GB2370459A

GB2370459A - Interference measurement

Info

Publication number: GB2370459A
Application number: GB0031360A
Authority: GB
Inventors: Thomas Keller
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2000-12-21
Filing date: 2000-12-21
Publication date: 2002-06-26
Also published as: WO2002051189A1; AU2002216265A1; GB0031360D0

Abstract

A CDMA receiver is arranged to measure the power present in the portions of a despread received signal that contain no information. This power measurement provides an estimate of noise/interference affecting the transmission of the signal received. The portions of the received signal that contain no information are either DTX symbols inserted in the transmitter responsible for generating the signal, or they are unused control information sections of secondary codes in a multicode transmission.

Description

INTERFERENCE MEASUREMENT

The invention relates to methods of, and apparatus for, assessing noise (including interference between channels) in a telecommunications system.

In a wireless communications system, it is desirable to quantify the extent to which transmissions on one channel interfere with transmissions on another channel. In a Code Division Multiple Access (CDMA) system, it is known to reserve a spreading code defining a test channel for interference measurements. No signal is transmitted on the test channel so that a receiver monitoring the test channel will detect only a signal due to noise plus interference from other channels in the system. The power on the test channel can be measured to quantify the degree of interference and noise.

One drawback with the interference measurement method discussed above is that a spreading code needs to be reserved, at least for the duration of the interference measurement. Thus, the test channel is unavailable for data transmission, resulting in the reduction of the transmission capacity of the system.

The present invention aims to provide for interference measurements in a wireless communications system which are less detrimental to the system's information transmission capacity.

According to one aspect, the invention provides a CDMA receiver arranged to identify portions of a despread received signal that contain no transmitted information, and use the identified portions to provide an indication of the noise and/or interference affecting the received signals.

According to a related aspect, the invention provides a method of assessing the quality of a transmitted spread signal, comprising identifying portions of a despread received signal that contain no transmitted information, and using the identified portions to provide an indication of the noise and/or interference affecting the received signal.

Hence, a spreading code does not need to be reserved for noise/interference measurements.

In a preferred embodiment, the power present in the identified portions is calculated so as to provide an indicator of noise and/or interference. Preferably, the received signal comprises a stream of soft decisions and these are squared to produce the power values.

In one embodiment, the identified portions are DTX symbols in a received signal.

In one embodiment, the received signal comprises a signal transmitted using several spreading codes, and the control sections (i.e., containing information to control the receiver) of the frame structure of at least one code are not used to transmit data. The unused control sections may comprise the identified portions used in the noise/interference estimation. By way of example only, certain embodiments of the invention will now be described with reference to the accompanying Figures, in which: Fig''re 1 illus - gates a signa! form..at; Figure 2 schematically illustrates a CDMA receiver; and Figure 3 schematically illustrates another CDMA receiver.

First Embodiment Figure 1 illustrates 1 power control period (PCP) of a multi-code down link (DL) transmission in UMTS frequency division duplex (FDD) mode. It will be apparent that only the primary code 10 is used to transmit dedicated physical control channel (DPCCH) information. The DPCCH portions of the secondary code 12 contain no transmitted information and hence should not contain any transmitter energy.

The receiver 14 of Figure 2 is configured to exploit the DPCCH portions of the secondary code to make measurements of interference affecting that code. The receiver 14 receives the UMTS FDD transmission via an antenna and supplies the received signal in parallel to descrambling/despreading units 16 and 18, which despread the primary and secondary codes respectively. Each of the descrambling/despreading units outputs a stream of soft decisions (SDs).

The SDs are supplied to respective demultiplexers 20 and 22, each of which functions to separate the dedicated physical data channel (DPDCH) SDs from the DPCCH SDs. The DPCCH SDs derived from the primary code are used for receiver control purposes (indicated by block 24) and the DPDCH SDs from the primary code are supplied to a bit rate processor 26 for subsequent processing. The DPDCH SDs from the secondary code are likewise sent to the bit rate processor 26 for subsequent processing. The demultiplexer 22 sends the DPCCH SDs of the secondary code, which contain no transmitted information, to estimator 26. The estimator 26 calculates a measure of the power at the receiver due to interference and noise by keeping a running average of the squares of the DPCCH SDs from the secondary code.

It is possible for several secondary codes to be used in the transmission. Each additional secondary code requires an additional descrambler/despreader and a demultiplexer in the receiver 14. The DPCCH SDs of all the secondary channels can be used in the averaging process to determine the interference and noise power estimate.

Second Embodiment The receiver 30 of Figure 3 is arranged to make use of DTX (Discontinuous Transmission) symbols inserted by a transmitter into the signal that it broadcasts. The transmitter that creates the signal inserts DTX symbols into the bit stream of the information that it is to transmit so as to direct the bit-rate of the signal to be transmitted to a desired value. The transmitter uses rate-matching and DTX insertion algorithms to achieve the desired bit-rate. The transmitter uses a threelevel code to represent the signal it transmits. Logical 0, logical 1 and DTX are represented in the bit stream by +1, -1 and 0, respectively.

In receiver 30, the transmitted signal is received by antenna 32 and converted to a stream 35 of soft decisions SDs by chip rate processing (CRP) section 34 which performs the functions of a rake receiver and also performs descrambling and despreading of the received signal. The stream of SDs contains the DTX symbols.

The stream of SDs is supplied to bit rate processing (BRP) section 36. Since the receiver has advance knowledge of rate matching and DTX insertion algorithms used by the transmitter when issuing the transmitted signal (this information is passed in advance to the receiver in a Transport Format Combination Indicator (TFCI)), the BRP section 36 is able to strip out the DTX symbols from the SD stream (in a process represented by demultiplexer 39).

The DTX symbols are not discarded but are passed to estimator 37. The remainder of the SD stream is passed on for subsequent processing within the BRP section 36, as signified by processing unit 38. The estimator 37 squares the DTX SDs it receives, and keeps a running total of the squared values. The total is used by the estimator 37 to calculate an average power due to the noise and interference affecting the transmitted signal.

Claims

1. A CDMA receiver arranged to identify portions of a despread received signal that contain no transmitted information, and use the identified portions to provide an indication of the noise and/or interference affecting the received signals.

2. A receiver according to claim 1, wherein the portions are identified from the format of the received signal.

3. A receiver according to claim 2, wherein the portions are DTX symbols included in the received signal.

4. A receiver according to claim 3, wherein the DTX symbols are included in the received signal by rate matching and DT insertion algorithms in the transmitter producing the received signal, and the receiver is arranged to use an algorithm to identify the DTX symbols.

5. A receiver according to claim 2, wherein the received signal comprises first and second components each transmitted using a respective spreading code, and the portions are unused control information sections of the second component.

6. A method of assessing the quality of a transmitted spread signal, comprising identifying portions of a despread received signal that contain no transmitted information, and using the identified portions to provide an indication of the noise and/or interference affecting the received signal.

7. A method according to claim 6, wherein the portions are identified from the format of the received signal.

8. A method according to claim 7, wherein the portions are DTX symbols included in the received signal.

9. A method according to claim 8, wherein the DTX symbols are included in the received signal by rate matching and DTX insertion algorithms in the transmitter producing the received signal, and an algorithm is used to identify the DTX symbols.

10. A method according to claim 7, wherein the received signal comprises first and second components each transmitted using a respective spreading code, and the portions are unused control information sections of the second component.

11. A CDMA receiver, substantially as hereinbefore described with reference to Figures 1 and 2 or with reference to Figure 3.

12. A method of assessing the quality of a transmitted signal, substantially as hereinbefore described with reference to Figures 1 and 2 or with reference to Figure 3.