GB2369018A

GB2369018A - Control channel interference cancellation for a CDMA receiver

Info

Publication number: GB2369018A
Application number: GB0027545A
Authority: GB
Inventors: Andrew Thurston
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2000-11-10
Filing date: 2000-11-10
Publication date: 2002-05-15
Also published as: GB0027545D0

Abstract

A CDMA (code division multiple access) rake receiver removes interference caused by control channels. The receiver estimates the amplitude of the control channel in a received signal and subtracts this estimation from the signal prior to recovering the spread channels. Preferably the estimation is a scaled locally held replica of the control channel. The effects of more than one channel may be cancelled, e.g. both the primary synchronisation channel (PSC) and the secondary synchronisation channel (SSC) may be subtracted from the signal. Application is to a UMTS radio system.

Description

RECOVERY OF INFORMATION FROM RECEIVED SIGNALS The invention relates to the recovery of information from received signals.

In particular, the invention relates to decoding of downlink signals based on OVSF codes in a UMTS radio system.

In a UMTS radio system, each base station periodically transmits two channels, the primary and secondary synchronisation channels (PSC and SSC respectively) to allow mobile terminals to synchronise to the incoming radio signals. However, the two synchronisation channels are non-orthogonal to the other channels that are transmitted at the same time and at the same frequency, i. e. the two synchronisation channels are not subject to spreading codes. The synchronisation channels act as interference to the other channels and may adversely affect the error rate of the demodulated (despread) received signal.

It is an object of the present invention to increase the precision of the demodulation of spread channels in a received signal in a CDMA-type communications system.

According to one aspect, the invention provides a CDMA receiver for receiving a signal containing at least one control channel and at least one spread channel, the receiver comprising means for estimating the amplitude of at least one control channel in the received signal and means for subtracting the or each estimated control channel from the received signal prior to recovering the spread channels from the received signal.

According to another, and related, aspect of the invention, there is provided a method of processing a received CDMA signal comprising at least one control channel and at least one spread channel, the method comprising estimating the amplitude of at least one control channel in the received signal and subtracting the or each estimated control channel from the received signal pnor to recovenng the spread channels from the received signal. Hence, by at least partially eliminating the control channel component or components from the received signal, the demodulation of the spread channel or channels may be improved.

Where there is one control channel for subtraction, the control channel is preferably a synchronisation channel conveying timing information to control the timing of operations in the receiver.

Where there is a plurality of control channels for subtraction, at least one of the control channels is preferably a synchronisation channel conveying timing information to control the timing of operations in the receiver.

In a preferred embodiment, the control channel (or one of the control channels where there are several) is a primary synchronisation channel (PSC) or a secondary synchronisation channel (SSC).

In one embodiment, the received signal contains two control channels, both of which are estimated and subtracted from the received signal before the spread channel or channels are recovered. Preferably, the two control channels are a primary synchronisation control channel (PSC) and a secondary synchronisation control channel (SSC).

In one embodiment, the estimation of a control signal comprises scaling a replica of the control signal to produce the estimate. The degree of scaling employed may be deduced by comparing a reference against the control channel recovered from the received signal.

To aid the understanding of the invention, certain embodiments will now be described by way of example only and with reference to the accompanying drawing which is a schematic diagram of a UMTS receiver.

A base station in a UMTS radio system periodically transmits two channels, the primary and secondary synchronisation channels (PSC and SSC respectively) to allow mobile terminals to synchronise to the incoming radio signals. The PSC comprises a synchronisation chip-sequence (the so-called primary synchronisation sequence) that is transmitted repeatedly at predetermined intervals. The SSC likewise comprises a secondary synchronisation chip-sequence that is transmitted repeatedly at predetermined mtervals.

The PSC and SSC are non-orthogonal to the other channels that are transmitted at the same time and frequency, i. e. they are not"spread". Hence, the synchronisation channels act as

interference to the other channels and may adversely affect the demodulated signal's error rate.

In order to correctly demodulate a spread signal in UMTS, it is necessary to have first located and decoded the primary and secondary synchronisation sequences in the overall signal. Since the PSC and SSC signals continue to be transmitted throughout a call, it is possible to estimate the level of the interfering chips of the PSC or SSC signals present in the received signal and to subtract the estimated chips from the overall signal. The accuracy of the estimated level of the PSC and SSC signals may be improved by repeating and averaging the estimate over several consecutive instances of these signals, though this relies on the level of the signal staying relatively constant throughout the estimation period.

Hence, this technique is particularly suited to slowing fading channels.

In Figure 1, RF signals arrive at the antenna 1 and are mixed to baseband I and Q components in the RF circuitry 2. The signals are converted to digital format in the A/D converters 3. Rays are extracted using rake fingers of the rake receiver 4 which locate the signal in time in the input signal and also track the timing of the incoming signal to maintain the signal lock. The signal is descrambled 5 and the resulting signal is passed on for dcsprcading, chmmcl decoding and data. extraction 6.

The PSC and SSC signals are present in the signal and hence they act as a noise-like interferer and consequently degrade the decoding and data extraction processes performed on the spread channels, resulting in a raised bit error rate (BER).

To remove the PSC and SSC, a level estimator 7 is included which estimates (by a process to be described later) the amplitude of the PSC and SSC chips in the signal. These estimated levels are used to scale locally held replicas of the PSC and SSC chip sequences 8 which are subsequently subtracted from the main signal m a summation node 9 to effectively cancel the PSC and SSC signals in the main signal In the receiver of Figure 1, the signal output by rake receiver 4 is correlated with a copy of the primary synchronisation sequence held within the receiver so that the receiver can synchronise its operation with the PSC in the received signal. The receiver also holds a local copy of the secondary synchronisation sequence and this is correlated with the signal

from the rake receiver 4 so that the receiver can be synchronised with the SSC in the received signal. These correlation processes provide the level estimator 7 with measurements of amplitude of the primary and secondary synchronisation sequences in the output of the rake receiver 4, and these measurements are used to scale the estimates of the primary and secondary synchronisation sequences of estimator 7 provided for subtraction from the main signal at 9.

Thus, the cancellation of the PSC and the SSC from the total signal is achieved prior to descrambling the received signal and prior to despreading the other channels. Consequently, the bit error rate (BER) of the other decoded channels no the radio receiver is improved.

Claims

CLAIMS 1. A CDMA receiver for receiving a signal containing at least one control channel and at least one spread channel, the receiver comprising means for estimating the amplitude of at least one control channel in the received signal and means for subtracting the or each estimated control channel from the received signal prior to recovering the spread channels from the received signal.
2. A receiver according to claim 1, wherein the estimating means is arranged to estimate at least one control channel by scaling a replica of the control channel.
3. A receiver according to claim 1 or 2, wherein at least one control channel is for synchronising the receiver with the received signal.
4. A receiver according to claim 1,2, or 3, wherein there are two control channels and the estimating means is arranged to estimate both control channels and the subtracting means is arranged to subtract both of the estimated control channels from the received signal.
5. A method of processing a received CDMA signal comprising at least one control channel and at least one spread channel, the method comprising estimating the amplitude of at least one control channel in the received signal and subtracting the or each estimated control channel from the received signal prior to recovering the spread channels from the received signal.
6. A method according to claim 5, wherein estimating the amplitude of a control channel comprises scaling a replica of the control channel.
7. A method according to claim 5 or 6, wherein at least one control channel is for synchronising the receiver with the received signal.
8. A method according to claim 5, 6, or 7, wherein there are two control channels and the 0 estimating step comprises estimating both control channels and the subtracting step comprises subtracting both estimates from the received signal.
9. A CDMA receiver, substantially as hereinbefore described with reference to the accompanying figure.
10. A method of receiving a CDMA signal, substantially as herein before described with reference to the accompanying figure.