MXPA99012057A - Transceiver prerotation based on carrier offset - Google Patents

Abstract

A wireless telephone system having aplurality of wireless handsets and a base unit, the base unit having a base transceiver. Each handset has a handset transceiver for establishing a wireless link over a shared channel with the base unit via the base transceiver, wherein the base transceiver transmits to a given handset transceiver a forward signal at a carrier frequency. Each handset transceiver has a receiver having a carrier tracking loop for detecting and removing a carrier offset from the forward signal;a transmitter for transmitting to the base transceiver a return signal;and an oscillator, independent of a base oscillator of the base transceiver on which the carrier frequency is based, for driving the receiver and transmitter of the handset. The handset transmitter comprises a prerotator that prerotates the return signal in accordance with the carrier offset detected by the carrier tracking loop so that the return signal will be received by the base transceiver with substant ially no carrier offset.

Description

PRERROTATION OF TRANSMITTER-RECEIVER BASED ON THE DISASSEMBLY OF THE CARRIER FIELD OF THE INVENTION The present invention relates to digital signal processing systems, and in particular, to communications between transmitter-receivers having independent oscillators.

BACKGROUND OF THE INVENTION The transmission of digital data from a transmitter to a receiver requires a variety of digital signal processing techniques to enable data to be transmitted by the transmitter, and to be successfully retrieved by the receiver. In digital wireless telephone systems, a wireless telephone headset unit communicates by means of digital radio signals with a base unit, which is normally connected by means of a standard telephone line to an external telephone network. In this way, a user can use the wireless headset to have a telephone call with another user through the base unit and the telephone network. Multi-line wireless telephone systems are being used in different situations, such as in businesses with many telephone users. These .l ^ á-d-auái & systems employ a headset that communicates with up to N handsets in a simultaneous manner, typically with digital communications schemes, such as an extended spectrum time division multiple access (TDMA). In an extended spectrum system, bandwidth resources are traded for performance gains, according to the so-called Shannon theory. The advantages of an extended spectrum system include a low power spectral density, better rejection of narrowband interference, integrated selective addressing capability (with code selection), and multiple access capability to the inherent channel. The spread spectrum systems employ a variety of techniques, including direct sequencing (DS), frequency hopping (FH), chirping systems, and hybrid DS / FH systems. In a time division multiple access system, a single radiofrequency channel is used, and each headset transmits and receives audio data packets, as well as data packets without audio, during dedicated slices or time slots within a cycle or time of multiple access of global time division. Other communication schemes include frequency division multiple access (FDMA), multiplexing / code division multiple access (CDM / CDMA), and combinations of these schemes, both complete and half-duplex. Different modulation schemes are used, such as amplitude / phase without carrier (CAP), and quadrature amplitude modulation (QAM). These digital data are often transmitted as modulated signals on a transmission medium, such as the radio frequency channel, in the form of binary bits of data. (Other transmission media frequently used for digital communications include twisted pair systems, which employ asymmetric digital subscriber cycle (ADSL) technology, or cable modem systems). The digital data is often modulated and transmitted in a complex form of digital data, where the transmitted data comprises symbols from which the original data can be reconstructed by the receiver. The complex data of digital symbols usually comprise real data (in phase, or "I"), and imaginary data (in quadrature, or "Q") (pairs I, Q). Each symbol of a pair I, Q can be a number of multiple bits, and represents a location of a constellation, mapped against a decision region, such as a quadrant. Each symbol is mapped or mapped to a prescribed coordinate in a four-quadrant grid-type constellation, using a look-up table (eg, a ROM). A prescribed number of symbols occupy the assigned areas in each quadrant, depending on the coding scheme. Depending on the number of bits / symbols of a given coding scheme, each quadrant of the constellation contains a number of symbols at prescribed coordinates with respect to the quadrature axes I and Q. For example, in the QPSK coding scheme, each sample it has one to four phase positions, one for each quadrant, so that each pair of symbols represents two data bits. To transmit a given input data value in a complex data system, the input data value to be transmitted is mapped to a pair of symbols or to a pair of I, Q coordinates of a corresponding constellation point on a constellation of complex signals having real and imaginary axes I and Q. These symbols I, Q, which represent the original data value, are then transmitted as part of data packets by means of a modulated channel. A receiver can retrieve the I, Q pairs, and determine the location of the constellation from them, and perform a reverse mapping to provide the original input data value, or a close approximation to it. In an extended spectrum system, each symbol is transmitted by a string of "sub-symbols" or "chips", derived by multiplying the symbol by a binary string of a pseudo-random number (PN). Therefore, these systems are characterized by a chip speed, which is related to the symbol rate by a so-called extension factor (a factor by which the original speed of the symbol data has been expanded). Extended spectrum systems can also be used, in general, to transmit any digital data, whether in complex format or not. As noted above, the transmission of digital data requires a variety of digital signal processing techniques to allow data to be transmitted by the transmitter, and to be successfully retrieved by the receiver. For example, first a communication link must be established, where the two transmitter-receivers are secured with one another, establish synchronization and other system parameters, and the like. The receiver side of a data transmission in an extended spectrum digital wireless telephone system employs a variety of functions to recover the data from a transmitted radio frequency signal. These functions may include: time recovery for symbol synchronization, carrier recovery (frequency demodulation), equalization, and gain control. The receiver includes symbol time recovery (STR), automatic gain control (AGC), carrier tracking (CTL) cycles, and equalizer cycles for each link. The recovery of time is the process by which the clock of the receiver (time base) is synchronized with the clock of the transmitter. This allows the received signal to be sampled at the optimal time point to reduce the opportunity for a splice error associated with the processing directed to the decision of the received symbol values. In some receivers, the received signal is sampled at a multitude of transmitter symbol rates. For example, some receivers sample the received signal at twice the symbol speed of the transmitter. In any case, the sampling clock of the receiver must be synchronized with the symbol clock of the transmitter. Equalization is a process that compensates for the effects of the alterations of the transmission channel on the received signal. More specifically, the equalization removes the intersymbol interference (ISI) caused by the alterations of the transmission channel. Intersymbol interference causes the value of a symbol given by the values of the previous and next symbols to be distorted. Carrier recovery is the process by which a received radio frequency signal, after changing its frequency to a lower intermediate band, the frequency is changed up to the base band to allow the retrieval of the base modulation band information. These and related functions, and the related modulation schemes and systems, are discussed in more detail in Edward A. Lee &; David G. Messerschmitt, Digital Communication, 2nd edition (Boston: Kluwer Academic Publishers, 1994). Because each transceiver operates on an independent oscillator, even when the frequencies are equal, the signal transmitted by a receiver is typically received with a "rotating" constellation, that is, a frequency offset of the carrier, which is detected and it is taken into account through the CTL. Accordingly, when a transmitter-receiver transmits at a given frequency of the carrier according to its local oscillator, the CTL of the receiving transceiver converts down to the lowest passband, and digitally removes the residual phase shift from the carrier Subsequently, the transceiver can regenerate the data stream embedded in the transmitted signal. Of course, when the second transmitter-receiver transmits the data back to the first transmitter-receiver, the first transmitter-receiver must also apply a CTL to remove the residual phase shift of the carrier. During initial insurance to establish a link, this process may delay the acquisition of an insured link, at both ends. Once both transceivers know the carrier phase shifts after the link is initially established, subsequent communications are not delayed either, because each receiving side can initiate the acquisition using the last recovered carrier phase shift. However, the initial link process may be delayed because each transceiver has independent oscillators. In addition, in a multi-line wireless telephone system employing a base unit and a plurality of headsets, each having a transceiver with an independent oscillator, such as a time division multiple access system, including after that the initial links are established, in order to avoid having to re-determine the correct carrier delay for each separate handset, when its slot is presented, and consequently, delaying its acquisition, the base must store and maintain the track of the carrier phase shifts for each of a plurality of links. This storage and tracking can be complex, costly, may cause delays, or may be otherwise undesirable, and yet, without it, the acquisition delay increases.

COMPENDIUM A wireless telephone system having a plurality of wireless headphones and a base unit, the base unit having a base transceiver. Each headset has a headset transceiver to establish a wireless link on a channel shared with the base unit by means of the base transceiver, wherein the base transceiver transmits up to a given headset transceiver, a signal towards forward to a carrier frequency. Because the base and each headset transceiver operate on independent oscillators, each headset transmitter receives the forward signal having an out-of-phase carrier. Each handset transceiver has a receiver that has a carrier tracking cycle to detect and remove the forward signal carrier phase shift; a transmitter for transmitting a return signal to the base transceiver; and an oscillator, independent of a base oscillator of the base transceiver, on which the carrier frequency is based, to drive the receiver and transmitter of the handset. The transmitter of the earphone comprises a prerrotator which pre-rotates the return signal in accordance with the phase shift of the carrier detected by the tracking cycle of the carrier, such that the return signal will be received by the base transceiver with substantially no carrier phase shift.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of the multi-line multiple-access, time-division, wireless telephone system in extended spectrum, in accordance with one embodiment of the present invention. Figure 2 is a block diagram illustrating the system of Figure 1, and the prerrotator of the earphone transmitter in greater detail, in accordance with one embodiment of the present invention. Figure 3 is a block diagram of a multi-line, frequency division multiple-access wireless telephone system in accordance with an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED MODALITY In the present invention, the transmitter-receiver of each headset of a multi-line wireless telephone system includes a prerrotator that pre-rotates the signal transmitted to the base unit of the system, in accordance with the phase shift of the carrier determined by the CTL of the transceiver receiver. This ensures that the signal received by the receiver of the base unit from each handset during, for example, the respective timeslots of a timeslot multiple access epoch, is received with virtually no carrier phase shift (or rotation). , thus streamlining the acquisition by the base unit, and also eliminating the need for the base unit to store and track the carrier phase shifts of each separate handset. These and other details and advantages of the present invention are described in more detail below. Referring now to Figure 1, a block diagram of the time division multiple access multi-line wireless telephone system 100 is shown, in accordance with one embodiment of the present invention. The time division multiple access system 100 comprises a base unit 110, which has the receiver and transmitter units 112 and 111, respectively, and is coupled to the external telephone network 116 by means of the telephone lines 115. The system 100 it also comprises N wireless headphones 120, 1202, ... 120N. Each has a transmitter and receiver unit (transceiver), such as transmitter 121 and receiver 122 of headset 120- ^ At any given time, some number (or none) of the headset is operating or is off-hook (i.e. the process of conducting a telephone call). Therefore, the system 100 provides a wireless network or link between the base station 110 and each headset 120i (1 <; i < N) In one embodiment, system 100 comprises 4 120-1204 headphones, all of which can be active in a simultaneous manner. In another embodiment, the system 100 comprises a different number of headphones, for example, N = 12, of which, for example, up to 8 may be active or operational at the same time. Each transmitter 121 modulates and transmits modulated signals. A variety of digital modulation formats can be employed in these systems, including: QAM, CAP, PSK (phase shift keying), PAM (pulse amplitude modulation), VSB (vestigial sideband modulation), FSK (click of frequency change), OFDM (orthogonal frequency division multiplexing), and DMT (discrete multitone modulation). In one embodiment, the present invention comprises a time division multiple access system for connecting multiple transceivers to a base station on a single radiofrequency channel. In particular, the system 100 employs a time division multiple access digital scheme, as described in more detail below. Referring now to Figure 2, there is shown a block diagram illustrating the system 100 of Figure 1, and an example earphone transmitter prerrotator in greater detail, in accordance with one embodiment of the present invention. Figure 2 shows, for illustrative purposes, the details of the prerrotator of transmitter 111 and receiver 112 of base unit 110, and transmitter 121 and receiver 122 of handset 121x. In particular, the base unit 110 comprises a base oscillator 215, which serves as a common oscillator to drive both the up converter 211 and the down converter 212, ie both round-trip channels. A signal or data message transmitted from the base unit to a handset can be referred to as a forward signal, and a signal or data message transmitted from a handset to the base unit can be referred to as a return signal. The base receiver 112 also comprises the unloader 216, and the numerically controlled CTL / oscillator (NCO) 217. The up converter 211 transmits a radiofrequency signal via the antenna 218, while the down converter 222 receives the signals from radiofrequency by means of the antenna 218. The earphone 120 comprises the local oscillator of the earphone 225, which serves as a common oscillator for driving both the upstream converter 221 and the downstream converter 222. The receiver of the earphone 122 also comprises the unrotator 226 , and the CTL / NCO 227. The up converter 221 transmits radiofrequency signals to the base unit 110 via the antenna 228, while the down converter 222 receives radio frequency signals from the base unit 210 via the antenna 228. In accordance with one embodiment of the present invention, the earphone transmitter 121 also comprises the transmitter predriver of the headset 229, which is coupled with the CTL / NCO of the headset receiver 227, to receive the carrier bearer phase out information. As will be appreciated, each headset 1202-20N is similarly configured to 120-2, with a headset transmitter prerrotator, such as the prerotator 229. Accordingly, during the initial acquisition of a link between the base unit 110 and any given handset , such as the receiver 120, the base unit 110 transmits a data packet in the "down link", which is secured by the receiver of the earphone 122. This signal transmitted by the base unit 110, has some delay of the carrier or rotation, because its local oscillator 215 is independent of the local oscillator of the handset 225. The CTL / NCO of the receiver of the handset 227 detects the phase shift of the carrier, and removes it digitally with the unloader 226. The same phase shift of the detected carrier , also used by the earphone transmitter prerrotador 229, to perform a reverse rotation on the "return channel link" or "link up" sent back to the base unit 110. In other words, the "return" channel is prerotated with a rotation that is the inverse of the rotation detected and removed from the "forward" channel. Accordingly, the base receiver 112 receives the signal from the handset transmitter 121, with virtually no rotation, thereby speeding up the acquisition of the signal. Accordingly, the base receiver 112 can be secured on the return channel more easily, due to its prerotation, and only needs to track the phase errors, because the frequency errors in the handset were removed. Subsequently, once the link has been established, the handset transmitter 121 continues to prerotate the signal transmitted with the prerrotator 229, thereby eliminating the need for the base unit 110 to store and track the carrier phase shifts of each separate handset. 1202-120N. In one embodiment, the prerrotator 229 comprises an NCO that can pre-rotate the return channel according to the last phase shift of the carrier frequency used in the receiver of the handset 122. Accordingly, in the present invention, a transceiver, such as an earphone , measures different parameters of the forward channel, including the offset of the carrier, and uses these parameters to pre-compensate the signal of the return channel, in order to improve the operation and operation of the receiver of the return channel. In an alternative embodiment, the base transmitter 111 also includes a prerrotator, which, while not expediting the acquisition of the initial link as does the use of the transmitter prerrotators of the headset 229, does eliminate the need for the headset receiver 122 to store and tracking the carrier phase shift of the base unit 110. One skilled in the art will recognize that the wireless system described above, in accordance with the principles of the invention, can be a cellular system, wherein the base unit 110 represents a station base that serves one of the cells in a cellular telephone network. In addition to the digital communications of a wireless telephone system, as described hereinabove, the present invention is also applicable to BPSK, QPSK, CAP, and QAM, for example, as well as to VSB modulation systems, such as used by the Grand Alliance High Definition Television (HDTV) system proposed for use in the United States. One skilled in the art will recognize what design changes are required to adapt the modulation system of the transmitter disclosed, to the desired modulation scheme, and will understand how to design the illustrated components to operate with the desired modulation scheme. In addition to implementing the present invention in a time division multiple access system, in alternative embodiments, the present invention is also applicable to other systems, such as FDMA, CDM / CDMA, and combinations of these schemes, both complete and half-duplex. For example, now referring to Figure 3, a block diagram of a multi-line, frequency division multiple frequency wireless telephone system 300 is shown, in accordance with an alternative embodiment of the present invention. When using different radiofrequency channels, identical oscillators will not always be used. However, if, as in the frequency division multiple access system 300, the forward and return channel oscillators (synthesizers) are secured to a common reference oscillator in both the base and the handset (oscillators 315, 325), then the frequency error is a function of the proportions of the synthesizer. Full duplex operation is also possible in this case. In this system, only one receiver is required to cancel the deviation of the carrier frequency; therefore, to implement the present invention in this frequency division multiple access system, one link can take the lead to acquire the channel, and the address of the other link would be secured quickly, because the frequency deviation would be removed. carrier, as will be appreciated by experts in this field. Those skilled in the art will also appreciate how to apply the present invention to other systems, such as CDM / CDMA. For example, in a CDM / CDMA system, full duplex operation is possible, because both channels may be operating concurrently in the same band. In the implementation of the present invention to a CDM / CDMA system, therefore, preferably a link (the base to the earphone, for example) is secured before the opposite link. Once the link from the base to the handset is established, the displacement of the carrier is measured and used in the return link. Using a prerrotator, the complexity of the system is distributed more evenly between the base and the headphones. It will be understood that those skilled in the art can make different changes in the details, materials, and configurations of the parts that have been described and illustrated above, in order to explain the nature of this invention, without departing from the principle and scope of the invention. invention as described in the following claims.

Claims (16)

1. A transmitter-receiver for communicating with a second transmitter-receiver transmitting a forward signal to the transmitter-receiver, the transmitter-receiver being characterized by: (a) a receiver having a carrier tracking cycle to detect and remove a phase shift the carrier of the forward signal; (b) a transmitter for transmitting a return signal to the second transmitter-receiver; (c) an oscillator, independent of the second oscillator of the second transmitter-receiver on which the carrier frequency is based, for driving the receiver and the transmitter, wherein the transmitter comprises a prerrotator that pre-rotates the return signal according to the phase shift of the carrier detected by the tracking cycle of the carrier, such that the return signal will be received by the second transmitter-receiver without substantially no carrier phase-shifting.

2. The transceiver of claim 1, characterized in that: the forward signal represents successive symbols at a first symbol rate; and the receiver is to receive samples that represent the forward signal. The transceiver of claim 1, characterized in that: the transceiver is a headset transmitter of a first wireless headset; the second transmitter-receiver is a base transceiver of a base unit; the first wireless headset and the base unit are part of a wireless telephone system further comprising a plurality of other wireless headsets, each headset comprising an earphone transmitter-receiver for establishing a wireless link on a channel shared with the base unit by means of the base transceiver. The transceiver of claim 3, characterized in that the wireless link is a time division multiple access (TDMA) link, wherein each handset communicates during a time slot exclusive to a multiple division access scheme of time that allocates time slots to headphones. The transceiver of claim 3, characterized in that the wireless telephone system is an extended spectrum system, wherein each successive symbol is a chip of a sequence of binary spread spectrum chips that represents a complex symbol. 6. The transmitter-receiver of claim 1, characterized in that the prerrotator is coupled to an output of the tracking cycle of the carrier, to receive the phase shift of the carrier detected by the tracking cycle of the carrier. The transceiver of claim 1, characterized in that the forward signal comprises a downlink data packet transmitted by the second transceiver before a communication link is established; and the return signal comprises a packet of return channel link data transmitted by the transmitter-receiver after the transmitter-receiver is secured on the forward signal. 8. In a transceiver that has a receiver with a carrier tracking cycle, a transmitter, and an oscillator coupled to the receiver and the transmitter, a method for communicating with a second transmitter-receiver that transmits a signal to the transceiver forward to a carrier frequency, characterized in the method by: (a) driving the receiver and the transmitter with the oscillator, characterized in that the oscillator is independent of a second oscillator of the second transmitter-receiver on which the carrier frequency is based; (b) receiving the forward signal with the receiver, and detecting and removing, with the tracking cycle of the receiver carrier, a shifting of the carrier from the forward signal; (c) preset with a transmitter prerrotator, a return signal according to the carrier phase shift detected by the tracking scan of the carrier, such that the return signal is received by the second transmitter-receiver without substantially no lag of the carrier; and (d) transmitting the pre-rotated return signal to the second transmitter-receiver. The method of claim 8, characterized in that: the forward signal represents successive symbols at a first symbol rate; and step (b) comprises the step of receiving samples representing the forward signal. The method of claim 8, characterized in that: the transceiver is a headset transceiver of a first wireless handset; the second transmitter-receiver is a base transceiver of a base unit; the first wireless headset and the base unit are part of a wireless telephone system further comprising a plurality of other wireless headsets, each headset comprising a headset transmitter-receiver for establishing a wireless link on a channel shared with the base unit by means of the base transceiver. The method of claim 10, characterized in that the wireless link is a time division multiple access link, wherein each handset communicates during a time slot unique to a time division multiple access scheme allocating slots of time to headphones. The method of claim 8, characterized in that the forward signal comprises a downlink data packet transmitted by the second transmitter-receiver before a communication link is established; and the return signal comprises a link data packet of the return channel transmitted by the transmitter-receiver after the transmitter-receiver is secured on the forward signal. 1

3. A wireless telephone system, characterized by: (a) a base unit having a base transceiver comprising a base oscillator; and (b) a plurality of wireless headsets, each earphone comprising a headset transceiver to establish a wireless link with the base unit, wherein the base unit transmits a signal forward to a carrier frequency to the headset transceiver, further characterizing the headset transceiver by: (1) a receiver having a carrier tracking cycle to detect and remove a forward signal carrier phase shift; (2) a transmitter for transmitting a return signal to the base transceiver; and (3) an oscillator, independent of the base oscillator, on which the carrier frequency is based, to drive the receiver and the transmitter, wherein the transmitter comprises a prerrotator that prerotates the return signal according to the carrier phase shift detected by the tracking cycle of the carrier, such that the return signal is received by the base transceiver without substantially no carrier phase-shifting. The system of claim 13, characterized in that: the forward signal represents successive symbols at a first symbol rate; and step (b) comprises the step of receiving samples representing the forward signal. The system of claim 13, characterized in that each wireless link is a time division multiple access link, wherein each handset communicates during a time slot unique to a time division multiple access scheme allocating slots. of time to headphones. The system of claim 13, characterized in that the forward signal comprises a downlink data packet transmitted by the second transmitter-receiver, before a communication link is established; and the return signal comprises a link data packet of the return channel transmitted by the transmitter-receiver after the transmitter-receiver is secured on the forward signal.