GB2384955A

GB2384955A - Using multipath interference cancellation in a spread-spectrum receiver to improve path timing estimates

Info

Publication number: GB2384955A
Application number: GB0202229A
Authority: GB
Inventors: Gideon Kutz; David Solomon; Yoram Or; Amir Chass
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2002-01-31
Filing date: 2002-01-31
Publication date: 2003-08-06
Anticipated expiration: 2022-01-31
Also published as: WO2003065605A1; TW200303655A; GB0202229D0; GB2384955B

Abstract

A path-timing estimator for a RAKE receiver finger relies on an interference cancellation process combined with a non-coherent tracker (7, 8). Estimates of interference produced by paths from other fingers are used to reconstruct an interference signal (1) which is subtracted (6) from the de-spread (5) signal. The estimator performs well even when paths are closely spaced.

Description

PATH TIMING ESTIMATION IN A SPREAD-SPECTRUM RECEIVER This invention relates to radio receivers and particularly, though not exclusively, to radio receivers for code-division multiple access (CDMA) cellular communications systems.

In a cellular communications system, a plurality of base stations provides a radio telecommunications service to a plurality of remote subscriber units often termed mobile stations. Each base station defines a particular geographical area or cell proximate to the base station to produce coverage areas.

Multiple access techniques permit the simultaneous transmissions from several mobile stations to and from a single base station. One type of multiple access technique is known as code division multiple access (CDMA), which employs spread-spectrum signalling. Individual users in the CDMA communications system use the same carrier frequency but are separated by the use of individual spreading codes. Hence, multiple communications channels are assigned using a plurality of spreading codes within the portion of radio spectrum, each code being uniquely assigned to a mobile station. In a direct sequence CDMA communication system, the signals are, prior to being transmitted, multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrow-band signal is thus spread and transmitted as a wide-band signal. At the receiver the original narrowband signal is re-generated by multiplication of the received signal with the same code. A signal spread by use of a different code will, at the receiver, not be despread but will remain a wide-band signal.

Traditional CDMA with direct spreading uses a signature sequence to represent one bit of information. Receiving the transmitted sequence or its complement (the transmitted binary sequence values) indicates whether the information bit is a 0 or a 1. The signature sequence usually comprises N-bits and each bit is called a chip.

The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The receiver correlates the received signal with the known signature

sequence of its own signature sequence generator to produce a normalised value ranging from-1 to +1. When a large positive correlation results, a 0 is detected.

When a large negative correlation results, a 1 is detected.

In CDMA communications systems, a RAKE receiver is commonly used as a lowcomplexity solution for a CDMA receiver. One of the functions of a RAKE is the separation of the multipath propagated signal components.

Multipath progapation arises due to a transmitted signal arriving at the receiver via a number of paths. For example, one received signal may pass direcity from a base station to a mobile station and another may be reflected off a building behind the mobile station and then back to the mobile station. There will be a time delay between reception of these two signals. Multipath effects result in a degradation (or fading) of the desired signal. The critical functionality of a RAKE receiver (which generally comprises a plurality of RAKE fingers), is the estimation of the properties of the communications channel. This estimation is comprised of two parts. Firstly, an estimation of the multipath delays (which is facilitated by finger management algorithms) is requried. Secondly, an estimation of the attenuation and phase of each of the paths (which is faciliated by the channel estimation algorithm) is also required.

An example of a RAKE receiver design is disclosed in US-B-6215814.

A signal from a remote transmitter travels through a multipath propagation channel via a multiplicity of paths. Each path is characterised by a delay and a channel coefficient. As the signal arrives at the receiver, it goes through a matched filter which is matched to the pulse forming filter of the transmitter. In a further stage, the signal is despread with a spreading code. The combined effect of the matched filtering with the de-spreading can be viewed as a convolution operation with the received signal.

Generally, a RAKE finger management algorithm detects and tracks various despreading codes timing which correspond to the most powerful paths of the channel.

One of the most important sub-modules in the finger management is the tracker.

The role of the tracker is to track the timing of each path with maximum accuracy.

The timing information supplied by the tracker serves as the offsets for the despreading of the various fingers. The tracker, therefore, has a very high impact on the overall performance of a RAKE receiver.

A known method employed for tracking individual paths in a multipath scenario relies on the use of a timing error detector (see for example IEEE 6th International Symposium on Spread-Spectrum Techniques and Applications. ISSTA 2000.

Proceedings, pages 278-282, volume 1, 2000, aMutlipath Resistant Co-herent Timing Error Detector for the DS-CDMA Applications"by G. Fock et al.

One known type of timing error detector (for use in an RAKE receiver) is the socalled"early/late"detector which operates on two classes of samples of the matched filter output. One is taken early and one is taken late with respect to the detection path. The output of this timing error detector is calculated as the difference between late and early output branches. This output can be used as the input to a tracking loop filter whose output is an updated path timing estimation.

As the task of a tracker is to keep track of changes in path delay during normal operation, each finger of a RAKE receiver has its own timing tracking loop.

It is usually assumed that the individual paths are well separated and that the timing error detector for each path is not influenced by the presence of other paths.

However, this is not true in the case of closely-spaced paths and in such cases significant degradation of the tracking process ensues.

Hence, there is a need for a tracking process which performs well in the abovementioned case of closely-spaced paths.

According to a first aspect of the present invention there is provided a path timing estimator for a spread spectrum receiver, the estimator including ;

means for despeading a received spread spectrum signal received from a remote transmitter via one of a multiplicity of signal paths to produce a despread signal, means for reconstructing an interference signal resulting from at least another one of said signal paths, and means for subtracting the reconstructed interference signals from the despread signals to produce a path timing correction signal.

According to a second aspect of the present invention there is provided a method for estimating path timing in a spread spectrum receiver, the method including the steps of; despeading a received spread spectrum signal received from a remote transmitter via one of a multiplicity of signal paths to produce a despread signal, reconstructing an interference signal resulting from at least another one of said signal paths, subtracting the reconstructed interference signals from the despread signals to produce a path timing correction signal, and updating path timimg using the correction signal in a timing control loop.

In the receiver, the results of the despreading process are biased because of the existence of other paths in vicinity of the path being despread. To cancel this effect, the interference from other paths is predicted and reconstructed and then subtracted from the despreader outputs (ie, the results of the correlation process). These corrected outputs are then used in a tracking process to update the paths timings.

The solution proposed by the present invention is based on a non-coherent tracker combined with an interference cancellation process.

A non-coherent tracker has the advantage of being more robust than a coherent tracker as is used in the aforementioned disclosure by Fock et al.

Preferably, the interference cancellation commences with the strongest paths (with highest energy), through to the weakest.

A preferred embodiment of the present invention, will now be described, by way of example only, and with reference to the Figure which is a schematic block diagram of a path timing estimator, for use in a rake finger, in accordance with the invention.

An interference reconstruction module 1 reconstructs interference from other paths using inputs on line 2 relating to the estimated path delay results from other fingers and using the outputs of a channel estimator 3. The channel estimator 3 receives inputs on line 4 relating to on-time de-spreading results from other fingers. The path timing estimator also includes a matched filter and despreading module 5, subtraction module 6, loop filter 7 and timing control module 8.

To alleviate tracker performance degradation due to the fact that the mutipathinduced interference does not have the same effect on the early and late correlation results because of their different timings, the invention incorporates the idea of a Serial Path Interference Canceller {SPIC).

Hence, it is desired to cancel the interference caused by the multipath with the following term;

hi is the complex amplitude of the ith path and is known from the conventional channel estimation algorithm. R is a known raised cosine function which can be

tabulated and stored in memory. N is a spreading factor and M is the number

of pilot signals. The only variable which is not known is t, the delay of the - ith path. Only its estimate L is known from former tracking results. However TL

can still be used in the cancellation process. This is motivated by the fact that

finger delays change slowly, (even when there is relative displacement between a mobile station and a base station for example) and therefore, for cancellation purposes, it can be assumed that the finger delays obtained from a previous tracking exercise are almost correct.

Whenever one or more path delays is changing from a previous tracking time to a current tracking time, (ie. TL t), the cancellation process may introduce

errors. These errors can propagate through the system and cause tracking errors in all fingers, because the cancellation process in each finger is dependant on the accuracy of the results provided from all other fingers. To avoid this error propagation, fingers are sorted prior to the cancellation process according to their on-time energy values. Cancellation takes place initially using results from the strongest finger, proceeding to the weakest. There are two reasons for this.

Firstly, the stronger paths are likely to drift more slowly than the weaker paths.

This means that their timing errors tend to be lower. Secondly, the interference from weak fingers is less likely to affect stronger fingers, even if timing errors are present in the weaker fingers. After the strongest finger has been tracked, its timing is updated, and this updated timing is used in the path interference cancellation for subsequent fingers. This idea of serial cancellation can be expressed by:

t < Where Ti is the estimated delay of finger i, in the tracking process t. If tracking

is done once per slot, then t represents the slot number. As described in this equation, the previous fingers effect on the current finger is cancelled using their updated timings estimations (t+1), while for the other, the former timing estimation (t) is used.

Claims

CLAIMS 1. A path timing estimator for a spread spectrum receiver, the estimator including ; means for despeading a received spread spectrum signal received from a remote transmitter via one of a multiplicity of signal paths to produce a despread signal, means for reconstructing an interference signal resulting from at least another one of said signal paths, and means for subtracting the reconstructed interference signals from the despread signals to produce a path timing correction signal.
2. A method for estimating path timing in a spread spectrum receiver, the method including the steps of; despeading a received spread spectrum signal received from a remote transmitter via one of a multiplicity of signal paths to produce a despread signal, reconstructing an interference signal resulting from at least another one of said signal paths, subtracting the reconstructed interference signals from the despread signals to produce a path timing correction signal, and updating path timimg using the correction signal in a timing control loop.
3. A method according to Claim 2 in which the step of subtrating commences with the strongest path proceeding through to the weakest path.
4. A method according to Claim 2 or Claim 3 in which the step of subtracting is represented by the relation ;

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5. A path timing estimator substantially as hereinbefore described with reference to the drawing.
6. A method for estimating path timing substantially as hereinbefore described wit reference to the drawings.