EP1356311A1 - Multipath searcher for cdma wireless communications - Google Patents

Abstract

An apparatus for performance improvement of a digital wireless receiver (108), having a plurality of signals, has a processing crcuit and a signal path searching circuit (214). The processing circuit provides a processed signal whereas the signal is received based on the serached results (timing of the path) of the searching unit (214). A method implementing the present invention is also described.

Description

MULTIPATH SEARCHER FOR CDMA WIRELESS COMMUNICATIONS

FIELD OF THE INVENTION

This invention relates to the field of wireless digital communications, and more particularly to a multipath searcher in the receiver for such signals.

BACKGROUND OF THE INVENTION

Wireless communications facilitates the delivery of information between the transmitter and the receiver without a physical wired connection. Such advantage translates to the freedom of mobility for the users and to the savings of wiring nuisance for the users. However, spectrum has become scarce resource as the usage of wireless communications for various applications becomes more popular. Therefore the efficiency of using spectrum presents challenges for the wireless industry. In order to maximize efficient spectrum utilization, various multiple access methods have been proposed to achieve the goal. First generation cellular communications systems, Advanced Mobile

Phone Services (AMPS) employed the Frequency Division Multiple Access (FDMA) method and provided voice communication services in the early days. Second generation cellular communications systems improved the spectrum efficiency by using more digital processing of signals and employed Time Division Multiple Access (TDMA) method in GSM and IS- 136 systems and Code Division Multiple Access (CDMA) method in IS-95 systems. While second generation systems typically provide two to five times voice capacity over the first generation systems, data capabilities of second-generation systems are very limited. Recent rapid commercial development of Internet and multimedia applications has created a strong demand for wireless cellular systems capable of providing sufficient bandwidth. In addition, further improvement of voice capacity in spectrum efficiency is in great demand as the spectrum allocated for service is very limited. This scarcity results in high licensing fees for the available spectrum.

Therefore there is a strong need to improve the system capacity and spectrum efficiency for wireless communication systems.

SUMMARY OF THE INVENTION One feature of the present invention is performance improvement of a digital wireless receiver, having a plurality of signals, a processing circuit and a signal path searching circuit. The processing circuit provides a processed signal whereas the signal is received based on the searched results (timing of the path) of the searching unit. A method implementing the present invention is also described.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawings in which: FIG. 1 is a stylized representation of a typical multipath channel model;

FIG. 2 is a block diagram of the rake receiver;

FIG. 3 is a block diagram of the multipath searcher; FIG. 4 is a block diagram representation of a multipath searcher based sliding window operation;

FIG. 5 is a block diagram representation of a multipath searcher based on full length FFT; and, FIG. 6 is a graphical representation of a multipath searcher based on shorter length FFT based on the "divide and accumulate" method.

DETAILED DESCRIPTION OF VARIOUS ILLUSTRATIVE EMBODIMENTS One embodiment of the present invention is an apparatus for performance improvement of a digital wireless CDMA receiver that combines the signal

• received from different multipaths. To identify the multipaths, a multipath

- searcher has to be implemented in the receiver to decide the timing delays of the multipaths. In a CDMA system, the possible delays in units of chip are limited. The set of all possible delays is often called the multipath search window. For each possible delay in the search window, the multipath searcher calculates the correlations between the received signal and an offset version of the locally generated sequence. Conventional implementation is to use either a correlator or a matched filter. Compared to the matched filter method, the correlator method requires less implementation complexity but is generally slower. This invention presents a novel design and implementation approach that offers both simplicity and fast processing speed. It uses the Fast Fourier Transform (FFT) to calculate the correlations for all the offset in the search window. It is based on the theory that the convolution of two sequences in time domain is equivalent to the product of their corresponding FFTs in frequency domain. In other words, the correlations can be calculated by the following:

1) take the FFT for both sequences;

2) multiply the two resulting sequences; and, 3) take the Inverse Fast Fourier Transform (IFFT) and shift the resulting sequence appropriately.

Both the FFT and IFFT can be implemented using the same hardware block, which we call a length n FFT block, where n is the length of the input sequences. In general, a shorter FFT lock, such as Vτ bits FFT block is much easier to implement and cost a low less. Also, in many systems, shorter FFT blocks are available because of their use in other application such as cell search. On the other hand, if the received data sequence needs to be accumulated over a long period, taking the FFT for longer sequences may be inevitable. Thus there creates a need to realize a long FFT using shorter FFT blocks. Available methods are such as the overlap-add and overlap-save methods can achieve this goal but often at an extra cost of redundant processing. This invention presents a novel "divide- and-accumulate" FFT solution. It is especially useful for multipath search due to that fact that only the correlations within a narrow window are relevant to the cell search. In other words, we do not need to compute the entire convolution of the two sequences, instead, only a portion of the convolution is necessary. The proposed method divides the received signal and the reference signal into segments of appropriate sizes and does the FFT and IFFT for these segments. The results are then accumulated, either coherently or non-coherently. Compared to conventional searcher method, this method offers identical performance with much lower implementation complexity.

In wireless communications, radio waves from a transmitter arrive at a receiver via several paths (multipaths) having different path lengths. The receiver combines the radio waves. However, the combining of the radio waves is not performed by coherent addition, as a result of which fading occurs. Various diversity schemes have been proposed to deal with such fading. One example is a rake receiver scheme. Rake reception is a technique, which involves identifying signals that have passed through multipaths and combining the signals (by maximum-ratio combining) upon weighting them for reliability, thereby improving the characteristic.

Digital cellular wireless communication systems using CDMA technology have been developed as next-generation mobile communications systems for implementing wireless Internet and multimedia communication. In such CDMA communication systems, transmission information from a plurality of channels or users is multiplexed by spreading codes that differ from one another and is transmitted via a transmission path such as a wireless link.

CDMA system concepts have been incorporated into the dominant third generation (3G) standards. As the whole wireless industry moves toward 3G development and deployment, CDMA systems are becoming increasingly more popular. Due to the ability to resolve multiple paths, CDMA systems usually employ a rake receiver in the signal reception process. This invention proposes a new architecture that improves the rake receiver design for CDMA systems. One major advantage of CDMA systems is their efficient usage of wide bandwidth signal. CDMA systems transmit wide bandwidth signal over the air from the transmitter to the receiver. Referring to Fig. 1 there is shown a typical multipath channel model, a variety of reflecting objects 106, such as geographical features (mountains, etc.), storage towers (water, gas, oil, etc.), and buildings as well as other objects cause the signal from the transmitter 102 to be split into multiple paths 104 and arrives at the receiver 108 with a different delay for different paths. Typically, each transmission path 104 has different length and different reflection condition and thus yields different delay and different channel attenuation in both the signal amplitude and phase rotation. Each transmission path gives the signal a different complex gain (wj) (signal strength) and a different corresponding delay (77, r_- ... τι ... τX).

The wide bandwidth that CDMA signal transmits helps resolve transmission path ambiguity and materializes the detection of signal at different paths. CDMA systems, such as the mobile receiver, therefore, incorporate a rake receiver to combine the signals from different paths. The rake receiver is usually equipped with several fingers with each finger demodulating and decoding the signal at a path with different delay. The rake receiver uses a soft combination of signals from different fingers to "rake" the received signal and reconstruct the transmitted signal. The "soft" combination at a rake receiver presents the intrinsic diversity gain spread over different paths. The rake receiver is thus one of the most critical subsystems in CDMA communication systems. The rake receiver detects the pilot signal and provides the channel estimate at each finger. Referring to Fig. 2 there is shown a block diagram of the rake receiver. A BTS transmitter 202 sends a signal over a channel 204 which is received by the User Equipment 206. The User Equipment 206 is comprised of a baseband RF Unit 208 and other modules 210. The other modules 210 include a fine time tracking module 212, a multipath searcher 214, a rake receiver 216 and a maximum ratio combining module. The received signal after the matched filter (matched to the transmitted pulse shaping), is then de-spread with a hypothesized delay τ. A typical de-spread operation is a multiplication with the product of the PN sequence (which is associated with the transmit base station and the user) and the specific channel orthogonal code (i.e., code for pilot code channel or traffic code channel), then followed by an integration over certain period. The hypothesized delay τ is unique to each finger so that multiple fingers are set up to capture the signal at multiple paths. The integration period for traffic channel is the symbol duration in the traffic channel and the integration period for pilot channel depends on the channel variation speed or the vehicular speed. The result of the integration at the /^th finger is a pilot signal and a traffic signal J_/.

The PN sequence is the pseudo random sequence. The sequence can be generated by a shift register with some binary add operations so the output sequence appears random. However, as long as the receiver knows the seed for the register and timing, it can re-generate the sequence.

Note that the received signal contains multiple delayed and distorted copies. To make sure each finger demodulating the signal from a given path, it has to know the proximate timing delay of the path. To identify these copies, we often correlate the received sequence with re-generated sequence in a sliding window fashion. Only when shifted locally generated sequence completely lines up with a multipath do we see a large gain (called spreading gain) and thereby we can identify the multipath. This is achieved by a multipath searcher, which searches for the possible delays up to a given resolution. Referring to Fig. 3 there is shown a basic functional block diagram for a searcher, which comprises talking data (such as pilot symbols 302, finding correlation over search window 304, finding peaks in correlation results 306 and multipath detection logic 308 producing multipath search results 310. Since in mobile communication systems, channel fading can rapidly change, the searching should be performed frequently. In the CDMA systems, the usual practice is to utilize pilot signals (i.e., known signal or training sequence either in the form of pilot code channel(s) or pilot symbols), which is transmitted together with the traffic signals to search for the multiple paths.

The determination of the time offsets of multi-paths is based on the correlation between the incoming samples of antenna data and the multiplication of locally generated scrambling code and channelization code at all offsets within the search window. The resolution of the offset is one half-chip. The correlation operation can be implemented using a sliding correlator or a complex code matched filter. Both methods will perform the same operation and therefore the same searching results. However a code-matched filter generates the results much faster with higher processing requirement; while a sliding correlator generates the results slower with lower processing requirement. The selection of implementation method is the trade-off between speed and the hardware complexity. In this paper, we propose an FFT based method for the searcher implementation that is simple and can generate the search result very fast. The sliding correlation operation is done between the locally generated sequence of the scrambling code and the multiple offsets of the input data as shown in Fig. 5. For each multipath offset, the result of the correlation represent the similarity of the offset input data with the reference scrambling code, in other words, represents the likelihood of the offset hypothesis being a real one. The larger the correlation is, the likely a multipath exists. Let N be the correlation length and let W be the search window length. Usually N » W. Referring to Fig. 4, the sliding mode operation is given as the following:

1. take N bits data from received signal 402;

2. take N bits local generated signal with n-samples delay (initially n is set at 0) 404;

3. find the correlation of the two sequences 406;

4. store the correlation results for offset n (Go to Step 5 if n = W, otherwise, increase n by 1 and go back to Step 2) 408; and,

5. multipath can be identified by identify the peaks based on the peaks in the correlation results 410.

The computation involved in the sliding correlation operation is that the correlation between 2 length 2N sequences need to be computed W times. Since the computation is fairly lengthy, when the sliding operation is implemented, multiple correlation units may operate in parallel to speed up the process.

The similar correlation operation can be achieved by FFT. Direct application utilizes full FFT with necessary zero padding, shown in Fig. 5 is the following:

1. take the 2N FFT of the N samples 502 input data plus N zero padding 504;

2. take the 2FFT of the N samples scrambling code 506 plus N zero padding 508; 3. multiply the two sequences 510;

4. find the inverse FFT of the product and take the non-overlap portion 12; and,

5. multipath can be identified by identify the peaks based on the peaks 514. The resulting sequence gives the correlation with offset ranging from 0 to

N-l. Since N>W, the results needed for multipath search are found all at once. This FFT method suffers from the following drawback:

1. Then it needs length 2N FFT block whose implementation is not as easy compared to a shorter length FFT block.

2. The computation is not simple, since N is much larger than W in general. The present invention provides an improved FFT approach in search implementation. Note that fact that the search window size W is usually much smaller than the accumulation window size N, it is not necessarily to perform full length FFT. Let M > W and M « N. We can use 2M bits FFT to find the correlations we are interested. Referring to Fig. 6, the details are given as the following:

Divide-accumulate FFT Method

1. Collect received N pilot symbols into sequence A 602, assuming M divides N.

2. Locally generate N reference symbols and put them into sequence B 604.

3. From sequence A 602, form block Al to An 606, n=N_accum/M as the following: Each block has 2M (note, not M) symbols and AiO)=A((i-l)*M+j). i=l n andj=l,...,2M.

4. Divide sequence B 604 into n blocks 608 each containing M samples and add M zeros to the end of the sequence, i.e., Bi(j)=B((i-l)*M+j). for i=l,...,n and j=l,...M. and Bi(j)=0, ), for i=l,...,n and j=M+l,...2M. 5. For each pair of blocks Ai (length 2M) and Bi (length 2M), do 2M bit

FFT 610. 6. For each pair of blocks Ai and Bi, multiply symbol by symbol their FFT 612 and take IFFT of the resulting product 614. 7. Accumulate the resulting IFFT sequences 616 symbol by symbol and shift M symbols to the left. The accumulation can be a simple summation, which corresponds to a coherent accumulation or a summation of LI or L2 norm, which corresponds to a non-coherent accumulation.

8. Multipath can be identified by identify the peaks based on the peaks 618.

Analytical evaluation of the present invention achieves identical searching results compared with the sliding winder correlator and the full FFT method. However, this only requires a length M FFT block and in implementations is preferable.

Although, the illustrative embodiment above is for the multipath search searcher of a CDMA system, the present invention is well suited for use to calculate the correlation of two long sequences over a narrow correlation window size. .

Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. For example, the received signal can be delay rather than the reference sequence. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications, which come within the scope of the appended claim, is reserved.

Claims

WHAT IS CLAIMED:

1. A method for implementation of a digital CDMA wireless rake receiver applying Rake fingers to process received signals comprising the following steps: determining the occurrence and timing of the possible multipaths by a searcher; assigning a Rake finger to process one of the multipaths by informing it the Rake finger timing of said one of the multipaths; generating a set of weights based on channel estimate; applying the weights to processed signal from each Rake finger; and, combining the weighted signals to produce a processed signal.

2. The method as recited in claim 1 wherein the received signal is correlated with the locally generated signal within the searcher.

3. The method as recited in claim 1 wherein in correlation of two long sequences is calculated over a narrow window.

4. The method as recited in claim 3 wherein the two long sequences are divided and zero-padded.

5. The method as recited in claim 3 further comprising computing a Fast Fourier Transfer (FFT) of the two long sequences are multiplying the FFT of the two long sequences.

6. The method as recited in claim 5 further comprising computing an inversely Fourier transformed (IFFT) of the multiplied product.

7. The method as recited in claim 6 further comprising accumulating the IFFT results.

8. The method as recited in claim 7 further comprising shifting and extracting the accumulated results.

9. A digital CDMA wireless rake receiver system for applying Rake fingers to process received signals comprising: means for determining the occurrence and timing of the possible multipaths by a searcher; means for assigning a Rake finger to process one of the multipaths by informing it the Rake finger timing of said one of the multipaths; means for generating a set of weights based on channel estimate; means for applying the weights to processed signal from each Rake finger; and, means for combining the weighted signals to produce a processed signal.

10. The system as recited in claim 9 wherein the received signal is correlated with the locally generated signal within the searcher.

11. The system as recited in claim 9 wherein in correlation of two long sequences is calculated over a narrow window.

12. The system as recited in claim 11 wherein the two long sequences are divided and zero-padded.

13. • The system as recited in claim 11 further comprising computing a Fast Fourier Transfer (FFT) of the two long sequences are multiplying the FFT of the two long sequences.

14. The system as recited in claim 13 further comprising means for computing an inversely Fourier transformed (IFFT) of the multiplied product.

15. The system as recited in claim 14 further comprising means for accumulating the IFFT results.

16. The system as recited in claim 15 further comprising means for shifting and means for extracting the accumulated results.