Method and apparatus for performing rate determination

1

METHOD AND APPARATUS FOR
PERFORMING RATE DETERMINATION

BACKGROUND OF THE INVENTION

I. Field of the Invention 5 The present invention relates to digital communications.

More particularly, the present invention relates to a novel and improved system and method for determining, at a receiver of a variable rate communication system, the rate at which data has been encoded for transmission. 10

II. 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 15 present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other tech- 2Q niques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901, 307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of 25 the present invention, the disclosure of which is incorporated by reference herein.

CDMA systems often employ a variable rate vocoder to encode data so that the data rate can be varied from one data frame to another. An exemplary embodiment of a variable 30 rate vocoder is described in U.S. Pat. No. 5,414,796, entitled "VARIABLE RATE VOCODER," assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein. The use of a variable rate communications channel reduces mutual interference by 35 eliminating unnecessary transmissions when there is no useful speech to be transmitted. Algorithms are utilized within the vocoder for generating a varying number of information bits in each frame in accordance with variations in speech activity. For example, a vocoder with a set of four 40 rates may produce 20 millisecond data frames containing 16, 40, 80, or 171 information bits, depending on the activity of the speaker. It is desired to transmit each data frame in a fixed amount of time by varying the transmission rate of communications. 45

Additional details on the formatting of the vocoder data into data frames are described in U.S. Pat. No. 5,511,073, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION," assigned to the assignee of the present invention, the disclosure of which 50 is herein incorporated by reference. The data frames may be further processed, spread spectrum modulated, and transmitted as described in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE 55 SYSTEM," assigned to the assignee of the present invention, the disclosure of which is also incorporated by reference herein.

Variable rate systems can be developed which include explicit rate information. If the rate is included as part of a 60 variable rate frame, then the rate is not recoverable until after the frame has already been properly decoded, at which point the rate has already been determined. Rather than including the rate in the variable rate frame, the rate could instead be sent in a non-variable rate portion of the frame. 65 However, only a few bits are typically needed to represent the rate, and these bits cannot be efficiently encoded and

2

interleaved in order to provide error protection for fading communications channels. Furthermore, the rate information is only available after some decoding delay or subject to error.

Alternatively, variable rate systems can be developed which do not include explicit rate information. One technique for the receiver to determine the rate of a received data frame where the rate information is not explicitly included in the frame is described in copending U.S. patent application Ser. No. 08/233,570, entitled "METHOD AND APPARATUS FOR DETERMINING DATA RATE OF TRANSMITTED VARIABLE RATE DATA IN A COMMUNICATIONS RECEIVER," filed Apr. 26, 1994 and assigned to the assignee of the present invention, the disclosure of which is herein incorporated by reference. Another technique is described in copending U.S. patent application Ser. No. 08/126,477, entitled "MULTIRATE SERIAL VITERBI DECODER FOR CODE DIVISION MULTIPLE ACCESS SYSTEM APPLICATIONS," filed Sep. 24, 1993 and assigned to the assignee of the present invention, the disclosure of which is herein incorporated by reference. According to these techniques, each received data frame is decoded at each of the possible rates. Error metrics, which describe the quality of the decoded symbols for each frame decoded at each rate, are provided to a processor. The error metrics may include Cyclic Redundancy Check (CRC) results, Yamamoto Quality Metrics, and Symbol Error Rates. These error metrics are well-known in communications systems. The processor analyzes the error metrics and determines the most probable rate at which the incoming symbols were transmitted.

Decoding each received data frame at each possible data rate will eventually generate the desired decoded data. However, the search through all possible rates is not the most efficient use of processing resources in a receiver. Also, as higher transmission rates are used, power consumption for determining the transmission rate also increases. This is because there are more bits per frame to be processed. Furthermore, as technology evolves, variable rate systems may utilize larger sets of data rates for communicating information. The use of larger sets of rates will make the exhaustive decoding at all possible rates infeasible. The decoding delay will not be tolerable for some system applications. Consequently, a more efficient rate determination system is needed in a variable rate communications environment. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.

SUMMARY OF THE INVENTION

The present invention is a novel and improved system and method for determining the transmission rate of communications in a variable rate communications system. Although the present invention may be used in many communications systems, it is particularly useful in cellular communication systems that use a variable rate vocoder for encoding and decoding speech at a plurality of discrete rates. Such communications systems include mobile telephone, personal communication devices, wireless local loop, and private branch exchange. The present invention is described in the context of a code division multiple access (CDMA) communication system but is equally applicable to other transmission formats.

The telecommunications industry association (TIA) has provided a standard for CDMA communications entitled IS-95-A Mobile Station—Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System, hereinafter IS-95-A. IS-95-A provides for the transmission of variable rate data. The present invention is described herein for the transmission of multiplex option 1 data which provides for the transmission of data at 9600, 5 4800, 2400, and 1200 bits/sec referred to herein as full, half, quarter, and eighth rates respectively.

Transmission of data in IS-95-A compatible systems is provided in 20 millisecond frames. A full rate frame contains twice as many bits as a half rate frame which contains twice 1Q as many bits as a quarter rate frame which in turn contains twice as many bits as an eighth rate frame. On the IS-95-A forward link, symbol repetition is introduced to occupy the full capacity of the outgoing frames. So each symbol in a half rate frame is provided twice within the outgoing frame, each symbol in a quarter rate frame is provided four times 15 and each symbol in an eighth rate frame is provided eight times.

Because a receiver can take advantage of the redundancy in the frame, frames transmitted at less than full rate are transmitted at lower energy than full rate frames. In the 20 exemplary embodiment, half rate frames are transmitted at half the energy of the full rate frames, quarter rate frames are transmitted at one quarter the energy of full rate frames, and eighth rate frames are transmitted at one eighth the energy of full rate frames. 25

In addition to transmitting frames of data, the transmitter in a variable rate communications system also transmits a reference signal at approximately the same carrier frequency as the data signal. The reference signal is transmitted at a constant energy. 30

At a receiver, each received frame of data is compared with the reference signal. More particularly, the ratio of the power of the received data frame to the power of the reference signal is compared with a predetermined ratio of the power of a data frame encoded at the maximum rate to 35 the power of the reference signal. Based on the relationship between the two ratios, the transmission rate of the received data frame can be determined prior to decoding. One use of the rate determination operation of the present invention is to provide a signal indicative of the transmission rate to the decoder for properly and efficiently decoding the received 40 data frame.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention will become more apparent from the detailed 45 description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 is a schematic overview of an exemplary CDMA cellular telephone system; 50

FIGS. 2a-2d illustrate, in a series of graphs, exemplary energy levels of the data frames at full, half, quarter, and eighth rates;

FIG. 3 is a block diagram of a variable rate receiving system with particular reference to the rate determination 55 features of the present invention;

FIG. 4 is a flow chart illustrating an exemplary embodiment of the processing steps involved in rate determination as performed by the processing elements of FIG. 3; and

FIG. 5 is a block diagram illustrating the elements of the 60 variable rate receiving system wherein a RAKE receiver is employed.

DETAILED DESCRIPTION OF THE

PREFERRED EMBODIMENTS 65

An exemplary cellular mobile telephone system in which the present invention is embodied is illustrated in FIG. 1. For

purposes of example this system is described herein within the context of a CDMA cellular communications system. However, it should be understood that the invention is applicable to other types of communication systems, such as personal communication systems (PCS), wireless local loop, private branch exchange (PBX) or other known systems. Furthermore, systems utilizing other well known transmission modulation schemes such as TDMA and FDMA as well as other spread spectrum systems may employ the present invention.

In FIG. 1, system controller and switch 10 typically include appropriate interface and processing hardware for providing system control information to the cell-sites. Controller 10 controls the routing of telephone calls from the public switched telephone network (PSTN) to the appropriate cell-site for transmission to the appropriate mobile unit. Controller 10 also controls the routing of calls from the mobile units via at least one cell-site to the PSTN. Controller 10 may direct calls between mobile users via the appropriate cell-site stations since such mobile units do not typically communicate directly with one another.

Controller 10 may be coupled to the cell-sites by various means such as dedicated telephone lines, optical fiber links or by radio frequency communications. In FIG. 1, two exemplary cell-sites, 12 and 14, along with two exemplary mobile units, 16 and 18, which include cellular telephones, are illustrated. Arrows 20a-20fc and 22a-22b respectively define the possible communication links between cell-site 12 and mobile units 16 and 18. Similarly, arrows 24a-24fc and arrows 26a-26fc respectively define the possible communication links between cell-site 14 and mobile units 18 and 16.

The cellular system illustrated in FIG. 1 may employ a variable rate data channel for communications between cell-sites 12, 14 and mobile units 16, 18. By example, a vocoder (not shown) may encode sampled voice information into symbols at four different rates, such as approximately 8,550 bits per second (bps), 4,000 bps, 2,000 bps, and 800 bps, based on voice activity during a 20 millisecond (ms) frame of data. As described above in the IS-95-A standard, each frame of vocoder data is formatted with overhead bits as 9,600 bps, 4,800 bps, 2,400 bps, and 1,200 bps data frames. As mentioned above, the highest rate data frame which corresponds to a 9,600 bps frame is referred to as a full rate frame; the 4,800 bps data frame is referred to as a half rate frame; a 2,400 bps data frame is referred to as a quarter rate frame; and a 1,200 bps data frame is referred to as an eighth rate frame. Although this example describes a set of four data rates, it should be understood that a different number of variable rates may be utilized instead.

Additional features of the variable rate data frames in a system utilizing a set of four rates are illustrated in FIGS. 2a-2d. As shown in FIGS. 2a-2d, the energy in a data frame is varied as the data rate of the data signal is varied. Further, when the data rate is lower than the maximum, in addition to lowering the energy, each data symbol in a frame is repeated a number of times as required to achieve a constant number of symbols in each frame to be transmitted. In FIG. 2a, a data frame, designated as a traffic packet, is shown to be encoded by symbols Pj-Pjg. The data frame of FIG. 2a has been encoded at full rate has the highest energy with no repetition of symbols. FIG. 2b shows that a half rate data frame has half of the highest energy with each symbol (Pj-Pg) repeated two times. FIG. 2c shows that a quarter rate data frame has a quarter of the highest energy with each symbol (Pj-P^ repeated four times. FIG. 2d shows that an eighth rate data frame has an eighth of the highest energy with each symbol ... repeated eight times. Although 5

FIGS. 2a-2d show that the fraction of energy is the same as the fraction of data symbols in a frame, it should be understood that different fractions of energy may be used instead.

In addition to encoding with data symbols, the data frames 5 are formatted with overhead bits, which generally will include additional bits for error correction and detection, such as Cyclic Redundancy Check (CRC) bits. The CRC bits can be used by the decoder to determine whether or not a frame of data has been received correctly. CRC codes are 1Q produced by dividing the data block by a predetermined binary polynomial as is described in detail in IS-95-A. Other methods of detecting whether a frame has been properly received include the Yamamoto Quality Metrics and the Symbol Error Rate. The Yamamoto metric is determined by comparing the differences in the metrics of remerging paths 15 in each step of the Viterbi decoding with a threshold and labeling a path as unreliable if the metric difference is less than a quality threshold. If the final path selected by the Viterbi decoder has been labeled as unreliable at any step, the decoder output is labeled as unreliable. Otherwise, it is 20 labeled as reliable. The Symbol Error Rate is determined by taking the decoded bits, re-encoding these bits to provide re-encoded symbols and comparing these re-encoded symbols against hard decision received symbols. The Symbol Error Rate is a measure of the mismatching between the 25 re-encoded symbols and the received symbols.

The formatted data frames undergo further processing, which include frequency upconversion to the radio frequency (RF) frequency band and amplification of the signals of data frames, before transmission. 30

When signals of the variable rate data frames are received by a mobile unit, such as mobile unit 16 or 18 of FIG. 1, the mobile unit must determine the rate of transmission in order to properly decode the signals. However, the rate of the received frame is not known by the mobile station a priori. 35 Further, it is not possible to determine the rate by looking at the absolute power of the received signal, even though the power is proportional to the rate of transmission. This is because of changes in the propagation path, such as fading and blocking. Fading occurs because a transmitted signal is 40 reflected from many different features of the physical environment. Consequently, a signal arrives at the receiver of a mobile unit with multiple reflected components. At the UHF frequency bands usually employed for mobile radio communications, including those of cellular mobile tele- 45 phone systems, significant phase differences in signals traveling on different paths may occur. The out-of-phase components may add destructively, greatly reducing the received signal power. Fading is explained in more depth in U.S. Pat. Nos. 4,901,307 and 5,103,459 mentioned above. Blocking 50 occurs because of a physical obstacle entering the line of sight propagation path.

Even though it is not possible to determine the encoded rate of the data frames by looking at the absolute power of the received data signal, the rate can be determined if the 55 fading characteristics are known. The present invention performs rate determination by comparing the power of a data signal with the power of a reference signal transmitted from the same source. The reference should be a signal which is transmitted at a fairly constant power at all times. 60 Further, since fading is frequency dependent, the reference signal should be transmitted at approximately the same frequency as the data signal. This way, the data and reference signals will exhibit similar fading characteristics, and the rate of the data signal can be determined in accordance 65 with the power of the data signal relative to the power of the reference signal.

6

In a preferred embodiment, the reference signal comprises a pilot signal as described in the previously mentioned patents U.S. Pat. Nos. 4,901,307 and 5,103,459. The use of a pilot signal in CDMA systems is well known. As disclosed in the aforementioned patents, a pilot carrier signal is used to provide a coherent phase reference for a communications link. In a CDMA cellular system, each cell or sector transmits a pilot signal of the same spreading code but with a different code phase offset. The phase offset allows the pilot signals to be distinguished from one another thus distinguishing originating cell-sites or sectors. Use of the same pilot signal code allows the mobile unit to find system timing synchronization by a single search through all pilot signal code phases. A pilot signal is also used as a time reference for demodulation of the digital speech signals transmitted by a particular cell-site.

Referring now to FIG. 3, an exemplary receiving system for receiving variable rate communications is illustrated. In a CDMA environment, for example, the receiving system of FIG. 3 may be implemented in a mobile unit in order to determine the data rate of signals transmitted from a cellsite. The present invention offers particular advantages in a mobile station because by determining the rate in advance of decoding, the exhaustive decoding at all rates can be avoided. This reduces power consumption in the decoding process which can extend battery life in the receiver. Also, the speed of rate determination is improved.

For purposes of discussing FIG. 3, the mobile unit in which the rate determination system is implemented will be referred to as mobile unit N, where mobile unit N may be illustrated by either mobile units 16 or 18 of FIG. 1. Variable rate data is transmitted to mobile unit N from system controller and switch 10 via one or more cell-sites. The cell-sites will be referred to as cell-site N' and are illustrated by either cell-sites 12 or 14 in FIG. 1. Although it is shown in FIG. 3 that the rate determination system is part of mobile unit N, it should be understood that the rate determination system may instead be implemented in a cell-site to determine the data rate of signals transmitted from a mobile unit. In addition, the rate determination system may also be utilized in other communications systems.

The variable rate receiving system illustrated in FIG. 3 includes receiver 30 for collecting cell-site transmitted signals. Signals received by receiver 30 include RF signals of the pilot and data signals transmitted by cell-site N. Receiver 30 amplifies and frequency downconverts the received signals from the RF frequency band to the intermediate frequency (IF) band.

The IF signals are presented to pilot demodulator 32 and traffic demodulator 34. The design and implementation of demodulators 32 and 34 are described in detail in U.S. Pat. No. 5,490,165, entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS," assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein. Pilot demodulator 32 demodulates the IF signal to produce the pilot signal transmitted by cell-site N, and presents the pilot signal to pilot power measurement element 36. Traffic demodulator 34 demodulates the IF signal to produce the data, or traffic, signal consisting of the symbols of one frame of data transmitted by cell-site N. Traffic demodulator 34 generates the data signal by despreading and correlating the IF signal with the pilot signal which identifies cell-site N. The data signal generated by traffic demodulator 34 is presented to traffic power measurement element 38. The data signal is also presented to decoder 40.