HK1028309A - Method and apparatus for determining the rate of received data in a variable rate communication system - Google Patents

Description

Method and apparatus for determining the rate at which data is received in a variable rate communication system

Technical Field

The present invention relates to a method and apparatus for determining the rate at which data is received in a variable rate communication system.

Description of the Related Art

The use of Code Division Multiple Access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques are known, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and AM modulation schemes, such as Amplitude Companded Single Sideband (ACSSB), CDMA has significant advantages over these other techniques. The use of CDMA techniques in multiple access communication systems is described in U.S. patent No. 4,901,307 (entitled "spread spectrum multiple access communication system using satellites or terrestrial repeaters," assigned to the assignee of the present invention), which is incorporated herein by reference. The use of CDMA techniques in multiple access communication systems is further disclosed in U.S. patent No. 5,103,459 entitled "system and method for generating signal waveforms in a CDMA cellular telephone system," which is assigned to the assignee of the present invention and incorporated herein by reference.

CDMA, which has the inherent nature of a wideband signal, provides a form of frequency diversity by spreading the signal energy over a wide bandwidth. Thus, frequency selective fading affects only a portion of the CDMA signal bandwidth. Space or path diversity is achieved by providing multiple signal paths over a synchronous link from a mobile subscriber through two or more cell sites. In addition, path diversity can be obtained by exploiting the multipath environment via spread spectrum processing by allowing signals arriving with different propagation delays to be received and processed separately. Examples of path diversity are described in U.S. patent No. 5,101,501 (entitled "method and system for providing soft handoff of communications in a CDMA cellular telephone system") and U.S. patent No. 5,109,390 (entitled "diversity receiver in a CDMA cellular telephone system"), both of which are assigned to the assignee of the present invention and are incorporated herein by reference.

CDMA systems typically employ a variable rate vocoder to encode data so that the data rate can be changed from one frame of data to another. An exemplary embodiment of a variable-rate vocoder is described in U.S. patent No. 5,414,796 (entitled "variable-rate vocoder", assigned to the assignee of the present invention and incorporated herein by reference). The use of variable rate communication channels reduces mutual interference by minimizing unnecessary transmissions when there is no useful speech to be transmitted. Algorithms are used in the vocoder to generate varying amounts of information bits per frame based on changes in speech activity. For example, a set of 4-rate (a rate set of four) vocoders may produce a 20 ms data frame containing 20, 40, 80, or 160 bits, depending on the activity of the speaker. Ideally, each data frame is sent in a fixed amount of time by changing the transmission rate of the communication. Additional details regarding the formatting of vocoder data into data frames are described in U.S. patent No. 5,511,073 entitled "method and apparatus for formatting data for transmission," which is assigned to the assignee of the present invention and incorporated herein by reference.

One technique for a receiver to determine the data rate of received data frames is described in co-pending U.S. patent application No. 08/233,570 entitled "method and apparatus for determining the data rate for transmitting variable rate data in a communications receiver," filed on 26/4/1994, assigned to the assignee of the present invention and incorporated herein by reference. Another technique is described in co-pending U.S. patent application No. 08/126,477 (entitled "multi-rate serial viterbi decoder for code division multiple access system applications," filed 24/9/1993, assigned to the assignee of the present invention and incorporated herein by reference). Yet another technique is described in U.S. patent application No. 08/730,863 entitled "method and apparatus for determining the rate at which data is received in a variable rate communication system," filed 10/18 1996, assigned to the assignee of the present invention and incorporated herein by reference. According to these techniques, each received data frame is decoded at each possible rate. Error metrics (metrics) are provided to the processor that describe the quality of the decoded symbols for each frame decoded at each rate. The error metric may include a Cyclic Redundancy Check (CRC) result, a Yamamoto quality metric, and a symbol error rate. These error metrics are known in the communication system. The processor analyzes the error metric and determines the most likely rate at which incoming symbols are transmitted.

Summary of The Invention

It is an object of the present invention to provide a novel and improved method and apparatus for determining the data rate of received data in a variable rate communication system.

In one aspect, the present invention provides a receiving system for determining a data rate of a received signal in a variable rate communication system, comprising: a decoder for receiving demodulated soft frame symbols and providing a decoded frame and a soft symbol error rate; a rate selector coupled to said decoder for receiving said soft symbol error rate, said rate selector providing an indication of said data rate of said received signal based on said soft symbol error rate.

In another aspect, the present invention provides a receiving system for determining a data rate of a received signal in a variable rate communication system, comprising: a decoder for receiving demodulated soft frame symbols and providing a decoded frame; a CRC check element coupled to said decoder for receiving said decoded frame and providing CRC bits; a re-encoder coupled to said decoder for receiving said decoded frames and providing re-encoded frames; a delay element for receiving the demodulated soft frame symbols and providing a delayed frame; a correlator coupled to said delay element and said re-encoder for receiving said delayed frames and said re-encoded frames, respectively, said correlator providing correlation values; a rate selector coupled to the correlator and the CRC check element for receiving the correlator value and the CRC bit, respectively, the rate selector providing an indication of the data rate of the received signal based on the correlator value and the CRC bit.

In another aspect, the present invention provides a method for determining a data rate of a received signal in a variable rate communication system, comprising the steps of: decoding the demodulated soft frame symbols to provide a decoded frame and a soft symbol error rate; calculating a normalized correlation metric from the soft symbol error rate; and indicating the data rate of the computed signal in accordance with the normalized correlation metric.

The present invention also provides a method for determining the data rate of a received signal in a variable rate communication system, comprising the steps of: decoding the demodulated soft frame symbols to provide a decoded frame; CRC checks the decoded frame to provide CRC bits; re-encoding the decoded frame to provide a re-encoded frame; delaying the demodulated soft frame symbols to provide a delayed frame; associating the delayed frame with the re-encoded frame to provide a correlation value; calculating a normalized correlation metric from the correlation value and the set of constants; and indicating the data rate of the received signal in accordance with the normalized correlation metric and the CRC bits.

The present invention may be embodied in a communication system having a transmitting system and a receiving system, wherein the receiving system determines at which of a plurality of data rates each frame in a signal is transmitted by the transmitting system. For example, if the transmission system employs four data rates, the receiving system decodes each frame in the received signal according to the four rates to produce four normalized correlation metrics, four Cyclic Redundancy Check (CRC) bits, and zero or more Yamamoto quality metrics. In the present invention, the highest normalized correlation metric is selected first, and the CRC bits for that data rate are checked. If the CRC checks, then the data rate is indicated as the received data rate. Otherwise, the next highest normalized correlation metric is selected and processing continues. If there is no CRC check, an erasure is indicated.

In a typical case, only the data rate corresponding to the highest normalized correlation metric is considered. The frame may be accepted or erased based on the CRC check and/or the Yamamoto quality metric. In some applications, CRC coding is not performed for all data rates. When this occurs, the Yamamoto quality metric may be employed in place of the CRC check, other metrics may be employed or the data rate determination process may rely solely on the normalization related metric.

It is an object of the invention to provide a reliable determination of the received data rate. A normalized correlation metric is calculated for each feasible data rate based on the correlation value and the correlation constant calculated for that data rate. And determining a correlation value according to the correlation between the demodulated soft frame code element and the recoded frame. The use of soft symbols enhances the quality of the normalized correlation metric over other metrics of the prior art, such as the Symbol Error Rate (SER), which only utilize the sign bits of the demodulated soft frame symbols. The correlation constants may be calculated using theoretical values, simulated or empirical measurements to provide robust characteristics. In addition, the energy-to-total-noise ratio E is input in a wide range per bit_b/N_tAnd in addition, the application effect on the normalized correlation measurement is better.

It is an object of the invention to minimize erroneous data rate indications that result in frame errors. In some communication systems (such as CDMA communication systems), frame errors are more disruptive than erasures. Thus, by comparing the normalized correlation metric to the correlation threshold, the system embodying the present invention can be optimized to minimize the frame error rate at the expense of a slightly higher erasure rate. The normalized correlation metrics below the threshold are discarded. In addition, the difference between the two highest normalized correlation metrics may be determined and compared to a difference threshold. If the difference is below a threshold, the normalized correlation metric may be discarded.

The present invention also improves the data rate determination process by utilizing the Yamamoto quality metric. During the decoding process, a Yamamoto quality metric may be determined for each decoded data rate. After the highest normalized correlation metric and CRC check have been selected, the Yamamoto quality metric for that data rate may be compared to a Yamamoto threshold. If the Yamamoto quality metric is below a threshold, the data rate may be discarded.

Drawings

The features, objects, and advantages of the present invention will become apparent from the following detailed description of embodiments of the invention when taken in conjunction with the accompanying drawings in which like reference characters identify correspondingly throughout and wherein:

fig. 1 is an exemplary block diagram of a transmission system of the present invention;

FIG. 2 is an exemplary block diagram of a receiving system of the present invention;

fig. 3 is an exemplary flow chart of the data rate determination process of the present invention.

Detailed description of the preferred embodiments

Referring to fig. 1-2, a transmitting system 100 transmits data to a receiving system 200. In an exemplary embodiment, the present invention is implemented in a wireless communication system in which spread spectrum modulated signals are employed for communication. Communication using spread spectrum communication systems is described in detail in the aforementioned U.S. Pat. nos. 4,901,307 and 5,103,459.

An exemplary block diagram of a transmission system 100 of the present invention is shown in fig. 1. The variable rate data source 110 provides frames of data at a variable rate to a Cyclic Redundancy Check (CRC) and tail bit generator 112. In an exemplary embodiment, the data source is a variable rate vocoder used to encode voice information at 4 variable rates, as described in the above-mentioned U.S. Pat. No. 5,414,796. The 4 data rates include full rate, half rate, quarter rate, and eighth rate, which may also be referred to as full, half, quarter, and eighth rates, respectively. For example, when used in a cellular telephone environment, a signal is sent at full rate to transmit speech (e.g., when the user is talking) and has been sent at eighth rate to transmit silence (silence) (e.g., when the user is not talking). The eighth rate saves the number of bits to transmit and thus saves power. In an exemplary embodiment, 90% of the signals transmitted by the transmission system 100 to the reception system 200 are transmitted at full rate or eighth rate. The half rate and the quarter rate indicate transition rates between the full rate and the eighth rate.

Generator 112 generates a set of CRC parity bits to provide error detection in receiving system 200, as is known in the art. In addition, generator 112 appends a series of tail bits to the CRC encoded frame. In an exemplary embodiment, generator 112 generates a set of CRC and tail bits according to the TIA/EIA/IS-95 Mobile station-base station compatibility standard for dual-mode wideband spread-spectrum cellular systems of the Telecommunications industry Association (hereinafter, referred to as the IS-95 standard). Generator 112 provides the encoded data frame to encoder 114 to encode the data into code symbols for error correction and error detection at receiving system 200. In the exemplary embodiment, encoder 114 IS a rate 1/2 convolutional encoder, as defined in the IS-95 standard for the forward link.

The code symbols from encoder 114 are provided to a symbol repeater 116, wherein the symbol repeater 116 repeats each symbol by R_txAt a sub-maximum rate R_tx1, half rate R_tx2, quarter rate R_tx4 and eighth rate R_tx8. The symbol repetition results in a fixed size frame of data (e.g., the same number of symbols per frame) at the output of the symbol repeater 116 regardless of the symbol rate input to the symbol repeater 116. Symbol repeater 116 may include a symbol truncation (puncturing) mechanismIt may truncate symbols to obtain other code rates, such as for full rate R_txRate 3/4 is 1.

The symbols from symbol repeater 116 are provided to interleaver 118, which rearranges the symbols according to a predetermined interleaving format. In an example embodiment, interleaver 118 is a block interleaver, the design and implementation of which is known in the art. The rearranged frames are then provided to a modulator 120 that modulates the frames for transmission. In an exemplary embodiment, modulator 120 is a CDMA modulator, the implementation of which is described in detail in the aforementioned U.S. patent nos. 4,901,307 and 5,103,459. The modulated data frames are provided to a transmitter (TMTR)122, which transmitter 122 upconverts, filters, and amplifies the signal for transmission via an antenna 124.

Within receiving system 200, a transmitted signal is received by an antenna 210 and provided to a receiver (RCVR)212, which filters, amplifies, and downconverts the received signal. The signal is then provided to a demodulator (DEMOD)214 that demodulates the signal. In an exemplary embodiment, demodulator 214 is a CDMA demodulator, the implementation of which is described in detail in the aforementioned U.S. Pat. nos. 4,901,307 and 5,103,459. Demodulator 214 also quantizes the signal into soft decision bits, which represent estimates of the transmitted symbols. In the exemplary embodiment, each received symbol is represented by 4 soft decision bits.

The received symbols are provided to a deinterleaver and buffer 216. Buffer 216 rearranges the symbols in the frame according to a predetermined rearrangement format known in the art. In an example embodiment, buffer 216 rearranges the symbols in a sequence that is the inverse of the sequence performed by interleaver 118. To the combination R_rxSymbol recombiner 218 of symbols provides rearranged symbols, where R_rxRepresenting the rate hypothesis decoded by the receiving system 200. In an example embodiment, for full rate R_rxFor half rate R ═ 1_rx2 for quarter rate R_rx4 and for one eighth rate R_rx8. Symbol recombiner 218 combines the energy of the code symbols transmitted during the multi-symbol time to provide a further correction to the transmitted symbolsAnd (4) performing good estimation. In an exemplary embodiment, the demodulated soft frame symbols from symbol reassembler 218 include 4-bit symbols for full rate, 5-bit symbols for half rate, 6-bit symbols for quarter rate, and 7-bit symbols for eighth rate. In embodiments where symbol repeater 116 includes a symbol truncation mechanism, symbol recombiner 218 includes a symbol insertion mechanism to replace the truncated symbols with zeros.

In an exemplary embodiment, demodulated soft frame symbols from symbol reassembler 218 are provided to a symbol metric table 220, wherein symbol metric table 220 converts symbols of varying bit numbers to symbols of a fixed bit number. Although different numbers of bits may be utilized and fall within the scope of the present invention, in an example embodiment, the output from the symbol metric table 220 comprises 4-bit symbols. Feeding the decoder 230 with a fixed number of bits for the soft-decision symbols simplifies the design of the decoder 230. In another embodiment, the symbol metric table 220 may be eliminated and the decoder 230 may be designed to decode symbols having different numbers of soft decision bits for each data rate (e.g., using all bit decoding provided by the symbol reassembler 218).

In the exemplary embodiment, decoder 230 is a multi-rate viterbi decoder, as described in detail in the above-mentioned co-pending U.S. patent application No. 08/126,477. Decoder 230 provides error correction for the frame of symbols using a predetermined set of hypothesized rates. In the exemplary embodiment, decoder 230 decodes symbols for each of the four possible rates to provide 4 separately decoded frames of data, each of which is provided to CRC check element 240. CRC check element 240 utilizes conventional techniques to determine whether the CRC parity bits for each frame are correct for the decoded data. CRC check element 240 performs a CRC check on the CRC parity bits in the four decoded frames to help determine whether the currently received frame was sent at full rate, half rate, quarter rate, or eighth rate. CRC check element 240 provides four CRC bits C for full rate, half rate, quarter rate, and eighth rate, respectively₀、C₁、C₂And C₃. In an example embodiment, a binary value of "1" for a given CRC bit indicates that the CRC bits match or are checked, while a binary value of "0" indicates that the CRC bits do not check.

The decoded frames from decoder 230 are also provided to a re-encoder 236, which re-encodes the data. In the exemplary embodiment, re-encoder 236 performs the same functions as encoder 114 in transmission system 100. Recoded frame from recoder 236Including binary sequences (which may represent "1" and "0" or "1" and "-1"), where the index i represents the data rate to be decoded. In an example embodiment, the demodulated soft frame symbols from symbol reassembler 218 are also provided to a delay element 232, where the delay element 232 provides the same amount of delay as experienced by symbol metric table 220, decoder 230, and re-encoder 236. The delayed frames from delay element 232 and the re-encoded frames from re-encoder 236 are provided to correlator 234. For each rate, correlator 234 performs a correlation of two frames, where the correlation can be mathematically described as:

whereinIs a re-encoded frame from the re-encoder 236_iIs the delayed frame from delay element 232, N is the number of symbols in the demodulated frame, R_iIs the decoded data rate andis the correlation between the re-encoded frame and the delayed frame. For each symbol in the frame, correlator 234 multiplies the re-encoded symbol with the demodulated and delayed soft frame symbols and accumulates the resulting products. In particular, when the symbols of the delayed soft frame correspond to those of the re-encoded symbols, the number of delayed soft frame symbols is added to the correlation sum for the soft frame symbol values, and if the symbols are different from those of the re-encoded symbols, the sum is subtracted from the correlation sum. If re-encoded frame and demodulationThe identity of the soft frame, which means that there is no error in the received data frame, thenIs a high value. However, if the re-encoded frame is not correlated with the demodulated soft frame (e.g., due to incorrect rate assumptions), thenIs a low value. Correlator 234 generates four correlation values: for full rate, half rate, quarter rate and eighth rate, respectively, of each received data frameAndthe CRC bits from CRC check element 240 and the correlation values from correlator 234 are provided to rate selector 250. Rate selector 250 determines at which of the four rates the currently received frame is to be transmitted.

CRC check element 240 provides four decoded frames to frame buffer 246 for storage, where each of the four frames is decoded under a different rate hypothesis. In accordance with the rate determined by rate selector 250, a control signal is provided to frame buffer 246, in response to which frames decoded at a predetermined rate are output, or if a (clear) erasure is declared, no frames are output. In an example embodiment, if an erasure is declared, frame buffer 246 outputs a signal representing the frame erasure. Although decoder 230, delay element 232, correlator 234, re-encoder 236 and rate selector 250 are shown as separate elements, these elements may be combined to form a single multi-rate decoder.

In the communication system of fig. 1-2, the signal transmitted by the transmission system 100 to the reception system 200 may change rapidly between a plurality of rates. In an example embodiment, the transmission system 100 does not include an actual indication of the rate of the currently transmitted signal within the transmitted signal. Such an indication may require additional overhead bits that may be used to transmit information. The transmission system 100 transmits frames at a current rate, which in the exemplary CDMA communication system may be one of four possible rates. The task of the rate selector 250 is to determine at which of the four rates the currently received frame was transmitted (e.g., whether the current frame was transmitted at full, half, quarter, or eighth rate) or whether an erasure was declared (e.g., the rate selector 250 cannot determine at which of the four rates the current frame was transmitted). Thus, frame buffer 246 outputs the correct data frame of the four decoded frames. The decoded data frame may be processed into an appropriate decoded signal, which may be provided to, for example, a vocoder, amplifier, or speaker (not shown in fig. 2).

In an exemplary embodiment, rate selector 250 operates in the method illustrated in the flow chart of fig. 3 to select the appropriate decoded frame to output to, or provide to, the user to declare the current frame as erased. Rate selector 250 calculates a normalized correlation metric for each decoded data rate. Using one of many embodiments, normalized correlation metrics can be computed, four of which are described below. Expressing the normalized correlation metric asWhere the subscript m denotes the examples used in the calculations. Other embodiments of calculating the normalized correlation metric are contemplated and are within the scope of the invention. Throughout the specification, normalized correlation metrics computed using various embodiments, such as those described below, are generally represented as

In a first embodiment, a normalized correlation metric is calculated according to the following equation

Wherein const (R)_i) Is a constant dependent on the decoded data rate and the energy of the received signal, andis a calculated value calculated according to equation 1. Next, the detailed description is given for const (R)_i) And (4) calculating.

In a second embodiment, if there is no symbol compression (e.g., no symbol metric table 220), then a normalized correlation metric may be calculated according to the following equation

WhereinRepresents the sum of the received symbols, which is related to the energy of the received data frame, and is nearly constant for all received frames at a given data rate,represents soft Symbol Error Rate (SER), and const (R)_i) Is a constant that depends on the data rate to be decoded and the energy that the received signal has, as used in equation 2. The soft SER is the sum of the soft symbol errors in the frame and can be computed as the overall normalization metric for the most likely path. In an example embodiment, the soft SER may be calculated according to a viterbi decoding process. In a viterbi decoding process, at each stage of the trellis, the state metric is normalized according to the best state metric. The soft SER may be computed by normalizing and summing the state metrics throughout the trellis and the final metrics in the trellis.

In a third embodiment, if there is symbol compression prior to the decoding process (e.g., there is a symbol metric table 220), then the soft SER may be scaled by a scaling factor in view of the compression. Using the following equation, a normalized correlation metric can be calculated

Wherein alpha is_iFor the rate R_iWhich allows for compression by the symbol metric table 220. May be based on const (R) in equations 2 and 3_i) Modifying the constant const3 (R) in equation 4_i) To account for compression by symbol metric table 220.

In a fourth embodiment, normalized correlation metrics may be usedThe approximation is:

since for a given data rate all received data framesApproximately constant (as described above) and made of const (R)_i) Partially biased. In addition, a scaling constant α may be used_iTo compensate for symbol compression by symbol metric table 220.

Fig. 3 shows a flow diagram of an example embodiment of the data rate determination process of the present invention. The rate determination process begins at state 302. In a first step, for all rate hypotheses to be considered, in block 304, rate selector 250 calculates and stores a normalized modification metric:and so on.

In the exemplary CDMA communication system, four normalized correlation metrics are calculated for four data rates. A normalized correlation metric can be calculated according to equations 2, 3, 4. In block 306, rate selector 250 receives and stores four CRC bits C for four rate hypotheses from CRC check element 240₀、C₁、C₂And C₃. In the exemplary embodiment, rate selector 250 then determines the data rate of the received signal using the four CRC bits and the four normalized correlation metrics.

In block 308, the rate selector 250 selects the highest normalized correlation metric in memory and, in block 310, determines the rate R corresponding to the normalized correlation metric_i. In block 312, rate selector 250 determines whether to perform a CRC check on the rate hypothesis. If the CRC check passes (e.g., CRC bit ═ 1), then rate selector 250 outputs R_iIs an indication of the received data rate (in block 314). If the CRC check is not successful, then rate selector 250 removes the normalized correlation metric and CRC bits for that rate hypothesis at block 316 (e.g., negates this rate hypothesis) at block 318, rate selector 318 then determines if all four rate hypotheses have been processed? (i.e., whether the memory is empty. If all four rate hypotheses have been processed, rate selector 250 outputs an erasure indication (in block 320). Otherwise, the rate selector 250 returns to block 308 and processes the next highest normalized correlation metric. This process is repeated until a valid CRC check is detected or all four rate hypotheses are unsuccessful.

The data rate determination process of the present invention outputs one of three possible indications: a correct rate indication, an erasure indication, or an erroneous rate indication. A correct rate indication typically results in the decoder 230 providing valid decoded data. The erasure indication indicates that the correct data rate cannot be determined and the mechanism for processing the erasure frame is activated. One method for handling erasures is to repeat the last known good decoded frame while no significant change from frame to frame data can be expected. Another method for handling erasures is to extrapolate the erased frame from the known well-decoded frames on both sides of the erased frame, thereby smoothing the erased frame. These mechanisms designed to handle erasures may degrade communication quality slightly, but do not result in significant quality degradation. However, erroneous rate indications by rate selector 250 typically result in erroneous decoded data, or frame errors, from decoder 230. Frame errors can cause severe degradation in the performance of the communication system.

In the exemplary CDMA communication system, the design goal for the frame error rate IS several orders of magnitude lower than the erasure rate, as suggested by "TIA/EIA/IS-98, recommended minimum performance standard for dual-mode wideband spread spectrum cellular mobile stations". Thus, indicating erasures (e.g., indicating data frames that cannot be properly decoded) is more desirable than error detection (e.g., indicating frames received at one rate when frames are actually being transmitted at another rate). The exemplary embodiments of the data rate determination process described above may be modified to minimize the probability of false rate detection at the expense of additional erasure indications. For example, the normalized correlation metric may be compared to a correlation threshold. If the normalized metric is below the threshold, the metric may be discarded. As a second example, the difference between the highest and second highest normalized correlation metrics is compared to a difference threshold. If the difference is below the threshold, both normalized correlation metrics are discarded and an erasure is indicated. In both examples, a low threshold results in lower erasures at the expense of a higher probability of error rate indication. On the other hand, a high threshold results in a higher erasure rate, but yields the benefit of a lower probability of error rate indication. It is within the scope of the invention to employ various thresholds.

In another embodiment of the data rate determination process, rate selector 250 may utilize a quality indicator to assist in determining the received data rate. In this embodiment, the quality indicator may comprise a set of Yamamoto quality metrics Y calculated in a Viterbi decoding process in the manner described in U.S. patent application No. 08/730,863, supra_i. The Yamamoto quality metric is a confidence metric (confidence metric) based on the difference between a selected path through the trellis and another nearest path through the trellis. Thus, the Yamamoto quality metric is a good indication of confidence that the decoded symbol is a true correct symbol. One use of Yamamoto quality metrics in rate determination is disclosed in the aforementioned U.S. patent application No. 08/730,863.

Although CRC detection is dependent onThe Yamamoto quality metric depends on the decoding process of the receiving system 200, but on the bits in each of the four decoded frames. In the exemplary embodiment, Yamamoto detector 248, together with CRC check element 240 and correlator 234, provides four Yamamoto quality metrics Y for each of the four possible rates_i: y for full rate, half rate, quarter rate and eighth rate, respectively₀、Y₁、Y₂And Y₃. While Yamamoto detector 248 is shown as a discrete component, Yamamoto detector 248 may be incorporated into detector 230.

Another embodiment of the data rate determination process is depicted in fig. 3. However, in indicating that the received data rate is R_i(as shown in block 314) prior to rate selector 250 determining the Yamamoto quality metric Y for the rate hypothesis_iCompared (in block 322) to a predetermined Yamamoto threshold, as shown by the dashed line. If Yamamoto quality measures Y_iBelow the threshold, the rate selector 250 outputs an indication R_iIs the received data rate (at block 324). Otherwise, rate selector 250 may output an erasure (not shown) or discard the data hypothesis and proceed to block 316 (shown as a dashed line).

In the present invention, other indicators or metrics may be combined with the normalized correlation metric, the CRC bits, and the Yamamoto quality metric to increase the accuracy of the data rate determination process. For example, the Symbol Error Rate (SER) assumed for each rate may be calculated and added to the data rate determination process using the method described in the above-mentioned U.S. patent application No. 08/730,863. It is within the scope of the invention for other metrics to be used in combination with the above metrics.

The present invention is described in detail with respect to a CDMA communication system including four data rates: full rate, half rate, quarter rate, and eighth rate. The present invention can be used in systems that include multiple rates. Further, each data rate may have any value, including a zeroth rate (e.g., no data transmission). It is within the scope of the invention to utilize any number of data rates and any rate values.

The above example and another embodiment of the data rate determination process assume that CRC coding is performed for all transmitted data frames. For some communication systems, this assumption is not valid. For systems in which CRC encoding is performed only for selected data rates, the data rate determination process may be modified to employ: 1) CRC check when available: 2) only normalized correlation metrics: 3) normalizing the correlation metric and the Yamamoto quality metric; or 4) normalize the correlation metric and some other metric.

The data rate determination process is described with respect to an exemplary CDMA communication system, and in particular, forward link transmissions. For the reverse link, a different convolutional encoder (e.g., rate 1/3) is utilized and symbol gating is applied instead of code element repetition. However, the demodulated soft frame symbols from the reverse link transmission are similar to those from the forward link transmission. Thus, the data rate determination process may be used for reverse link transmissions and fall within the scope of the present invention.

I. Derivation of correlation constants

In the present invention, a normalized correlation metric is derived assuming an Additive White Gaussian Noise (AWGN) channel and a known received powerWith these assumptions, the constant const (R) for the forward link can be derived_i) The calculation is as follows:andwhere R is the full rate frame (R) to be decoded by the decoder 230_x1) the associated code rate (e.g., rate 1/2 or 3/4); e_b/N_tIs the energy-per-bit-to-noise ratio of the received signal, i ═ 0, 1, 2, and 3 for full rate, half rate, quarter rate, and eighth rate, respectively; n is the length of the code sequence after symbol repetition and truncation (e.g., data frame length); and A_sIs the amplitude (false) of the transmitted symbolConstant full rate i ═ 0).

Equation 7 assumes that the received E is prior to the rate determination by the data selector 250_b/N_tIs known a priori. Thus, if E_h/N_tIs not known, then A cannot be accurately determined_s. In particular, under fading conditions, A_sA larger amount of change (e.g., 10dB) is possible. However, in the present invention, at E_b/N_tFour constants const (R) in a wide range_i) The difference therebetween does not change much. Thus, even if E of the signal is received_b/N_tAnd A_sThe invention does not known to perform well. All that is required is to receive E_b/N_tIs estimated.

Equation 6 is derived assuming that all four data rates are transmitted with equal probability (e.g., a probability of 0.25 for each of the four data rates). In the exemplary CDMA communication system, all data rates are not transmitted with equal probability. In fact, the transmission probability at full rate and eighth rate can reach 0.90. Furthermore, it can be assumed that for a given rate i, the probability of a data sequence is equal to 2^-NR/2’Equation 6 is derived in the case of (1). Equation 6 can be further modified to account for data frames that contain additional overhead bits (e.g., CRC and code tail bits) and to apply the actual probabilities for the data sequences at each of the four data rates.

As noted above, some systems do not provide CRC encoding for all data rates. For these systems, the constants may be adjusted to account for the absence of a CRC check. For example, a constant may be increased to achieve a higher degree of confidence before a particular data rate is designated as the received data rate. Different adjustments to equations 6 and 7 are contemplated and are within the scope of the present invention.

In the present invention, equations 6 and 7 are used to guide the determination of the normalized correlation metric calculated in equations 2 and 3The constant const (R) used_i). The constant const (R) may also be derived by simulation, empirical measurements, or other methods_i). Using different constants const (R) derived by various methods_i) Falling within the scope of the invention.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (37)

1. A receiving system for determining a data rate of a received signal in a variable rate communication system, comprising:

a decoder for receiving demodulated soft frame symbols and providing a decoded frame and a soft symbol error rate;

a rate selector coupled to said decoder for receiving said soft symbol error rate, said rate selector providing an indication of said data rate of said received signal based on said soft symbol error rate.

2. The receiving system of claim 1, wherein the decoder is a viterbi decoder.

3. The receiving system of claim 1 or 2, further comprising:

a CRC check element coupled to said decoder for receiving said decoded frame and providing CRC bits; and

wherein said rate selector is further coupled to said CRC check element for receiving said CRC bits, said rate selector providing an indication of said data rate of said received signal based on said soft symbol error rate and said CRC bits.

4. A receiving system for determining a data rate of a received signal in a variable rate communication system, comprising:

a decoder for receiving demodulated soft frame symbols and providing a decoded frame;

a CRC check element coupled to said decoder for receiving said decoded frame and providing CRC bits;

a re-encoder coupled to said decoder for receiving said decoded frames and providing re-encoded frames;

a delay element for receiving the demodulated soft frame symbols and providing a delayed frame;

a correlator coupled to said delay element and said re-encoder for receiving said delayed frames and said re-encoder frames, respectively, said correlator providing correlation values;

a rate selector coupled to the correlator and the CRC check element for receiving the correlator value and the CRC bit, respectively, the rate selector providing an indication of the data rate of the received signal based on the correlator value and the CRC bit.

5. The receiving system of claim 4, wherein the decoder is a Viterbi decoder.

6. The receiving system of claim 5 wherein said viterbi decoder is a rate 1/2 convolutional decoder.

7. The receiving system of claim 5 wherein said viterbi decoder is a rate 3/4 convolutional decoder.

8. The receiving system of claim 5 wherein said viterbi decoder has a code rate, said code rate being obtained by truncation.

9. The receiving system of claim 5 or 8, wherein said viterbi decoder has a code rate, said code rate being obtained by symbol repetition.

10. The receiving system of claim 5 wherein said viterbi decoder has a code rate, said code rate being obtained by symbol gating.

11. The receiving system of claim 5 wherein said viterbi decoder has a code rate in accordance with IS-95 standard.

12. The receiving system of any of claims 4 to 12, wherein the CRC check element conforms to the IS-95 standard.

13. The receiving system of claim 5 or any of claims 6 to 12, further comprising:

a Yamamoto detector coupled to said decoder for receiving said decoded frames, said Yamamoto detector further coupled to said rate selector for providing a Yamamoto quality metric; and

wherein the rate selector provides an indication of the data rate of the received signal based on the correlator value, the CRC bits, and the Yamamoto quality metric.

14. A method for determining a data rate of a received signal in a variable rate communication system, comprising the steps of:

decoding the demodulated soft frame symbols to provide a decoded frame and a soft symbol error rate;

calculating a normalized correlation metric from the soft symbol error rate; and

indicating the data rate of the received signal in accordance with the normalized correlation metric.

15. The method of claim 14, further comprising the steps of:

indicating erasure according to the normalized correlation metric.

16. The method of claim 14 or 15, wherein said decoding step is a convolutional decoding step performed with a viterbi decoder.

17. The method of claim 15 or 16, further comprising the steps of:

the CRC checks the decoded frame to provide CRC bits, and the indicating step is further based on the CRC bits.

18. The method of any of claims 14 to 17, wherein the decoding step is a convolutional decoding step performed using a viterbi decoder.

19. A method for determining a data rate of a received signal in a variable rate communication system, comprising the steps of:

decoding the demodulated soft frame symbols to provide a decoded frame;

CRC checks the decoded frame to provide CRC bits;

re-encoding the decoded frame to provide a re-encoded frame;

delaying the demodulated soft frame symbols to provide a delayed frame;

associating the delayed frame with the re-encoded frame to provide a correlation value;

computing a normalized correlation metric from the correlation value and the set of constants; and

indicating the data rate of the received signal as a function of the normalized correlation metric and the CRC bits.

20. The method of claim 19, further comprising the steps of:

indicating an erasure as a function of the normalized correlation metric and the CRC bits.

21. The method of claim 19 or 20, wherein said decoding step is a convolutional decoding step performed by a viterbi decoder.

22. The method of claim 21 wherein said step of convolutional decoding is performed by a rate 1/2 viterbi decoder.

23. The method of claim 20 or 21, wherein the decoding step is performed by a viterbi decoder having a code rate obtained by truncation.

24. The method of claim 20 or 21, wherein the decoding step is performed by a viterbi decoder having a code rate obtained by symbol repetition.

25. The method of claim 20 or 21, wherein the decoding step is performed by a viterbi decoder having a code rate obtained by symbol gating.

26. The method of claim 20 or 21, wherein the decoding step is performed by a viterbi decoder having a code rate obtained by truncation and symbol repetition.

27. The method of claim 20 or 21, wherein the decoding step IS performed by a viterbi decoder having a code rate conforming to IS-95 standard.

28. A method according to any one of claims 19 to 27, wherein the CRC check step IS performed in accordance with IS-95 standard.

29. A method according to any one of claims 19 to 28, comprising the steps of:

selecting a highest normalized correlation metric; and

wherein said indicating said data rate step is based on said highest normalized correlation metric and said CRC bit corresponding to said highest normalized correlation metric.

30. The method of claim 29 further comprising the step of comparing the highest normalized correlation metric to a correlation threshold, and wherein the step of indicating the data rate is further based on the result of the step of comparing.

31. The method of any one of claims 19 to 30, further comprising the steps of:

calculating a difference between the highest normalized correlation metric and the next highest normalized correlation metric to provide a difference;

the difference is compared to a difference threshold and the step of indicating the data rate is further based on the result of the step of comparing.

32. The method according to any of claims 19 to 30, further comprising the step of calculating a Yamamoto quality metric based on said decoding step, and wherein said indicating step is further based on said Yamamoto quality metric.

33. The method of claim 32, further comprising the steps of:

comparing the Yamamoto quality metric to a Yamamoto threshold, and the indicating step is further based on the result of the comparing step.

34. A method according to any one of claims 19 to 33, wherein the set of constants is derived from theoretical calculations.

35. The method of any of claims 19 to 33, wherein the set of constants is derived by simulation.

36. A method according to any one of claims 19 to 33, wherein the set of constants is derived by empirical measurements.

37. A method as claimed in any of claims 19 to 33, wherein the set of constants is adjusted for data rates which do not provide CRC coding.