. parts in several views. The drawings are not necessarily presented to scale, the illustration of the principles of the invention being emphasized on the contrary. Figure 1 is a block diagram of functions of an exemplary mobile communication system in which the present invention can be usefully employed; Figure 2 is a block diagram of functions of an exemplary radio station transceiver where the present invention can be usefully employed; Fig. 3 is a block diagram of functions illustrating further details of the scattering and modulation blocks illustrated in Fig. 2; Figure 4 illustrates a unit circle diagram that illustrates four four-phase heats in a complex plane; Fig. 5 is a flowchart illustrating exemplary procedures for providing a spreading code from an optimal spreading code family 5 S (2) in accordance with the present invention; Figure 6 is a schematic diagram illustrating with additional details the code generator shown in Figure 2; Figure 7 is a schematic diagram illustrating a spread scattering code generator in accordance with a fixed extended symbol example mode; and Figure 8 is a schematic diagram illustrating an exemplary extended scatter code generator in accordance with an exemplary embodiment of symbols newspaper extensions; Figure 9 is a block diagram of functions that • illustrates exemplary procedures in accordance with an extended spread code embodiment of the present invention; and Figure 10 is a graph showing the performance of fixed and periodic extended spread codes; DETAILED DESCRIPTION OF THE DRAWINGS In the following description, for purposes of explanation and not limitation, specific details such as particular modalities, procedures, techniques, etc. are presented.

The object is to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention can be practiced in other embodiments that depart from these specific details. For example, while the present invention is sometimes described in the context of a mobile radio station using uplink spreading codes, the present invention can be applied equally to other radio stations, for example radio base stations , and obviously to any dispersion spectrum communication system. In other cases, detailed descriptions of well-known methods, interfaces, devices and signaling techniques are omitted so as not to obscure the description of the present invention with unnecessary details. The present invention is described in the context of the universal mobile telecommunication system (UMTS) 10 presented in figure 2. An external network, oriented to connections, representative, illustrated as a cloud 12 can be for example the Public Switched Telephone Network (PSTN) and / or the Integrated Services Digital Network (ISDN). An external-oriented fed with no connections, representative illustrated as a cloud 14 can be, for example, the Internet. Both Core networks are connected to corresponding service nodes 16. The PSTN / ISDN connection oriented network 12 is connected to a service node oriented connection illustrated as a center node Mobile Switching (MSC) 18 that offers switched services per circuit. In the existing GSm model, the MSC 18 is connected through an interface A to a Base Station Subsystem (BSS) 22 which in turn is connected to a radio base station 23 through the interface TO' . The network oriented offline 14 Internet is connected to a General Packet Radio Service node (GPRS) 20 adapted to provide packet switching type service. Each of the radio core service nodes 18 and 20 connects to a UMTS Radio Access Network (URAN) 24 through a radio access network (RAM) interface. URAN 24 includes one or more radio network controllers 26. Each RNC 26 is connected to several base stations (BS) 28 and to other RNCs in the URAN 24. In the preferred embodiment, radio access is based on Multiple access by code division, broadband (WCDMA) with individual radio channels assigned using CDMA spreading codes. WCDMA provides broad bandwidth for multimedia services and other high-speed demands as well as robust features such as diversity transfer and RAKE receivers to ensure high quality. Each mobile station 24 receives its own spreading code so that a base station 20 identifies transmissions from this particular mobile station as well as for the mobile station to identify transmissions from the base station contemplated for this mobile station among all other transmissions and noise present in the same area. A CDMA radio station transceiver 30 in which the present invention can be employed is shown in FIG. 2 in function block format. Those skilled in the art will note that other radio transceiver functions employed in CDMA transceivers not particularly relevant to the present invention are not illustrated. In the transmission branch, bits of information to be transmitted are received by a separator 32 that disperses the information bits in the available spectrum of frequencies, (for broadband CDMA this frequency band could be for example 5 MHz, 10 MHz, 15 MHz or more), in accordance with a spreading code generated by a spreading code generator 40. The controller 44 determines which spreading code is to be supplied by the code generator 40 to the spreader 32. The spreading code provided by The code generator 40 corresponds to a radio channel in a CDMA communication system. Because a very large number of code symbols (sometimes referred to as "chips") can be used to encode each bit of information, (based on the current data rate in a variable data rate system, for example a WCMA system), the dispersion operation considerably increases the data rate thereby expanding the signal bandwidth. The dispersion signal is provided to a modulator 34 that modulates the dispersion signal in the RF carrier. An oscillator 42 generates a carrier of radio frequencies appropriate to a frequency selected by the controller 44. The modulated RF signal is then filtered and amplified in an RF processing block 36 prior to its transmission through the radio interface by means of an antenna 38. Similar but diverse operations are performed in the reception branch of the transceiver 30. An RF signal is received by the antenna 38 and filtered in an RF processing block 150. The processed signal is then demodulated to extract the baseband signal from the RF carrier in a demodulator 48 employing a suitable RF carrier signal provided by the oscillator 44. The demodulated signal is concentrated in a concentrator 46 in accordance with a code selected by the controller 44 and generated by the code generator 40. The concentrated signal corresponds to the information bits received in the baseband which are d Then they are typically processed additionally. While individual function blocks are displayed on the station transceiver radio 30, those skilled in the art will observe that these functions can be performed by individual equipment circuits, through a suitably programmed digital microprocessor, through an application-specific integrated circuit (ASIC), and / or by one or more digital signaling processors (DSPs). Figure 3 schematically illustrates additional example details of the separator 32 and the modulator 34. A similar scheme would apply to the demodulator 48 and the concentrator 46 with opposite functions in the reverse direction. Quadrature phase shift manipulation (QPSK) is used for both data modulation (performed by separator 32) and dispersion modulation (performed by quadrature modulator 34). Figure 4 illustrates four quadrifast points in the unit circle corresponding to the complex plane defined by an actual axis I and an imaginary axis Q. The four quadriplegic alphabetic values correspond to where The exemplary separator 32 in FIG. 3 includes two biphasic information streams (+/- 1) to be demodulated separately, such as, for example, a data stream of traffic and a stream of control data, which are input to respective multipliers 52 and 54 in order to be dispersed and multiplexed by IQ. The traffic and control data streams are scattered by different channelization codes and then represented in branches I and Q. The channelization codes are used to separately identify and distinguish the streams of real and imaginary information in the receiver, still if there is an imperfect I and Q phase synchronization in the receiver. In the situation where several streams of traffic and control data must be transmitted in parallel from a single mobile user, (eg, multiple code transmissions - provided for very high data rates), several orthogonal channelization codes they are used to make the necessary parallel code channels. Channeling codes can be based on what is known as Orthogonal Variable Scattering Factor (OVSF) codes that maintain orthogonality even if different scattering factors are employed. Channeling codes are common for all mobile stations. The information streams I and Q represent the real and imaginary parts of a complex data stream to be transmitted through a CDMA radio channel. In the present description, real information streams Separate imaginary and corresponding different channelization codes have been employed in order to generate a complex signal to be dispersed using a corresponding radio CDMA spreading code. However, the signal does not have to be complex. In fact, the present invention can be used to disperse any type of information signal. The scatter code generated by the scatter code generator 40 is used by the complex multiplier 60 to disperse the complex information signal. The complex multiplier 60 in a QPSK data modulator effects a complex multiplication between the complex data stream I + jQ and a complex dispersion code (eg, temporarily assigned to a mobile station) to provide the dispersion signal output to the modulator 34. A quadrature modulator 34 divides the scattered signal into real current (I) and imaginary current (Q) which are processed by a corresponding pulse formation filter 62, 64, for example a cosine filter raised to the root, and then it is provided to respective mixers 66 and 68 which also receive in-phase and quadrature versions of the RF carrier. The quadrature signals of the modulated carrier are summed in adder 70 and sent to RF processing block 36. As mentioned above, the number of scatter codes CDMA that is used to distinguish users of mobile stations, particularly in the uplink direction from a mobile station to a base station must be large enough to allow more 5 mobile stations to communicate at the same time in the same geographical area . On the other hand, the number of scatter codes can not be too large; otherwise there would be excessive interference generated between mobile stations to have an acceptable communication. The The present invention offers a set of dispersion codes with an optimal balance: a relatively large number of scatter codes with only minimal periodic cross-correlation between any pair of dispersion codes in the family. 15 By way of comparison, parameters of several families of biphasic and quadriplegic scattering codes are shown in Table I below. The size of the alphabet corresponds to the number of different values that each code symbol can assume. In the case of biphasic codes, the size of alphabet is 2; in the case of quadriplegic codes, the alphabet size is 4. The sequence length (L) is the number of code symbols ("chips") in each code and for all families of codes in table I is equal to 2m-l, where m is a positive integer whose possible values can be restricted according to how the family is built particular of code. The family size (M) is the number of codes in a particular family of scatter code. The greater the size of the M family, the greater the capacity. The maximum absolute cross-correlation (Cmax) is the maximum periodic cross-correlation between two scatter codes in the family of scatter codes. TABLE 1 Code family Size of Dispersion Restriction Length Alphabet (p) sequences (L) of length Gold 2 2m-l m = 1 mod 2 Gold 2 2m-l m = 2 mod 4 Similar to gold 2 2m-l m = 0 mod 4 Reciprocal similar 2 2m-lm = 0 mod 2 to Gold Small Kasami 2 2m-lm = 0 mod 2 Big Kasami 2 2m-lm = 2 mod 4 Big Kasami 2 2m-lm = 0 mod 4 Medium Kasami 2 2m-lm = 0 mod 2 Very large Kasami 4 2m-lm = 0 mod 2 Family S (0) 4 2m-l No Family S (l) 4 2m-lm > 3 Family S (2) 4 2m-l m > 5 Code family Family size Cross correlation of maximum absolute dispersion (M) ÍC max) Gold L + 2 1 + > / 2 > / L + 1 Gold L + 2 1 + 2 ^ L + 1 Similar to gold L + 1 1 + 2 ^ L + 1 Reciprocal similar L + 2 2 L + 1 - 1 to Gold Kasami small L + 1 1 + L + 1 Kasami Grande ( L + 2) VL + 1 1 + 2 L + 1 Big Kasami (L + 2) VL + 1-1 1 + 2 ^ L + 1 Medium Kasami (L + 2) 2VL + 1 1 + 4 ^ / L + 1 Very large Kasami (L + 2) 2VL + 1 2 (1 + 4 L + 1) Family S (0) L + 2 1 + L + 1 Family S (l) (L + 2) (L + 1) 1 + 2 / L + 1 Family S (2) (L + 2) (L + l) 2 1 + ^ L + 1 Based on this analysis several characteristics of these families of codes, the inventor determined that the S family (2) of dispersion codes offers the optimal compromise between the largest number (M) of scatter codes (L + 2) (L + l) 2, and the lowest cross correlation 1 + 4 * L + 1. In other words, for scattering codes S (2), the relationship between the number of scatter codes and the type of cross correlation is optimized for a given length of scatter code L. The families of scatter code S (l) and S (2) you get average The generalization from the construction of the family S (0) of cruadriphasic scattering codes. The family of dispersion codes S (2) includes the family S (l) which is the subset of (L + 2) (L + 1) scatter codes that are obtained by combining sequences of different components a (n) and b (n). The scattering code families S (2) and S (l) include the family S (0), which is the subset of (L + 2) scatter codes obtained by different initial states of a shift register of component sequences a (n). The scattering code family S (0) has the same number of scatter codes as in the Gold scatter code family, but the S (0) family has a lower cross-correlation by at least a factor of * v2. In order to provide a better understanding of the present invention, the construction of the S (2) family of scattering codes is described below. If h (x) = xm + lx "1-1 + ... + hm_? X + h, where H *, xe Z, is a primitive pomial in Z4 of degree m, where Z4 is the set of integers. { . 0,1,2,3.}., Ie, the integer ring of modulo 4. A list of all primitive pomials over Zj. To the gram m = 15 can be found in "On a Recent 4-Phase Sequence". Design for CDMA "(on a recent quadriplegic sequence design for CDMA), Hammons et al., IEICE Trans. Commun., Volume E76-B, number 8, pages 804-813. ) in Z4, in accordance as defined by h (x) as ar (n) = hla (n - \) - h2a (n - 2) - ... - hma (n - m) (mod 4), n = m. (I) produces a quaternary sequence of period L = 2 ™ - 1. The recurrence above can be implemented using a shift register with feedback connections. There are L + 2 cyclically distinct sequences that can be obtained from the recurrence defined in equation (1) by selecting an appropriate initial state of recurrence, that is, the shift register. The initial state Xr is a vector of m elements, which can be represented as The L + 2 initial states t0, ti, t:, ..., tL - i can be selected in accordance with the following algorithm tr = [(yr '"-,), 7 { r' n-2), ..., r (rr 7 (yr)], (2) where (mod (?). od 4) (mod x), (3) ro = l n = 2. ? 2 = 3 / 3 = l-, 4 = l- 2 ...., r¿ + I = l _ ^ - «, (mod h (x) .mod 4) (4) The set of sequences (ar (n).} Defined by equations (l) - (4) represents the family S (0) of sequences whose parameters are given in table I. The set of sequences (ar (n) } of length 255 can be generated by a primitive polynomial of degree 8 on ZJ The primitive polynomial of degree 8, which provides the simplest feedback connections of the generator of the corresponding shift register, is in accordance with the following: (x) = x9 + x? + 3x3 + x2 + 2x + 1 The family S (l) of sequences (yu (n).}., u = 0, 1, ..., (L + 2) (L + 1) -1, is the generalization of the family of sequences S (0) obtained by combining the quaternary sequences from the set (a (n).}., R = 0, 1, 2, ... , (L + 1), with the binary sequences { B3 (n).}., S = 0, 1, 2, ..., L, of the same length The exact algorithm is provided by the following relationship : W (ma / 4), n = 0, l ..., L-l. (6) The sequences of bs (n) are obtained through a linear recurrence on Z;, which is defined through the polynomial g (x) = xe + g? Xe ~ * + ... + ge-? X + 1 as bt. (n) = g, b (n- \) + g2b (n-2) + ... + b (n-e) (mod 2). n = e, (7) where e < m is a minimum integer that satisfies (3.2) mod (2m -1) = 3. The polynomial g (x) is related to the polynomial h (x) and is obtained from the polynomial g (x) ', provided by , 2 * \ (g (x) = (x - x - (r3) 2) (x - (x'f) ... [x - (3) 2 '"' (mod h (x). Mod 2 ), (8) in accordance with the following relationship 0 e < m (9) g (X > r? (x) + g (?) '| m0¿2. e = m For h (x) given by equation (5), corresponding g (x) is equal to 5 g (x) = x8 + x7 + x5 + x + 1 The set of L + 1 different sequences (but not cyclically different) of bs (n) is defined through the appropriate initial states of the recurrence defined in equation (7). The L + 1 initial states d;, di, d2, ..., dL, are defined as The current initial state [bs (m-l), b3 (m-2), ..., bs (0)] is provides by the corresponding polynomial coefficients ds defined by equation (11) in accordance with the following íf.6j ("- t)? -, +», (- i) I-J + * - + * - W The family S (2) of sequences (zv (n).}., V = 0, 1, 2, ..., (L + 1) (L + l) 2 - 1, is an additional generalization using families S (0) and S (l) is obtained by combining the sequences from the previously defined sets (ar (n).}. And (bs (n).} With an additional set { Ct (n).}. of L + a binary sequences, according to the following relation:? (n) = ar (n) + 2b: (n) + 2cl (n) (mod 4), ¿V (12 )? = (Ü L-.

An enumeration algorithm for the set S (2) can be defined by v = r.2 (L -:) (+1) + s.2 l + tr = 0, l, 2, ..., L + ls = 0, 1, 2, ..., L t = 0, 1, 2, ..., L The sequences ct (n) are obtained through a linear recurrence in Z2, which is defined by the polynomial f (x ) = xe + t-vJ ~ x + ... + fe-? X + 1 as c > (n) = fic, ("- l) + f2cl (n - 2) + ...- rcl (n - e) (mod 2) n = e, (I4) where e < m is a minimum integer that satisfies (5.2e) mod (2m -1) = 5. The polynomial f (x) is related to the polynomial h (x) and is obtained from the polynomial f (x) ', provided by in accordance with the following relationship • / (*) • h (x) + For h (x) provided by equation (5), corresponding f (x) is equal to f (x) = x8 + x7 + x5 + x4 + 1 The set of L + 1 different (but not cyclically distinct) sequences ct ( n) is defined by the appropriate initial states of the recurrence (14). These initial states are already defined by equation (11). The above constructions for the dispersion code family S (2) produce the quaternary codes with elements belonging to the set. { 0,1,2,3} . To obtain complex quadriphasic scattering codes that have a constant envelope, with real and imaginary parts being biphasic values + 1 / V2, that is, with the elements that belong to the set the following transformation is applied J * A ») r2 (") =, • e - (18) With this mathematical explanation of how the S (2) family of scatter codes is constructed, reference is now made to a Mobile Call routine (block 80) illustrated in a function block format in Figure 5. Initially, a station mobile requests a traffic channel (TCH) by sending a traffic channel request through a random access channel (RACH) (block 82). The random access channel has one or more corresponding spreading codes that the mobile station uses to transmit and receive in the random access channel. In response to the request of the mobile station, the base station sends through the random access channel to the mobile station the number "v" of a dispersion code zv (n) from the scattering code family S (2) (block 84) corresponding to an assigned radio channel. Zv (n) is defined in equation (122), and v define in equation (13) above. Using the dispersion code number v, the mobile station determines the ordinal numbers r, syt that uniquely identify the initial states of shift registers used to generate the three sequences of components ar (n), bs (n) and ct (n) defined above in equations (1), (7) and (14), respectively. These three component sequences are combined to provide a quaternary dispersion code S (2) corresponding zv (n) in accordance with equation (12) (block 88). The quaternary dispersion code S (2) is then projected to a corresponding quad-phase dispersion code (block 90) and used to scatter / concentrate (according to the transmission or reception operation currently performed in the mobile station) the information using the code of quadriplegic dispersion generated (block 92). Figure 6 illustrates an example shift register implementation of a code generator 40 for generating quadriphasic dispersion codes S (2) (and concentration) in accordance with an exemplary embodiment of the present invention. A code generator 40 includes three linear, feedback, 100, 102, and 104 shift registers. Each shift register includes eight memory elements (shifting stages) 0-7. At the beginning of each chip interval, the content of each memory element is moved (run) to the memory element adjacent to the right. The productions of the memory elements are multiplied by the coefficients of the respective recurrence equation and then module 4 (or 2) is added. The result of the duma is stored in the memory element further to the left at the beginning of the subsequent chip interval. The shift register 104 implements the linear recursion ar (n) defined in equation (1). The shift register 102 generates the sequences bs (n), and the shift register 100 generates the sequences ct (n) in accordance with equations (7) and (14). The outputs of the shift registers 100 and 102 are multiplied by two in respective multipliers 106 and 108. Each of the three sequences produced by the three corresponding shift registers is added in the adder 110 in order to generate a quaternary code S (2) which is then converted into a corresponding quadriphasic dispersion code S (2) through a representation device 112. Obviously, the output of the quaternary code S (2) depends on the actual initial state established in the shift registers that they are determined in accordance with equations (2), (3), (4), (11). These initial states can be entered into the appropriate shift registers by the transceiver controller 44 that establishes the appropriate values of the adjustable parameters in the transmitter and in the receiver, both in the transceivers of ^ mobile station and base station. Although in a preferred embodiment, the dispersion code generator 40 is implemented employing shift registers that generate the necessary S (2) shift codes as required, these scatter codes S (2) could be generated in advance, stored in memory, and ^^ recovered using a table reference function. Thus, the present invention offers a family of four-phase CDMA scattering codes that provide a maximum number of CDMA spread codes of a particular length that have a minimal cross-correlation. At the same time, these scatter codes have an alphabet of small signaling which is very convenient for the practical implementation of the separator and the concentrator. ^ fe Even if the S (2) family of scatter codes can be randomly assigned, a preferred mode assigns the codes from the S (2) family = more helpful. As shown above, the scatter code family S (l) and S (0) are subsets of the S (2) family of codes and have better cross-correlation properties, and therefore produce less interference among mobile users. Table 1 above shows that the family of codes S (l) and the family of codes S (0) have one half and one quarter of the maximum absolute cross-ratio compared to the family of S codes (2), respectively. In this preferred embodiment, the number of codes provided by the family S (2) is used by the mobile communication system, but specific subsets of codes S (2) are assigned to particular base stations or sectors of particular base stations. Accordingly, depending on the number of mobile users in a particular area of a CDMA cellular network, the quality of service is improved, that is, there is less interference between mobile users connected to the same base station or to the same station sector of base. For example, the mobile communication system may employ scatter codes S (2) of a length L = 255. A first base station BSO receives the code subset S (2) which is defined by the sequences of components having indexes r = 0, 1, 2, ..., 256; s = l; t = 0. In other words, BSO is assigned the "pure" S (0) family of codes. A second neighbor base station BSl receives another subset of scatter codes S (2) defined by the component sequences corresponding to the indices r = 0, 1, 2, ..., 256; s = l; and t = 0, the codes of the second base station are very similar to the pure S (0) codes, (the S codes (0) are multiplied chip by chip with a sequence of components common b? (n)), and have essentially the same characteristics. As a result of this subset assignment of S (2) codes, the cross-correlation between these assigned S (2) codes for each base station is the same as for the S (0) family of codes, that is, a smaller one. cross-correlation between the codes compared to what is observed in the case of the S (2) family in general. ^^ Using this strategy to assign subset code S (2), the mutual interference between mobile stations connected to the same base station is minimized, and the interference between the base stations is also limited in accordance with the properties of the S (2) codes. The subset code assignment strategy S (2) can be defined generally as follows: each base station (or base station sector) ^ fc has at least L + 2 scatter codes from the S family (2) defined by three sequences of components that have an index r = 0,1,2, ..., L and indices s and t that are for each base station (or each base station sector), that is, the indices s and t have different integer values for different base stations. While this scheme of assigning subset codes is profitable insofar as it reduces the correlation cross between mobile users in a base station / base station sector compared to the general S (2) family of codes, transfer situations require certain special conditions. For the duration of a call, the mobile station maintains in same spreading code assigned at the beginning of the call by the base station / source base station sector even when the mobile station changes from the original base source station during the transfer to a destination base station. Using the code assigned by the source base station while connected to the destination base station can produce more interference than what is obtained in the case of the S (0) family. But this interference is still not greater than I define it by the set of codes S (2). In the transfer situation where the source base station assigned a particular spreading code to the mobile station, the source base station can not assign the same spreading code before the transferred mobile station has terminated the call to avoid the situation in which two mobile stations receive the same code. One way to erase this is for the source base station to assign a finished time marker to each available scatter code. The finished time marker is set, which means that the code can be assigned to another mobile station only if a predefined time interval has elapsed since the code was assigned. Alternatively, the marker has a finished time value that is not 0 only when the mobile station is in transfer with the time interval starting at the time of transfer. In any case, the same code can not be assigned to two mobile stations at the same time when the mobile stations are connected to neighboring base stations. The construction of the S family (2) or another family of scatter codes produces scatter codes each having a length L = 2m - 1. Therefore, each code length is not a power of two. However, in a CDMA system that supports different data rates in the same physical radio channel according to the currently operating service, the dispersion code length must be expressed as a multiple of each dispersion factor in the CDMA speed system multiple The dispersion factor is the number of chips (several chips are used to scatter a bit of data) within the data symbol. One way to implement multiple data rates is the use of these data rates that allow the corresponding scattering factors (SF) to be expressed as follows SF (k) = L2 ' where the variable k is proportional to the data rate. Also, since the number of chips within the data symbol must be a whole number, the length of the scatter sequence must be a power of two. Accordingly, spreading code sequences belonging to the S (2) family can be extended with a quaternary symbol for optimal use in a multi-rate CDMA system. The present invention solves that need by providing a dispersion code extension without increasing the maximum cross-correlation between scatter codes in the dispersion code family, with minimum complexity of equipment implementation. In a preferred embodiment which attempts to reduce the complexity of the equipment implementation, the scatter code symbol is added to the end of the original spreading code in order to extend the code by a symbol. Obviously, the length of the original spreading code can be extended by adding a code symbol in other locations in the original code. In other words, extended spread codes can be obtained by the addition of an additional scatter code symbol, after the L symbols of the original, not extended spread code of a length L = 2m-1. In an example mode fixed code extension, the Additional scattering code symbol is fixed, that is, it is the same for all scatter codes. In the case of quaternary codes such as for example the family S (2) of scatter codes, the additional scatter code symbol 5 may have four possible values. The particular code symbol value, i.e., the chip value can be selected to minimize the mutual cross-correlation between extended sequences in a set, i.e., the S (2) sequences. • 10 An example of the fixed code extension mode is shown in FIG. 7 for the original dispersion codes S (2) of length 255 where similar reference numbers refer to similar elements of FIG. 6. code 40 'in figure 7 includes a comparator 120 connected to the outputs of each memory element of the shift register 104 that generates the sequence of components ar (n). In addition, a corresponding register 122 containing the initial state tr of the shift register 104 is connected to the remaining available inputs of the comparator 120. In addition, a switching block 124 is connected to an input terminal to the output of the adder 110. The other input terminal is connected to the value x of 'fixed code symbol and the switch output is is connected to the display device 112. The outputs of the comparator 120 suspend the shifting operation of the three registers 100, 102 and 104 and control the state of the switch 110. In operation, the comparator 120 detects the end of an original dispersion code S (2) by detecting the end of the sequence of components ar (n). Only the sequence of components ar (n) has the same period as the dispersion code S (2). The other two component sequences, bs (n) and ct (n), have shorter periods contained in the period of a scatter code S (2), and therefore, are not used to detect the end of the scatter code S (2). The end of the ar (p) component sequence is detected by detecting the subsequent periodic occurrence of the same state of the shift register 104 that was loaded in the register 104 upon initialization of the operation of the code generator 40. during the initialization of the code generator 40, the three shift registers 100, 102, and 104 are loaded with the corresponding initial states and then released to operate in parallel. However, only the internal state of the shift register 104 is monitored by the comparator 120. When the end of the original spreading code is detected by the comparator 120, the comparator 120 generates a shifting suspension operation during the next Dispersion code symbol cycle. At that time the extension symbol x, which can be any value of the set of values 0, 1, 2 or 3, is added to the end of the code when the switch 124 is momentarily connected to the terminal x in accordance with a output of comparator 120. During that time, the internal states of the three shift registers remain unchanged. As a result, the scatter code S (2) is extended by a symbol to obtain a total of 256 symbols that is a power of 2, that is, 28 ** = 256. After the chip interval inserted, three shift registers they begin to move from the corresponding initial states without actually recharging these initial states. Figure 8 shows an example of a periodic code extension mode in which the original spread code is extended by a single chip whose value is the same as the value of the first symbol in the original spread code. The structure and operation of the code generator 40"illustrated in Figure 8 is similar to what is described above for the code generator 40 'illustrated in Figure 7. However, the switch 124 is not used; an external source "x" is used that supplies the additional chip, on the contrary, when the end of the original dispersion code is detected by the comparator 120, the corresponding output of the code generator 40 represents the extended chip value (256ava). Displacement in all shift registers is suspended during the next chip cycle such that the same state, equal to the initial state, appears in the first chip cycle of the next dispersion code period. After the chip interval inserted, the three shift registers continue to move from their corresponding initial states without actually recharging these initial states. Accordingly, an additional symbol code equal to the first symbol in the original spreading code is inserted after the last symbol in the original spreading code without the addition of equipment. This same periodic extension can be implemented using a module counter, module 256 in this example where L = 255, (more generally the counter module is equal to the extended spread code period), which indicates the end of the scatter code extended. In operation, the shift registers are reinitialized as usual at the end of the code period and generate as the next chip output the first chip of the compliance code as determined by the initial states of the shift registers. But after the production of this first chip, (thus extending the dispersion code generated by a chip), the counter generates an output that causes the shift registers to reload their respective initial states in such a way that they restart the extended code generation operation. Reference is now made to an extended code routine (block 200) illustrating an example procedure in accordance with the present invention. Initially, a family of original scatter codes is generated, each code having a length L (block 202). For each generated spreading code, the end of this original spreading code is detected (block 204). Linear feedback and shift operations in the code generator are suspended momentarily (block 206). A decision is made in block 208 in the sense of itself in the fixed spread code extension procedure or the periodic scatter code extension procedure described in the two above example modes are selected. In the case of a periodic scatter code extension, a scatter code symbol equal to the first symbol in this code is added to the end of the scatter code (block 210). In the case of a fixed spread code extension, a fixed code symbol is added to the end of the scatter code (block 212). The code extension process is repeated for each generated code (block 214). Obviously, once you has made the decision as to the particular type of extension, the decision in block 208 no longer requires to be taken. In the case of the dispersion code family S (2), both 5 extension procedures described above can be carried out easily and with minimal equipment. These extended S (2) codes provide the flexibility to optimize multi-rate communications ^^ while allowing the greatest number of users to balance with a minimum cross-correlation between extended codes. Since it is difficult to predict the cross-correlation properties of theoretically extended codes, the following dispersion code performance evaluation S (2) is performed numerically. 15 The performance of fixed and periodic spreading code extensions is considered to be a time in combination with the figure ^ 10 and is based on the calculation of the probability of error in the average bits Pe in a multiple access system with K concurrent users. The calculation of error probability in the bits is implemented based on a numerical evaluation of an analytical formula that includes (K-2) times the convolution of the cross-correlation probability density function between pairs of codes in the following manner: where E is the data bit energy (dispersion sequence) with No is the additive white Gaussian noise power spectral density, and f? (Z) is the multiple access interference probability (PDF) density function. The function f? (Z) is obtained by (K-2) times the convolution of the cross-correlation PDF in pairs of code fpa? R (z '), that is, A BPSK data modulation format and time shift between users that correspond to the integer multiples of the code symbol period (chip) were considered in such a way that the probability density function of cross correlation. The cross-correlation probability density function was obtained by counting all the different values of the real part of odd and even cross correlations within a given set of scatter codes. The cross-correlation probability density function was evaluated for the spread codes S (l) extended (forming a subset of dispersion codes S (2)) of length L = 32. It was found that both the fixed extension approaches as newspapers they have approximately the same performance. The probability of errors in the average bits Pe for K = 4 concurrent users using periodically extended S (l) sequences of length 32, as well as for sequences S (l) extended by a fixed symbol (equal to 3) is shown in the figure 10. Comparing the performance between the non-extended and extended S (l) sequences, one would expect, based on the maximum absolute period cross-correlation (Cmax), that the non-extended dispersion codes would have a better performance because they have a value Cmax minor. If the number of users is K = 4, the scatter codes periodically extended produce a slightly higher frequency of errors in the average bits. However, when the number of users is raised to K = 6, extended codes surprisingly produce a lower frequency of bit errors on average than scatter codes not extended. This last relationship remains valid for all other user numbers ^ greater than 6 which represents another advantage of the extended spread code embodiments of the present invention. The explanation can be found in the properties of the odd cross correlation function that dominantly influences the shape of the cross-correlation probability density function of pairs of ^^ code fpair (z '), both in the case of scatter codes not extended or extended. The form of the multiple access interference probability density function f? (Z), which directly determines the frequency of errors in the average bits, is influenced by both the form of the function fpa? R (z ') and 15 by the number of autoconvolutions of fpair (z ') in equation (20) that is, by the number of concurrent users. While the present invention has been described in relation to a particular embodiment, those skilled in the art will recognize that the present invention is not limited to the specific embodiments described and illustrated herein. Different formats, modalities and adaptations in addition to the illustrated and described, as well as many modifications, variations and equivalent arrangements can also be used to implement the invention. Accordingly, while the present invention has been described in relation to its Preferred embodiments, it should be understood that this disclosure is illustrative only and presents only one example of the present invention and is only for the purpose of providing a clear disclosure of the invention. Accordingly, the present invention is limited only by the spirit and scope of the appended claims.

Claims (5)

1. CLAIMS In a mobile direct dispersion spectrum (DSSS) mobile communication system where several mobile radio stations communicate with one or more base stations located in corresponding geographic areas on a radio channel, each radio channel corresponding to one of a set of scatter codes, one or more of the radio stations, comprising: a scatter code generator that provides quaternary scatter codes from a family of quaternary sequences of length L = 2m-1, where m is an integer greater than or equal to 5 that has code elements from an alphabet. { 0,1,2,3} , generated by the fact of adding module 4 three component sequences that include a first quaternary sequence of components a, a second binary sequence of components b, and a third binary sequence of components c where the binary sequences of components b and c are multiplied by two before the addition module 4; a separator that disperses an information signal to be transmitted by the mobile radio using one of the quaternary scatter codes to provide a scattered signal; Y
2. a concentrator that concentrates a signal received using one of the quaternary dispersion codes. • The radio station according to claim 1, further comprising: a modulator that modulates the signal dispersed in a radio carrier, and a demodulator that demodulates a received radio signal and provides the signal demodulated to the radio. concentrator.
3. 3. The radio station according to claim 1, wherein the family of quaternary scatter codes provides a maximum number of scatter codes of a particular length

   15 that have a minimal cross-correlation. The radio station according to claim 3, wherein the size of the quaternary scatter code family is (L + 2) (L + l) 2 and the maximum absolute cross-correlation value for

   20 the family of quaternary dispersion codes is

   25

   The radio station according to claim 1, wherein the first quaternary component sequence is defined by:

   ar (n.) = hla (n-l) -h2a (n-2) -...- hma (n-m) (moa 4), n = m,

   where h is a first polynomial, the second quaternary sequence of components is defined by:

   bs (r.) = gxb (n-l) + g2b. { n-2) + ... + b (n-e) (mod 2), n = e,

   where g is the second polynomial, the third quaternary sequence of components is defined by:

   ct (n) = f? Ct (n-?) + f2c, (n-2) + ... + c, (n-e) (mod2), n = e,

   where f is a third polynomial. The radio station according to claim 5, wherein the code generator includes first, second and third feedback shift registers that generate the first, second and third component quaternary sequences, respectively, and where the corresponding initial states of the first , second and

   Third shift registers are determined in accordance with the redefinition of each of the dispersion codes in the family of quaternary dispersion codes. 7. The radio station according to claim 1, wherein the radio station is a mobile radio station and one of the base stations assigns one of the scatter codes to the mobile radio station in response to a request. to

   10 'A communication involving the mobile radio station. The radio station according to claim 1, wherein the dispersion code generator projects the scatter code

   15 quaternary to a four-phase dispersion code. 9. The radio station in accordance with the

   • claim 1, where the code generator extends the length of the quaternary dispersion codes generated. 10. The radio station according to claim 9, wherein the generated quaternary scattering codes are extended by a quaternary symbol in such a way that the quaternary scattering code length has a power of 2. 25 11. In a multiple access mobile communication system

   by code division (CDMA), wherein several mobile radio stations communicate with one or several radio base stations that are in corresponding geographic areas through a 5-radio channel, each radio channel corresponding to one of a set of scatter codes, one or more of the radio stations, comprising: a code generator that selectively provides four-phase dispersion codes

   10 determined from a quaternary dispersion code set S (2) having a maximum quaternary dispersion code number with a minimum cross-correlation and extending the length of quaternary dispersion codes S (2) to support

   15 multi-rate communications in the CDMA mobile communication system; ^^ a separator that disperses an information signal to be transmitted by the radio station using one of the four-phase dispersion codes assigned to the

   20 radio station to provide a dispersion signal; and a modulator that modulates the dispersion signal in a radio carrier. 12. The radio station in accordance with the

   25 claim 11, further comprising:

   a demodulator that demodulates a CDMA signal received from the radio carrier, and a concentrator that concentrates the received CDMA signal using the dispersion code

   5 quadriplegic in order to provide a received information signal. -13. The radio station according to claim 11, wherein information signals to

   ^^ transmit are assigned in a stream of data

   10 reals and in an imaginary data stream, the radio station further comprises: a real channel separator that disperses the actual data stream using a real channelization code; 15 an imaginary channel separator that disperses the imaginary data stream using an imaginary channel ^ ^ code; and a combiner that combines the outputs of the real and imaginary channel separators to generate a complex signal, wherein the separator disperses the complex signal using the four-phase dispersion code assigned to the mobile radio. 14. The radio station according to claim 25, wherein the modulator is a modulator

   of manipulation of quadrature phase shifting (QPSK) and where the complex signal is divided into real and imaginary components that are then entered into the real and imaginary inputs of the QPSK modulator. 15. The radio station according to claim 14, wherein the code generator projects the quaternary dispersion code to a four-phase dispersion code. 16. The radio station according to claim 11, wherein the code generator extends the length of quaternary dispersion codes S (2) periodically. The radio station according to claim 11, wherein the quaternary dispersion codes S (2) are extended by a quaternary symbol in such a way that the dispersion code length S (2) is a multiple of each factor of multiple dispersion employed in the CDMA mobile communication system. 18. In a code division multiple access (CDMA) communication system where several communication devices communicate using assigned communication channels, each channel corresponding to a set of CDMA spread codes, one method

   which comprises: generating a family of dispersion codes w? quaternary S (2) original, each quaternary dispersion code S (2) original has a predetermined length 5, and extend the length of the original Quaternary dispersion codes S (2) originating from the quaternary scattering code family S (2) )

   ^ originals by a code symbol to generate a

   10 family of CDMA scatter codes. The method according to claim 18, wherein the extension further comprises: detecting the end of one of the original quaternary dispersion codes S (2), and adding the code symbol to the end of one of the original quaternary dispersion codes S (2). • The method according to claim 18, wherein the added code symbol is the same for

   20 all quaternary dispersion codes S (2) original in the family. The method according to claim 18, wherein the code symbol added to the quaternary dispersion code S (2) original is the same as

   25 the first code symbol in the original code.

   22. The method according to claim 18, wherein the code symbol is added at the end of one

   Wf of the original quaternary dispersion codes S (2) periodically. 23. The method according to claim 18, wherein the original quaternary scattering code family S82) are quadriplegic codes, and each aggregate code symbol used to extend the

   ^^ quaternary dispersion code S (2) original has

   10 four possible values. 24. The method according to claim 18, wherein the aggregate code symbol is selected to minimize the cross-correlation between CDMA spreading codes. 25. A CDMA code generator that provides CDMA spreading codes, comprising: one or several shift registers of

   • feedback that have m stages, where m is an integer, where a departure from a last stage is

   20 feedback to an input of a first stage, the output - of the feedback shift register or of the various feedback shift records corresponding to one of a family of Quaternary Scatter Codes

   25 S (2) of length L = 2m-l, and

   an electronic circuit for adding an additional code symbol to the quaternary dispersion code S (2) to provide an extended quaternary dispersion code S (2) corresponding to one of the CDMA scattering codes. 26. The CDMA code generator according to claim 25, further comprising: a comparator that detects one end of a code and points to the electronic circuit to add the

   10 additional code symbol at the end of a code. 27. The CDMA code generator according to claim 25, further comprising: a counter that generates a counter output at the end of an extended code period that causes the

   15 shift records are adjusted to respective initial states. ^ • ^ 28. The CDMA code generator according to claim 25, wherein the added code symbol is the same for all codes in the

   20 family. 29. The CDMA code generator according to claim 25, wherein the code symbol added to the original code is the same as the first code symbol in the original code. 25. 30. The CDMA code generator in accordance with

   claim 25, wherein an additional code symbol value is selected to reduce the cross-correlation between the CDMA spreading codes. 31. The CDMA code generator according to claim 25, wherein the family of quaternary sequences of length L2m-1, where m is an integer greater than or equal to 5 having code elements ^^ from a Alphabet { 0,1,2,3} , generated by the

   The fact of adding module 4 three sequences of components that include a first quaternary sequence of components a, a second binary sequence of components b, and a third binary sequence of components c where the shift register of

   15 feedback or the various feedback shift records include a first

   ^ feedback shift record, a second feedback shift record and a third feedback shift record

   20 that generate the first, second and third component quaternary sequences, respectively, with the output of the second shift register and the third shift register multiplying by two before the module 4 sum of the first, second, and

   25 third component quaternary sequences.

   32. The CDMA code generator according to claim 32, wherein the code generator projects the extended quaternary sequence into complex quadriphasic CDMA scatter codes. 33. In a mobile communication system that includes several base stations for communication with mobile stations and employs dispersion codes from a particular scattering code family for radio communications between mobile stations and base stations, a method who understands; assigning a first subset of the particular family of scatter codes to a first base station; and assigning a second subset of the particular family of scatter codes to a second base station, where the scatter codes in the first subset and in the second subset have lower cross-correlations than the scatter codes in the particular family of codes dispersion.
4. 4. The method according to claim 33, wherein the particular scattering code family corresponds to the S (2) family of codes and the first

   subset and the second subset are associated with one or both of the families of codes S (0) and S (l). 35. The method according to claim 34, wherein each of the first subset of codes of

   The dispersion and second subset of dispersion codes is defined by three component sequences in such a way that a first sequence of components includes an Index of r = 0,1,2 .., L + 1, ^ where L is the length of scatter code, and one

   10 or more of the indices for the second sequence of components and the third sequence of components are different from the first base station and second base station. 36. The method according to claim 33, further comprising: for a particular call, assigning to a mobile station ^^ associated with the first base station an assigned-code from the first subset of scatter codes; 20 associate a marker with the assigned code; set the marker to a first value when the mobile station is involved in the call; set the marker at a second value after the expiration of a prescribed time; and 25 prohibit the assignment of the assigned code to another

   mobile station until after the expiration of the prescribed time. 37. The method according to claim 36, further comprising: setting the marker at the first value at the beginning of the call, and measuring the prescribed time from the beginning of the call. ^ - 38. The method according to claim 36, in

   10 wherein the mobile station employs the assigned code for the duration of the call even when the call is transferred to the second base station. 39. The method according to claim 38, further comprising: setting the marker at the first value when the mobile station is involved in the call; ^^ measure the prescribed time from a time associated with the transfer; and if the mobile station has not been in a transfer 20 during the call, the dialer is set to the second value at the end of the call. 40. In a mobile communication system that includes a base station that has several sectors to communicate with mobile stations and employs codes

   25 scatterings from a family of codes

   particular dispersion for radio communications between the mobile stations and the base station, a method comprising: assigning a first subset of the particular family of scatter codes to a first base station sector; and assigning a second subset of the particular family of scatter codes to a second base station sector, wherein the scatter codes in the first subset and in the second subset have lower cross correlations than the scatter codes in the particular family of dispersion codes. 41. The method according to claim 40, wherein the particular scattering code family corresponds to the code family S (2) and the first subset and the second subset are associated with one or both of the code families S (0) and S (l). 42. The method according to claim 41, wherein each of the first dispersion code subset and second dispersion code subset is defined by three component sequences such that a first sequence of components includes an index of r = 0,1,2 .., L + 1,

   where L is the dispersion code length, and one or more of the indices for the second sequence of components and the third sequence of components are different for the first base sector and for the second base station sector. 43. The method according to claim 40, further comprising: for a particular call, assigning to a mobile station associated with the first base station sector an assigned code from the first subset of scatter codes; associate a marker with the assigned code; set the marker to a first value when the mobile station is involved in the call; set the marker at a second value after the expiration of a prescribed time; and prohibit the assignment of the assigned code to another mobile station until after the expiration of the prescribed time. 44. The method according to claim 43, further comprising: setting the marker at the first value at the beginning of the call, and measuring the prescribed time from the beginning of the call.

   45. The method according to claim 43, wherein the mobile station employs the assigned code for

   • the duration of the call even when the call is transferred to the second sector of the base station. 46. The method according to claim 45, further comprising: setting the marker to the first value when the mobile station is involved in the call; ^^ Measure the prescribed time from a time

   10 associated with the transfer; and if the mobile station has not been in a transfer during the call, the dialer is set to the second value at the end of the call.

   fifteen

   twenty

   25

   SUMMARY OF THE INVENTION Optimal code sequences are generated for use in dispersion and concentration functions in a code division multiple access (CDMA) communications system. Particularly, a family of quadriplegic scattering codes is used which offers a maximum number of scatter codes to achieve a high capacity in the CDMA communication system while having at the same time a minimum peak cross-correlation between any two scatter codes within the family to ensure that cross-correlation interference is maintained at acceptable levels. This family of optimal quadriphasic scattering codes is the S (2) family of quadriplegic code sequences of length L = 2 m "1, where m is an integer greater than or equal to
5. 5. The size of the S family (2) of quadriplegic scatter codes is (L + 2) (L + l) 2, and the maximum cross-correlation is 1 + (L + 1). Dispersion codes are preferably assigned to base stations that use specific subsets of codes of the S family (2) that have the same cross-correlation properties of the S (0) and / or S (l) family of codes. The scatter codes are profitably extended by one or several code symbols as necessary or otherwise desirable, for example, to support variable speed services, it is desirable to employ

   Dispersion codes whose length can be expressed as an integer multiple of each scattering factor in the mobile communication system. Since individual spreading codes have a length of 2m -1, a code symbol is added to the generated spreading code.