GB2371716A

GB2371716A - A radio receiver and method of searching or acquiring signals

Info

Publication number: GB2371716A
Application number: GB0102277A
Authority: GB
Inventors: Diego Giancola
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2001-01-29
Filing date: 2001-01-29
Publication date: 2002-07-31
Also published as: GB0102277D0

Abstract

A direct sequence code division multiple access (CDMA) radio receiver 10 comprises a first and second antenna each of which is sensitive to a different signal polarisation; first and second signal paths 13, 14 with a correlator 20, 24 each, associated with one of the antennae; a code generator 17 for providing a code to vary the phase of the code in a search and or acquisition mode and a signal detector 16, connected to both signal paths 13, 14, for detecting the signal strength produced. A polarisation diversity method is used with orthogonal antennae with possible opposite circular polarisation and a time delay with switch option to allow synchronous measurement is used, instead of using a combiner.

Description

A radio receiver and method of searching or acquiring signals

This invention relates to a direct sequence code division multiple access radio receiver, and to a method of searching or acquiring signals in a direct sequence code division multiple access receiver.

On power-up of a direct sequence (DS) code division multiple access (CDMA) radio receiver, it is necessary to search the code space (the different phases of the code between code repeats) for one or more signals. This is true whether or not the receiver includes a rake receiver (a receiver in which plural rays are detected and subsequently combined). Once a signal has been acquired, the receiver locks onto the signal and demodulates signals communicated thereby. It is common, especially where receivers are expected to operate in channel fading environments, to search for new or stronger signals whilst the receiver is already locked onto one or more signals. For this purpose, one or more channels of the receiver may be allocated for searching.

Searching involves scanning the code space, by varying the phase of a demodulating code applied to a correlator in the searching channel, and detecting the strength of signals provided at certain phases of the code. The code varying is conventionally carried out in a stepwise fashion, with each code phase being dwelled on for only a short period of time. The same occurs with signal acquisition, although the receiver is only approximately locked onto the frequency of the code when signals are being acquired initially. The efficiency of the searching or acquisition operation is determined by the time for which each code phase is dwelled on, and on the length of the code.

In accordance with a first aspect of the invention, there is provided a direct sequence code division multiple access radio receiver, comprising: first and second antennas, the antennas being primarily sensitive to signals having a different polarisation to those which the other antenna is primarily sensitive to;

first and second signal paths each including a respective correlator, each signal path being associated with a respective one of the antennas ; a code generator for providing the correlators with a code and for varying the phase of the code in a search and/or an acquisition mode; and a signal detector connected to outputs of both the signal paths for detecting the strength of signals provided thereby.

A radio receiver constructed according to this invention is able to search and/or acquire signals with the additional diversity of polarisation diversity. Compared to an equivalent conventional receiver, a receiver constructed according to this aspect of the invention provides an improved probability of signal detection for a given search time, or reduces the search time for a given signal detection probability.

In accordance with a second aspect of the invention, there is provided a method of searching or acquiring signals in a direct sequence code division multiple access radio receiver, the method comprising: providing first and second signals from respective ones of first and second antennas the antenna being primarily sensitive to signals having a different polarisation to those which the other antenna is primarily sensitive to; applying the first and second signals to respective ones of first and second signal paths each including a respective correlator; providing the correlators with a code ; varying the phase of the code; and detecting the strengths of signals provided at outputs of the signal paths.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which :- Figure 1 shows part of a radio receiver according to a first embodiment of the invention; and Figure 2 shows part of a radio receiver according to a second embodiment of the invention;

Referring to the drawings, Figure 1 shows, part of a radio receiver 10 which comprises first and second inputs 11, 12, a first signal path 13, a second signal path 44, a signal combiner 15, a signal detector 16 and a code generator 17. The first signal path 13 includes, in sequence, an input buffer 18, a downsampler 19, a serial correlator 20 and a modulus square operator 21. Corresponding elements 22-25 are present in the second signal path 14. The code generator 17 provides a code to each of the serial correlators 20,24. The radio receiver 10 may be part of a radiotelephone, or part of a base station for communicating in a wideband CDMA System, such as the Universal Mobile telecommunications system (UMTS). The code generated by the code generator 17 is an OVSF code, which is specific to the channel between the receiver 10 and a UMTS base station or radiotelephone with which it is communicating, multiplied with a code which is specific to the base station. As will be appreciated, the individual codes and the resultant code are pseudo-random noise (PN) codes. The code generator 17 may be arranged to provide different phases of the code on its outputs.

The signal inputs 11,12 are connected to respective ones of first and second antennas (not shown) which together form an antenna arrangement. These antennas are preferably patch with orthogonal polarisation angles to one another so that the first antenna is responsive primarily to signals having vertical polarisation and the second antenna is primarily responsive to signals having horizontal polarisation.

The first and second inputs 11,12 are connected to different antennas of the Figure 1 antenna arrangement via respective RF downconverters and analogue-to-digital converters (not shown).

Signals transmitted from the UMTS base station are polarised in a particular plane. However, once these signals are reflected, the polarisation cannot be assumed to be the same as the polarisation present on transmission. Further signal reflection may cause further polarisation changes.

The elements in the signal paths 13,14 correlate the signals received at their respective inputs 11,12 with the code provided by the code generator 17. The output of each the

modulus square operators 21, 25 is indicative of the strength of the correlated signal of its respective signal path 13, 14. The strength of each correlated signal is indicative of the amplitude of the signals received at the respective input 11,12 at the same code phase as the code provided by the code generator 17. The combiner 15 combines the output signals of the modulus square operators 21,25, and provides the result to the signal detector 16. Since the signal paths 13,14 are the same, and the phase of the code is the same for each signal path, the outputs of the square modulus operators 21,25 contain signal strength information for different polarisation planes.

The receiver 10, therefore, is sensitive to signals at more than one type of polarisation, i. e. the receiver 10 is sensitive to signals in both the E and the H polarisation planes.

Thus, the receiver 10 has two extra degrees of freedom compared to an equivalent conventional receiver. Since signals on the two different polarisation axes suffer independent fading, the probability of identifying a signal in the code space is approximately doubled for a given time period.

Figure 2 shows part of a modified form of radio receiver 30. Reference numerals have been retained from Figure 1 for like elements. The radio receiver 30 includes a delay element 31 interposed between the input buffer 22 and the downsampler 23 in the second signal path 14. Rather than including a combiner, the radio receiver 30 includes a switch 32, which is controllable to connect alternately the outputs of the modulus square operators 22,25 to the signal detector 16.

The delay of the delay element 31 is equal to one half of the oversampling factor (OSF) of the receiver 30. The OSF is the sampling rate of the analogue-to-digital converters (not shown) which are interposed between the antennas and the inputs 11,12. This delay is selected to allow synchronous measurements from the two antennas.

Where signals are circularly polarised, a single reflection causes a reversal in the direction of circular polarisation. Where the signal transmitted by a transmitter for reception by a receiver is circularly polarised, an antenna arrangement having first and second antennas of opp (site circular polarisation directions is used in place of the

antenna arrangement mentioned above. im

Claims

Claims

A direct sequence code division multiple access radio receiver, comprising : first and second antennas, the antennas being primarily sensitive to signals having a different polarisation to those which the other antenna is primarily sensitive to; first and second signal paths each including a respective correlator, each signal path being associated with a respective one of the antennas; a code generator for providing the correlators with a code and for varying the phase of the code, in a search and/or an acquisition mode; and a signal detector connected to outputs of both of the signal paths for detecting the strength of signals provided thereby.
2. A receiver as claimed in claim 1, in which a signal combiner is connected to the outputs of the signal paths, and the signal detector is connected to an output of the signal combiner.

n
3. A receiver as claimed in claim 1, in which a switch is arranged to connect alternately the outputs of the first and second signal paths to the signal detector.
4. A receiver as claimed in claim 3, comprising a delay element interposed in one of the signal paths such that the first and second signal paths provide significantly different delays.
5. A receiver as claimed in either preceding claim, in which the antennas are mounted generally orthogonally.
6. A receiver as claimed in any of claims 1 to 4 in which the antennas are of opposite circular polarisation directions.
7.-nce to, 7. A receiver substantially as shown in. and/or as described with reference to, figure 1 or figure 2 of the accompanying drawings. c
8. A method of searching or acquiring signals in a direct sequence code division multiple access radio receiver, the method comprising : providing first and second signals from respective ones of first and second antennas, the antennas being primarily sensitive to signals having a different polarisation to those which the other antenna is primarily sensitive to; applying the first and second signals to respective ones of first and second signal paths each including a respective correlator ; providing the correlators with a code; varying the phase of the code ; and detecting the strengths of signals provided at outputs of the signal paths.