JP2001177450A - Station and method for eliminating disturbance from received information including walsh code - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a station that can receive information including Walsh coding in a plurality of channels and eliminate disturbance from the received information and to provide a method for eliminating the disturbance. SOLUTION: The station is provided with a memory that stores a received signal including a Walsh code in each of a plurality of channels and a signal that is reflected and refracted and with a plurality of 1st signal paths coupled with the memory. Each signal path receives the signal in the memory. The signal passing through the signal path is subjected to a delay and inverse fast Walsh transform and the Walsh code is eliminated from the signal. The phase shift and attenuation in each path can be estimated and the conjugation of the estimation is multiplied with the signal from which the Walsh code is eliminated to produce path outputs of each channel. By combining them, estimation of data from which disturbance is eliminated is obtained. The disturbance can furthermore be eliminated by providing a plurality of 2nd signal paths corresponding to the 1st signal paths and conducting the above processing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a method of jamming cancellation in a CDMA system, in particular in relation to such a system having a plurality of mobile stations.

[0002]

2. Description of the Related Art A code division multiple access (CDMA) communication system is a communication system in which data is transmitted according to a specific code, whereas in time division and frequency division systems, the transmission is for transmission and reception, respectively. , And the difference between communication frequency bands. In CDMA,
All frequencies and all time slots are available, so that communication with a particular receiver is performed by matching the code applied to the message sent to the code of the selected receiver. Will be The selected receiver receives only the communication whose transmission code matches the receiver code. In such a CDMA system, the codes of all receivers in the system must be orthogonal to other receivers in the system to avoid distortions such as crosstalk.

During transmission from a base station to a mobile station, signals transmitted to different mobile receivers in the system may be orthogonal to one another as initially required, but the transmitted signals may be transmitted through a transmission path. A problem arises in that the building and / or other reflecting and / or refracting media therein can be reflected and / or refracted one or more times. These reflections and / or refractions shift the initially orthogonal signals so that they are no longer completely orthogonal and are received by other receivers at varying intensities, thereby causing other receivers to To interfere with. The quality of the received signal is a function of the amplitude of the transmitted signal,
It is divided by a function of the sum of noise and interference. Since noise is generally not variable in established facilities, minimizing disturbances is desirable and generally essential for improving signal quality, and therefore for improving received signal quality. Clearly a necessary approach.
Current commercial CDMA systems have no mechanism to eliminate such types of interference.

Interference cancellation in CDMA systems increases capacity, reduces the need for power control, or
It has been widely studied as a method for improving the bit error rate in a receiver, and has been widely used for wideband code division multiple access (WCD).
MA) is of particular interest in connection with systems. One type of jamming cancellation is multi-user detection, where the receiver demodulates information from many different transmitters. This erasure is usually performed at the base station, which must receive signals from all mobile units in the cell of the base station, but such erasure can be performed at the mobile unit as well. . The optimal jamming elimination method is described in IEEE Transactions on Inf in January 1996.
operation Theory), Volume IT-32,
No. 1, pages 85 to 96, S.I. S. Verdu, "Minimum Probability of Errors in Asynchronous Gaussian Multiple Access Channels (Minimum pro
availability of error for asy
nchronous Gaussian multip
le-access channels). This procedure can be used to find an upper bound on the performance gain that can be achieved by jamming cancellation. The optimal interference cancellation is based on maximum likelihood sequence estimation (ML
SE), the MLSE makes this solution uneconomical due to its exponentially increasing complexity with the number of users, making it cost efficient and power efficient. Cannot be executed. Jamming cancellation can be used at the base station to cancel the effects of other mobile units within a single cell, and at the mobile station it can be used to eliminate jamming signals from the current base station or other base stations. Can be used to erase

[0005] The term "receiver" as used herein refers to both a device capable of performing only reception and a device capable of performing transmission and reception.
Is generally a mobile station.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a generally simple and relatively economical multistage system for canceling the above-mentioned disturbances.

[0007]

SUMMARY OF THE INVENTION The invention described herein is used in a mobile station to cancel interference caused by multiple paths from a base station, wherein the received signal is transmitted. It arrives as a pristine signal and as a reflected and / or refracted signal. The approach taken is related to multi-user detection (MUD), which is a type of interference cancellation in which signals on many channels are demodulated at the receiver. MUD in such a situation relates to the demodulation of many channels transmitted by one base station.

[0008] Briefly, a multiple rake finger (multi) for receiving a signal having a Walsh code.
A mobile station having a single rake finger is provided. Each rake finger is a correlator that demodulates one multipath component of the received signal. (The basis vector of the Hadamard transform can be generated by sampling a Walsh function that takes only two values ± 1 to form a perfect orthogonal basis for the square integrable function. The Hadamard transform can also be generated by Jane A. K. (J
ain, A .; K. ) "Basics of Digital Image Processing (F
undentals of Digital Im
Age Processing ", also known as the Walsh-Hadamard transform, as described in Prentice-Hall (1989). After performing a standard procedure to remove the long code used by the CDMA system (the long code is a known pseudo-random sequence that takes two values ± 1, which modulates the signal to be transmitted, The mobile station applies this signal to an inverse Walsh transform circuit (IFWT), which removes the appropriate Walsh code from each channel, making it appear to other users in the system as Gaussian noise. This allows all channels transmitted from the base station to be demodulated simultaneously, producing a separate output for each channel. For each rake finger, a single channel response is generated for each channel by a pilot symbol (a training symbol known at the receiver that can be used for purposes such as synchronization, channel estimation, and signal-to-interference ratio estimation). Is estimated from The channel estimate is the average of the received pilot symbols to estimate the amplitude and phase of the channel. The output of each IFWT is multiplied by the complex conjugate of the channel response, and after the combination of the maximal ratios, each output is sent to the data decision block (finding the constellation point closest to the received amplitude, which symbol For example, in the case of {1 + j, 1-j, -1 + j, -1-j} in QPSK modulation, if the received symbol is 0.8 + 1.1j, the data decision block is 1 + j. Determined to have been transmitted) and the data decision block makes symbol decisions on the respective channels, which are estimates of the transmitted data.

[0009] Interference cancellation is performed by transferring the estimated data in each channel to a Walsh transform circuit, in which the estimated data is converted into orthogonal channels using Walsh codes for the respective channels. Is re-spread. This signal is
Multiplied by the downlink long code. At each rake finger, the reproduced signal is multiplied by an estimate of the channel response and delayed by the channel delay of each finger. For each rake finger,
Reproduction signals from all other fingers are subtracted from the original signal received by the finger. The subtraction of the regenerative interference from the original signal is then fed back to the rake finger, where the signal is again recirculated through the circuit described above a desired number of times, each recirculation further removing the interference. After the desired number of iterations, the estimated data from the last iteration on the channel assigned to the mobile device is
Taken as the output of the receiver.

[0010] The standard implementation of despreading and respreading operations in a receiver for an M channel where each channel has a spreading factor N requires MN operations. Since the despreading and re-spreading operations are the most computationally intensive parts of the jammer, reducing the complexity of these operations can make the jammer more cost-effective and power-efficient. . The present invention uses inverse fast Walsh transform (IFWT) and fast Walsh transform (FWT) to reduce computational requirements at the receiver. Since all receive channels are time synchronized and follow the same multipath, inverse fast Walsh transform (IFWT) is used to despread Walsh codes, and fast Walsh transform (FWT) reconstructs Walsh codes. Can be used to spread. Both of these transformations are Nlog ₂
(N) times of addition and subtraction. A second contribution of the present invention is that channels with multiple rates can be despread and respread using a modified FWT. The number of required operations is Nlog
₂ (N) or less. Less than,
The multi-speed FWT will be described.

When demodulating multiple channels where each channel has a different Walsh code, an IFW is used to remove the Walsh code from each channel.
T can be used. This IFWT can be performed with Nlog ₂ (N) additions and subtractions, where N is the length of the spreading sequence. In a 256 chip sequence,
The number of operations required for different diffusion coefficients is shown in the following table.

[0012]

[Table 1]

If all the channels to be detected have the same spreading factor N, the IFW with the correct spreading factor
T can be used to separate all channels.
However, if the channels have multiple rates and thus different spreading factors, the number of operations increases if direct implementation is performed. For example, if 32 and 128
If the spreading factor is present in some receive channels, the direct approach of applying the IFWT with lengths of 32 and 128 requires 1280 + 1792 = 3072 operations to process 256 chips And

[0014] The mobile station may respread the demodulated data into orthogonal channels using FWT. FWT also has Nlog ₂ (N) complexity. 1 and 2
Are the FWs for Walsh codes of length 8, respectively.
Figure 2 shows one version of the circuit diagram of T and IFWT.
These flow graphs are not traditional flow graphs for FWTs and IFWTs, but have been selected to provide a simple-to-implement multi-rate FWT. All signals go from left to right, and all operations are additions or subtractions. The two branches that intersect at a node represent an addition, except when the lower branch is marked with "-1" representing a subtraction at that node. Lowercase letters indicate complex values before Walsh spreading, uppercase letters indicate complex values after Walsh spreading. The Walsh codes used are standard Walsh codes, and the eight Walsh codes shown in FIGS. 1 and 2 are shown below, where each row is one Walsh code And the lower case symbol to the left of each code is the data point assigned to the Walsh code.

[0015]

[Table 2]

It is easy to extend the single-rate FWT and IFWT shown in FIGS. 1 and 2 to a multi-rate version. 3 and 4 show examples of an 8-point multi-speed fast Walsh transform (MFWT) and an 8-point inverse multi-speed fast Walsh transform (IMFWT). If a pair of data points is 1 /
With a diffusion coefficient of 2, the first butterfly is omitted. Similarly, if the four data points have a spreading factor of 1/4 of the maximum spreading length, the first two butterflies are omitted. This concept is extended to shorter spreading codes. The Walsh codes shown in the above example are of lengths 2, 4, and 8, and are given below. Both x2 and x4 use the same Walsh code of length 4. The first 4 chips are x2
And the second four chips transmit x3. Similarly, x4, x5, x6, and x7 all use the same Walsh code of length two.

[0017]

[Table 3]

IFWT requires only a number of operations corresponding to (or less than) the maximum spreading factor. Thus, if there were 32 and 128 spreading factors as in the previous example, the number of operations required to process 256 chips would be 1792 or less. In a multi-speed FWT, the number of operations can be reduced because some butterflies are not required. For Walsh codes with a low spreading factor, inputs using the same Walsh code must be in index bit-reversed order as demonstrated by x4, x5, x6, and x7 in FIG. This is a standard bit-reversal order used in most fast transforms, such as the fast Fourier transform (FFT). Walsh codes with low spreading factors must also be properly scaled at the output of FIG. The output corresponding to a spreading factor of 1/2 of the maximum length must be multiplied by 2, and an output having a spreading factor of 1/4 of the maximum length must be multiplied by 4. The same pattern is used for short Walsh codes. Since these coefficients are always a power of 2, multiplication is easily performed by performing a bit shift to the left.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 5, assuming operation in CDMA, signals received from a base station at a mobile unit are stored in a memory. This received signal is
Includes unreflected and refracted signals and the same reflected and refracted signals from each channel corresponding to each mobile unit being served. The signals in all channels have the same standard long code and a unique Walsh code associated with each channel. Furthermore, the delay in receiving all significant maxima in the received signal, which is generally the main signal and the signal that has undergone major reflection and / or refraction, is determined in a standard manner, and the signal in memory is Applied to a plurality of delay fingers. Each delay finger is part of a path having a plurality of channels, each path corresponding to a different Walsh code, and the number of channels corresponding to the number of available Walsh codes being used. Each path has substantially the same components, and there is one delay finger for each maximum value of interest. The maximum value of interest may be, for example, a maximum value exceeding a certain amplitude, but criteria other than the maximum value may be used. The long code is removed from each channel of each path, and the signal of each channel of each path is then removed to remove the Walsh code for that channel from the signal.
After receiving the inverse fast Walsh transform (IFWT), subtracting the long code from the transmission signal passing through each delay device and subtracting the Walsh code of the channel,
It will be left at each IFWT output. This IFWT output for each channel in each path is then multiplied by the conjugate of the channel estimate in that path. This channel estimation compensates for the phase shift in that channel and gives the correct weight so that a maximum ratio combination can be made. The channel estimate is a number with magnitude and phase that represents the attenuation and phase shift of the signal in each path resulting from the output of the IFWT, and is obtained by averaging the pilot symbols. The outputs of all the same channels in each path are then combined in a maximal ratio combiner that adds the energy received from all paths individually for each channel. The output of this maximum ratio combiner is sent to data decision blocks along multiple paths, one path for each channel, and one data decision block for each channel is added to the received signal. Estimating which symbol was transmitted by finding the closest constellation point. The output of each data decision block is a data estimate, which is the output for the received signal, or it can be recycled for additional correction.

In the case of recycling, the data from each channel is regenerated by applying the fast Walsh transform (FWT) for that channel to the output of each channel and then combining all the signals. Spread. The combined signal is multiplied by the next longer code, and the combined signal is applied to the same sequence of delay fingers as previously described, and the signals along each path are used earlier in each path. It is delayed using the same delay device as was done. The delayed signals in each path are then each multiplied by the channel estimate at each finger, as before, and reintroduced previously removed attenuation and phase shift. The signal outputs after the multiplication of the channel estimates are added in an adder to provide total rejection. This total playback disturbance is subtracted from the original signal stored in the memory, resulting in the original signal minus the disturbance determined by a single pass through the disturbance cancellation circuit. In each path, the jamming estimate for that finger is added to the signal minus jamming, and the sum is now stored in the first memory.
By switching appropriately from the delay device at the k-th finger to the input from the delay device for each finger instead of the signal obtained from the memory. This jamming can be recycled one or more times as desired, with each recycling allowing more jams to be continuously erased.

The circuits required to perform each of the above functions are well known, for example, Viterbi A. J. (Vit
erbi, A .; J. ) “CDMA: Principles of Spread Spectrum Communication (CDMA): Principles of S
read Spectrum Communicat
ion) ", Addison-Wesley, Reading (A
Ddison-Wesley, Reading), Mass. (1995), the contents of which are incorporated herein by reference.

While the invention has been described with reference to specific embodiments, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore intended that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

With respect to the above description, the following items are further disclosed. (1) A station capable of receiving information including Walsh coding on a plurality of channels and eliminating interference from the received information, including: A memory for storing a received signal comprising a Walsh code in and a reflected and / or refracted version of the received signal, (b) a first plurality of signal paths selectively coupled to said memory. Wherein each of the signal paths coupled to the memory receives the signal stored in the memory, and wherein each of the signal paths includes: Signal path,
(I) a first delay circuit corresponding to one of a plurality of different time interval conditions of the received signal; and (ii) an inverse fast Walsh transform circuit for removing a Walsh code corresponding to the path from the received signal. (Iii) a circuit for generating an estimate of the phase shift in each path and the attenuation of the received signal in the path, (iv) removing the Walsh code in each of the paths, A circuit for multiplying a conjugate to generate a path output in each of the channels; and (v) a combiner for generating a data estimate by combining the path outputs in each of the channels.

(2) The station according to item 1, wherein the station is a mobile station. (3) The station according to (1), wherein the interval condition is a predetermined maximum value in the received signal. (4) The station according to (2), wherein the interval condition is a predetermined maximum value in the received signal.

(5) A Walsh code corresponding to each of the channels is applied to the data estimation in the channel, and a second plurality of signal paths corresponding to the first plurality of signal paths, 2. The station of claim 1, further comprising circuitry for providing said second plurality of signal paths, wherein said second plurality of signal paths includes: A second delay circuit corresponding to one of the plurality of different time interval conditions of the received signal for delaying the data estimation comprising:
(Ii) a circuit for estimating the phase shift in each path and the attenuation of the received signal in the path;
i) a circuit for determining and subtracting the total reproduction interference from the received signal; and (iv) for each path to the first delay circuit, excluding the received signal and excluding the received signal−the total reproduction interference + the estimated interference. Circuit to apply.

(6) The station according to item 5, wherein the station is a mobile station. (7) The station according to (5), wherein the interval condition is a predetermined maximum value in the received signal. (8) The station according to (6), wherein the interval condition is a predetermined maximum value in the received signal.

(9) In a station capable of receiving information including Walsh coding in a plurality of channels, a method for eliminating interference from the received information, the method including the steps described in the following items; Providing a memory for storing a received signal including a Walsh code in each and a reflected and / or refracted version of the received signal; (b)
A first plurality of signal paths selectively coupled to the memory, wherein each of the signal paths coupled to the memory receives the signal stored in the memory. Providing a plurality of signal paths;
(C) performing the following steps in each of the signal paths; (i) delaying the received signal according to one of a plurality of different time interval conditions of the received signal; (ii) Applying an inverse fast Walsh transform corresponding to the channel to the received signal in the channel for removing a Walsh code corresponding to the channel from the received signal, (iii) a phase shift in each path Generating an estimate of the attenuation of the received signal in the path; and (iv) conjugate of the estimate to the signal in a respective channel of the respective path after removal of the Walsh code in the respective path. Multiplying by each other to generate a path output in each of said channels; (v) The step of generating a data estimation by combining the path output of the channel.

(10) The method according to item 9, wherein the station is a mobile station. (11) The method according to (9), wherein the interval condition is a predetermined maximum value in the received signal. (12) The method according to (10), wherein the interval condition is a predetermined maximum value in the received signal.

(13) applying a Walsh code corresponding to each of the channels to the data estimation in the channel and providing a second plurality of signal paths corresponding to the first plurality of signal paths; Wherein each of the second plurality of signal paths performs the steps described in the following items, further comprising:
(Ii) delaying the data estimate with the applied Walsh code by a delay corresponding to one of the plurality of different time interval conditions of the received signal; Generating an estimate of the phase shift in the path and the attenuation of the received signal in the path; (iii) determining and subtracting total reproduction interference from the received signal; and (iv) the first delay circuit. Applying the received signal−the total reproduction interference + the estimated interference, excluding the received signal, for each path.

(14) The thirteenth station, wherein the station is a mobile station.
The method described in the section. (15) The method according to (13), wherein the interval condition is a predetermined maximum value in the received signal. (16) The method according to (14), wherein the interval condition is a predetermined maximum value in the received signal.

(17) A station capable of receiving information including Walsh coding in a plurality of channels and capable of canceling interference from the received information, and a method of erasing. A first memory is provided for storing a received signal including a Walsh code in each of the plurality of channels, and a reflected and / or refracted version of the received signal, and is selectively coupled to the memory. A plurality of signal paths are provided, each of which is coupled to the memory for receiving the signal stored in the memory. In each of the signal paths, the received signal is delayed according to one of a plurality of different time interval conditions of the received signal.
For the received signal in each of the channels,
An inverse fast Walsh transform corresponding to the channel is applied to remove a Walsh code corresponding to the channel from the received signal. An estimate of the phase shift in each path and the attenuation of the received signal in the path is generated, and after removal of the Walsh code, the signal in each channel of each path is multiplied by the conjugate of the estimate. , A path output in each channel is generated. The path outputs of each of the channels are combined to generate a data estimate. The station is preferably a mobile station, and the interval condition is preferably a predetermined maximum value in the received signal. Interference is further eliminated by applying a Walsh code corresponding to each channel to the data estimation in the channel and providing a second plurality of signal paths corresponding to the first plurality of signal paths. Each of said second plurality of signal paths delays said data estimate having an applied Walsh code by a delay corresponding to one of said plurality of different time interval conditions of said received signal. Do. An estimate of the phase shift in each path and the attenuation of the received signal in that path is generated, and the total reproduction interference is determined and subtracted from the received signal. For each path to the first delay circuit, the received signal minus the total reproduction interference + the estimated interference is applied, excluding the received signal.

[Brief description of the drawings]

FIG. 1 shows one version of a schematic diagram of a FWT for a Walsh code of length eight.

FIG. 2 IFWT for a Walsh code of length 8
2 shows one version of the circuit diagram of FIG.

FIG. 3 shows an example of an 8-point multi-rate fast Walsh transform.

FIG. 4 shows an example of an 8-point inverse multiplex rate fast Walsh transform.

FIG. 5 is a flow diagram of an embodiment of a jamming cancellation circuit according to the present invention.

Continued on the front page (72) Inventor Srinath Hosl United States Texas, Dallas, Shadybrook Lane 6441, number 2152 (72) Inventor Timothy M, Schmiddle United States Texas, Dallas, Beltline 5850, number 1414 F-term (reference) 5K022 EE02 EE35 5K046 AA05 EE47 EE56 5K059 CC03 DD35 EE02 5K067 AA02 CC24 EE02 GG11 HH22 HH23 KK15

Claims (2)

[Claims] 1. A station capable of receiving information including Walsh coding on a plurality of channels and eliminating interference from the received information, including: (a) a plurality of channels; A memory for storing a received signal comprising a Walsh code in each of the following, and a reflected and / or refracted version of the received signal: (b) a first plurality of signal paths selectively coupled to said memory. Wherein each of the signal paths coupled to the memory receives the signal stored in the memory, and wherein each of the signal paths comprises: A plurality of signal paths; (i) a first delay circuit corresponding to one of a plurality of different time interval conditions of the received signal; (Iii) a circuit that generates an estimate of the phase shift in each path and the attenuation of the received signal in that path, and (iv) the circuit that generates an estimate of the phase shift in each path and the attenuation of the received signal in that path. Multiplying the signal by the estimated conjugate after removal of the Walsh code;
(V) a combiner that combines the path outputs of each of the channels to generate a data estimate. 2. A method for eliminating interference from received information at a station capable of receiving information including Walsh coding on a plurality of channels, the method comprising the steps of: a. Providing a memory for storing a received signal comprising a Walsh code in and a reflected and / or refracted version of the received signal; (b) a first plurality of signals selectively coupled to said memory. A path, wherein each of the signal paths coupled to the memory comprises the first plurality of signal paths receiving the signal stored in the memory; and (c) each of the signals. Performing the following steps in the path: (i) one of a plurality of different time interval conditions of the received signal; Delaying the received signal, (ii) performing an inverse fast Walsh transform corresponding to the channel on the received signal in each of the channels to remove a Walsh code corresponding to the channel from the received signal. Applying; (iii) generating an estimate of the phase shift in each path and the attenuation of the received signal in the path; (iv) removing each of the Walsh codes in each of the paths. Multiplying the signal in each channel of the path by the conjugate of the estimate to generate a path output in each of the channels; and (v) combining the path outputs of each of the channels to generate a data estimate.