MXPA00000220A - Rate detection for variable rate cdma communication system - Google Patents

Abstract

Variable rate data transmissions are accommodated having BRI transmitted in the same frame as the data itself. When Walsh code spreading is employed, the despreading process can be split into two separate despreading operations, with buffering between the two operations. Buffering is made of the intermediate symbols between the despreading stages, and can be made at the maximum user data rate instead of the much higher chip rate. In this way, the size of the buffer in the receiver can be significantly reduced.

Description

SPEED DETECTION FOR A VARIABLE RATE CDMA COMMUNICATION SYSTEM This invention, in general, relates to variable rate data transmissions and, more specifically to techniques for efficiently detecting data transmission at a variable rate when transmitting information at an explicit bit rate. At present, cellular radio communication systems have been developed that use spectrum-spread or multipath modulation (CDMA) and multipath modulation techniques. In a common CDMA system, a data stream of information to be transmitted is superimposed over a much higher bit rate data stream sometimes referred to as a spreading or dispersion code. Each symbol of the scatter code is commonly known as a chip. The signal of the information and the signal of the dispersion code are usually combined by multiplication in a process sometimes called coding or dispersion of the information signal. Each information signal is assigned to a unique dispersion code. A plurality of encoded information signals are transmitted as modulations of the radiofrequency carrier waves and are received together as a composite signal in a receiver.

Each of the coded signals is superimposed on all other coded signals, as well as the signals related to noise, frequency and time. By correlating the composite signal with one of the unique scatter codes, the corresponding information signal can be isolated and decoded. As radio communication is more widely accepted, it will be desirable to provide different types of radiocommunication services to meet consumer demand. For example, it is possible to support facsimile, e-mail, video, Internet access, etc. through-radio communication systems. In addition, it is expected that users may wish to have access to different types of services at the same time. For example, a videoconference between two users will include voice and video support. Some of these different services will need relatively higher data rates compared to the voice service that has traditionally been provided by radio communication systems, while other services will require service at a variable data rate. Thus, it is anticipated that the systems of. Future radio communications will need to support communications at a high data rate as well as communications at a variable data rate. Some techniques have been developed to perform communications at a variable data rate in CDMA radiocommunication systems. From the perspective of data transmission at variable rates, these techniques include, for example, discontinuous transmission (DTX), variable dispersion factors, multi-code transmission and coding for forward error correction, variable (FEC). For systems that use DTX, the transmission occurs only during a variable portion of each frame, that is, a defined period of time to transmit a block of data of a certain size. The relationship between the portion of the frame used for the transmission and the total time of the frame is commonly known as the occupation cycle? For example, if it is transmitted at the highest possible rate, that is, during the entire frame period,? = 1, while for transmissions at rate 0, for example, during a pause in the conversation,? = 0. DTX is used, for example, to provide transmissions at a variable data rate in systems designed in accordance with the United States standard entitled "Compatibility Standard for Mobile Station-Base Station for Staggered Spectrum Cell System," Broadband, in double mode ", TIA / EIA Interim Standard TIA / EIA / IS-95 (July 1993) and its revision TIA / EIA Interim Standard TIA / EIA / IS-95-A (May 1995). These standards that determine the characteristics of the cellular communication systems of the United States are promulgated by the Association of Telecommunication Industries and the Electronic Industries Association in Arlington, Virginia. Dispersion factor variation is another known technique for providing communication at a variable data rate - As already mentioned, spread spectrum or spread spectrum systems scatter data signals across the available bandwidth by multiplying each one of the data signals with scatter codes. By varying the number of code-symbols or chips by data symbols, i.e., the scattering factor, while maintaining the rate of the fixed code symbols, the effective data rate can vary in a controllable manner. In common implementations of the variable dispersion factor approach, the dispersion factor is limited by the ratio SF = 2 x SFmin, where SFmi "is the minimum allowable dispersion factor corresponding to the highest allowed user rate. Another known technique for varying the rate of transmitted data is commonly referred to as a multi-code transmission. According to this technique, data is transmitted using a variable number of spreading codes where the exact number of codes used depends on the instantaneous user bit rate. An example of the multi-code transmission is described in the application of the US Patent Serial No. 08 / 636,648 entitled "Systems and Methods DS-CDMA in Compressed Mode, Multicode", filed on April 23, 1996, the description of. which is incorporated herein by reference. Yet another technique for varying the speed of data transmitted in radiocommunication systems includes varying the FEC. More specifically, the early error correction (FEC) coding rate varies using code drilling and rep * etition or switching between different rate codes. In this form, the user rate varies while the bitrate of the channel remains constant. The experts in the art will appreciate the similarities between the variation of the FEC and a variable dispersion factor as mechanisms to carry out the transmission at a variable rate. Regardless of the specific technique adopted in a radio communication system to provide variable data transmission capacity, the receiver must know the specific data rate at which a signal is transmitted to adequately detect and decode the received signal. Methods for informing the receiver of the instantaneous data rate of a received signal generally fall into two categories, systems that explicitly transmit the bit rate information (BRI) together with the transmitted signal, and systems that provide the receiver with the capacity to determine "blindly" the rate at which the data have been transmitted, for example, by trying different rates and looking for a cyclic, correct redundancy check (CRC), US Patent No. 5,566,206 to Butler et al. It provides an example of blind detection of the rate.Both methods of transmission of the explicit BIR as the blind detection of the rate has certain drawbacks.For example, blind detection of the rate gives rise to relatively complex receivers due to the need of additional circuits / logic to correctly identify one of a plurality of possible data transmission rates. I explicit also creates design aspects. For example, the BRI can be sent in the data frame before the data frame that it describes or in the same frame that it describes. If the BRI is transmitted in the previous frame, an additional delay of one frame will be introduced in the transmitter. That is, as soon as the data for a frame is available in the transmitter, the BRI for this frame is calculated and transmitted, while the transmission of this data frame is delayed until the next frame period. This additional delay may be undesirable for low delay services such as speech, especially for long frame lengths.

On the other hand, if the BRI is transmitted in the same frame as the data, the receiver needs to store the received signal in temporary memory until it has detected and decoded the BRI for this frame. This solution originates additional temporary storage in the receiver, and therefore, additional cost and complexity. Therefore, it would be desirable to create new techniques and systems to allow explicit rate information to be transmitted in the same frame that it describes, while minimizing the amount of temporary storage needed in the receiver.

COMPENDIUM These and other problems of the above communication systems are solved by the invention of the applicants, wherein variable rate data transmissions using a variable dispersion factor are accommodated having BRI transmitted in the same frame as the data itself. If Walsh code dispersion is employed, the de-dispersion process can be divided into two separate de-scattering stages, with temporary storage between the two stages. Temporary storage can then be done at the maximum user data rate instead of the much higher chip rate. In this way, the size of the temporary memory can be significantly reduced.

In a first demodulation step it can be carried out using a common first code for all physical channels, based on the maximum rate of the user data and the property of the Walsh code words that code the stem of the common roots. After the BRI is decoded, for example, from a physical control channel, this information is then provided to a second demodulation stage that determines the individual codewords associated with each physical-channel to be decoded.

BRIEF DESCRIPTION OF THE DRAWINGS The characteristics and objectives of the invention of the applicants will be understood by reading this description together with the drawings, in which: FIGURE 1 is a block diagram representation of an exemplary receiver structure in which realize the present invention; FIGURE 2 illustrates variable dispersion in two physical channels with common randomization; FIGURE 3 is an exemplary code tree; FIGURE 4 illustrates two-stage de-dispersion according to an exemplary embodiment of the present invention; and FIGURE 5 illustrates a more detailed version of the block diagram of FIGURE 4.

DETAILED DESCRIPTION Although this description has been written in the context of cellular communication systems that include portable telephones or mobile radiotelephones, those skilled in the art will understand that applicants' invention can be applied to other applications in communications. In accordance with exemplary embodiments of the present invention, CDMA systems can support "services at variable bit rate, such as voice, by providing control information in each frame that specifies the instant data symbol rate for this plot. To perform this in a regular time interval, the physical channels can be organized into frames of equal length. Each frame carries a whole number of chips and a whole number of bits of information. Using this exemplary frame structure, the bit rate control information may be provided for each CDMA frame by transmitting this information on a separate physical channel. The physical channels carrying the data and the control information can be defined as physical data channel (PDCH) and physical control channel (PCCH), respectively. The dispersion code, the symbol rate or the equivalent scattering factor of the PCCH are known a priori for the receiver. Many potential advantages are attributable to variable rate transmission. For example, it is possible to reduce the interference for some users of the system since the chip rate remains constant and a lower bit rate provides a higher dispersion factor, thus allowing a lower transmission power. Those skilled in the art will readily appreciate how this possibility of varying the information rate in a CDMA system can be advantageously used to vary other parameters. The structure of an exemplary receiver (usable, for example, in a base station or a mobile station) is illustrated in Figure 1. The received signal is first processed to produce complex baseband samples by the processor 10. The signal is then distributed to the signal processing branches including the control channel and RAKE demodulators of data channel 12 and 14, respectively. Although only one PDCH signal processing branch is illustrated in Figure 1, it will be appreciated by those skilled in the art that it is possible to include a plurality of these branches in the receivers according to the present invention. The demodulators "12 and 14 are also provided with the corresponding spreading codes for the PCCH and PDCH by units 16 and 18. As already described, the PCCH frame contains pertinent information about the PDCH structure transmitted at the same time and therefore, the PCCH information to be decoded before the PDCH can be demodulated Thus, a video buffer 20 upstream of the PDAK demodulator RAKE 14 delays the input of the baseband signal thereto so that the PDCH demodulator RAKE 14 receives the scattering factor of the PDCH before decoding This information related to the rate at which the data was transmitted in the frame-by-frame PDCH is provided by the PCCH-22 decoder A PDCH decoder 24 is also provided with current below to decode the PDCH as is well known in the art The size of the video buffer 20 can be reduced to the minimum according to the modalities exemplary embodiments of the present invention as described below. To fully appreciate how the size of the buffer 20 can be reduced, a brief description of the variable scattering factors and the orthogonal codes is first provided. As mentioned in the above, variable rate services can be supported by dispersing a data flow with a variable dispersion factor. For example, consider a service that requires a first data rate (lower) during a first period, and a second (higher) data rate during a second period in which it is being supported using a PDCH between a mobile station and a station base. During the first period, it is possible to select a first scatter code based on the first data rate. During the second period, it is possible to select a second scatter code to disperse the frames to be transmitted to the second data rate. Since the second data rate is greater than the first data rate, the second dispersion code will be shorter than the first dispersion code. In addition, a plurality of variable rate PDCHs can be handled in a similar manner. For example, a number of data streams can be dispersed at chip speed using Walsh codes of different lengths, followed by summation and scrambling. Figure 2 represents these dispersion and scrambling operations, exemplified by two physical channels. In this, a first data stream is supplied to the multiplier 30 having a data rate of Ri equal to the chip rate Rc divided by the scattering factor SFi for this data stream. This data stream is dispersed with a code word Ci having a length that is selected such that the result of the multiplier 30 is a physical channel at the chip rate Rc. In the same way, a second data stream is supplied to the multiplier 32 having a second data rate R2 equal to the chip rate Rc divided by a second scattering factor SF2. This data stream is dispersed with a second codeword C2 having a length that is selected to give rise to a physical channel at the chip rate Rc. The two physical channels are summed in the adder 34 and then the compound signal is scrambled with a scrambling code Ccsr in block 36. The resulting signal is sent, for example, to transmit to the signal processing circuits and finally coupled to an antenna. The rate of the data streams may be limited to such an interval that the dispersion factors used are greater than or equal to a predetermined Sfmin. The Walsh codes used for dispersion in multipliers 30 and 32 may be observed in a way Similar to a tree, as illustrated in Figure 3. Codes on the same level in the tree are orthogonal and have the same scattering factor. If a physical channel is scattered with a first code in the tree, and another physical channel is dispersed with another code that is (1) not equal to the first code, (2) not to the left of the first code in the path to the root of the tree, and (3) not in the sub-tree that has the first code as root, the scattered physical channels will be orthogonal. Each physical channel is assigned to a dispersion code of the tree, with dispersion factors corresponding to the respective data rates. As the data rate varies for a specific PDCH, a code of a different level of the tree will be assigned. For example, the increase in data rates, the code selection will move to the left in the tree, while the decrease in data rates, the code selection will move to the right, thus, a PDCH rate Common variable will usually move up and down along a certain path in the code tree as your data rate varies. As can be seen in Figure 3, any given code in the tree is used to construct the codes to the right of it (that is, beyond the root). Thus, it can be considered that any given code consists of lower level codes that are in the direction of the root of the code tree. Applicants have recognized that this code ownership can be used to reduce the transient storage requirements in the receiver. In "the receiver that is described with respect to Figure 1, the received signal is descrambled and desdispersed, however, before the desdispersion of a frame, the receiver needs to know the dispersion factor used for the transmission of this frame. that the BRI is transmitted to the same frame according to the exemplary embodiments of the present invention, the signal needs to be stored, however, the applicants have recognized that the de-scattering with the larger common part of all the Walsh codes that are available for a particular connection establishment it can be performed without transient storage, ie before the BRI is decoded.Therefore, the receiver branch illustrated in Figure 1 including the temporary memory 20 and the PDCH demodulator RAKE 14 can be modified as illustrated in Figure 4. In this, the signal is correlated with a first code in the root of the sub-tree in the all the possible codes in block 50 to partially disperse the received symbols. This process is performed for all multipath beams, the RAKE combination is made and the intermediate symbols obtained are stored in temporary memory. Once the BRI has been decoded by the PCCH decoder 22, the intermediate symbols of the temporary memory 20 are correlated with a second code in the block 52 to obtain the unprocessed, undispersed bits. The code used for the second de-scattering step is easily identified from the code tree. For example, the de-dispersion of the code (+ 1 + 1-1-1-1-1 + 1 + 1) can be performed by first disperse with (+ 1 + 1) in block 50 followed by desdispersion with (+ 1-1-1 + 1) in block 52. Anotalternative is to disperse with (+ 1 + 1-1-1) in block 50 followed by (+ 1-1) in block 52. The first combination gives rise to intermediate storage of the symbols at a rate hig and tfore needs a larger buffer 20. However, in any case, the temporary storage according to the present invention is done at a significantly better rate than the chip rate. For example, in a system that uses a variable dispersion factor between 64 and 256, temporary storage is done at a rate 64 times lower than the chip rate. Figure 5 depicts an exemplary de-scattering process according to the present invention for two codes in a two-finger RAKE receiver. First, in blocks 60 and 62 the received signal is submitted to the process to reverse the randomization that was performed in block 36 of Figure 2. Then, in blocks 64 and 66, the received signal is partially despread using the code common, that is, the code in the far left portion of the code tree that is common to all the variable dispersion factor options for this specific transmission. The resulting desdispersed partial signals are integrated into blocks 68 and 70, respectively, and modified based on the channel estimates in blocks 72 and 74. These latter two operations are well known to those skilled in the art with respect to processing of signals from the RAKE receiver. The resulting partially demodulated signals are then summed in block 76 and entered into the temporary memory 20 w they are stored until bit rate information can be provided from the PCCH decoder. However, since the signals have been partially desdispersed before being stored in the temporary memory 20, the storage data rate is much lower than the chip rate, providing the designer the opportunity to reduce the size of. the temporary memory "18. Once the BRI information is available for this frame, then it is possible to select the second codes C" and C2"to complete the de-scattering operation in blocks 78 and 80, respectively. Once again, the desdispersed signals are integrated into blocks 82 and 84 and subsequent processing is performed as is well known in conventional RAKE receivers. It will be understood that the invention of the applicants is not limited to the specific modalities described in the foregoing and that persons skilled in the art can make modifications to it. The scope of the invention of the applicants is determined by the following clauses, and any and all modifications that fall within the scope are proposed to be included in it.

Claims (20)

1. A receiver consisting of: the means for receiving a scattered spectrum signal that includes at least two physical channels; the means to partially disperse one of the at least two physical channels using a first code; the means for temporary storage of the partially disperse signal; and the means to disperse the stored signal using a second code.

The receiver of claim 1, wherein the spread spectrum signal is received at a chip rate and the means for temporary storage stores the partially despread signal at less than the chip rate.

3. The receiver of claim 1, wherein at least two physical channels include a control channel and a first data channel.

4. The receiver of claim 1, wherein one of the at least two physical channels can be received at any of a plurality of data rates and the first code is selected based on the plurality of data rates.

The receiver of claim 3, further comprising: means for demodulating and decoding the control channel to obtain bit rate information associated with the first data channel; and the means for supplying bitrate information to the medium to disperse the stored signal using at least a second code.

6. The receiver of claim 5, wherein the second code is selected based on the bit rate information.

The receiver of claim 3, wherein at least two physical channels further include a second data channel.

The receiver of claim 7, wherein the means for partially descrambling also de-disperses the second data channel using the first code.

The receiver of claim 8, further comprising: means for demodulating and decoding the control channel to obtain the bit rate information associated with each of the first and second data channels; and wherein the means for de-dispersion selects the second code and a third code to disperse the first and second data channels, respectively, based on the bit rate information.

10. The receiver of claim 9, wherein the rate information of. bits includes scattering factors for each of the first and second data channels.

11. The receiver of claim 5, wherein the bit rate information is a scattering factor.

The receiver of claim 8, wherein the first code is selected based on the code bits that are common to the first and second data channels. - 13.

A method to disperse a data frame consists in the steps of: (a) disperse the data frame using a first code; (b) storing in temporary memory an output of step (a); (c) determine a rate at which a data frame was transmitted; and (d) disperse the stored output using a second code, the second code is selected based on the determined rate.

The method of claim 13, wherein step (b) further comprises the step of: storing the output at a rate that is less than a chip rate of the data frame in temporary memory.

15. The method of claim 13, wherein step (c) further comprises the step of: obtaining the rate. Dispersing and decoding a control channel.

16. The method of claim 13, wherein step (a) further comprises the step of: determining a first code based on a plurality of rates at which the data frame can be transmitted.

17. A receiver consisting of: a. Dispersing first to disperse a received spread spectrum signal; a temporary memory connected to the first desdispersor for storing an output thereof; and a second de-disperser connected to the temporary memory to disperse the stored output.

18. The receiver of claim 17, wherein the first desdisperser desdisperses the scattered spectrum signal received using at least two branches, each branch uses the same first code to disperse the received spread spectrum signal.

19. The receiver of claim 17, wherein the second de-disperser dis- disperses the stored output using at least two branches, each branch using a code that is different from the other codes used in at least two branches. The receiver of claim 17, wherein the first descrambler reverses the randomness of the received spread spectrum signal using a scrambling code.