Interference Rejection Combining

Interference Rejection Combining 1 Introduction The purpose of Interference Rejection Combining (IRC) is to improve performance on the uplink in radio environments limited by interference. The frequency space in a cellular system like GSM is very limited. Therefore one of  the concepts is to have the same carrier frequency covering different areas. areas. The limiting factor in tight frequency reuse is interference from other cells operating on the same frequency. Interference causes bit and frame errors, which makes the speech quality decrease. If the errors occur on the control channels, they can lead to missed or dropped calls. IRC minimizes the disturbance from an interferer by combining the signal received on diverse antennas and suppressing the interfering signal. Unlike the basic RX diversity feature, IRC assumes that the noise in the received signals consists of both white Gaussian noise as well as signals from interfering transmitters. This means that in those cases a correlation exists between the received interfering signals, and IRC can improve the signal more efficiently than the basic RX diversity feature. From an operator point of view, IRC can be used to increase capacity in a radio network limited in the uplink, improve speech quality, and increase data throughput in the uplink. From a subscriber point of view, IRC gives improved speech quality, fewer dropped calls, and increased data throughput due to fewer re-transmissions.

2 Capabilities The quality of a channel depends on the signal strength of the carrier and the strength of interfering signals. It is often expressed as carrier/interference carrier/interference ratio (C/I) and a higher ratio means a better channel. IRC is a method to increase the channel quality by using receiver antenna diversity and interference rejection. IRC is ideal for situations with a strong interferer in which the noise consists of  interfering signal(s) in addition to white Gaussian noise. This means that a correlation (the interferer) will exist between the received signals and IRC is able to suppress the interfering signal and thereby increase the quality of the received signal. This situation, with interfering signals, is typical for high capacity areas (urban areas) where tighter frequency reuse is in place. IRC requires more processing power than basic RX diversity and is thus only available on dTRU and EDGE sTRU.

3 Technical Description

3.1 General IRC is configured on a per cell basis and in order to configure IRC, the cell must also be configured for RX Diversity. It is possible to mix both transceivers capable of IRC and those not capable of IRC in the same cell. When IRC is activated, those transceivers capable of IRC will perform interference rejection using the IRC algorithm, the other transceivers will continue to utilize basic RX diversity. The selection of method is based on the IRC parameter. If IRC is selected, it will be used for the transceivers supporting IRC. If IRC is not selected, basic RX diversity will be used on all transceivers.

3.2 Algorithm The IRC algorithm excels when a strong co-channel interferer is present, meaning that the noise (which consists of both Gaussian white noise and the interfering signal) will be correlated in the signals received on the different antenna branches. The interference rejection is based on estimates of the spatial and temporal correlations of the noise. These correlations are used to construct digital filters that suppress the interference and yield a received signal with a higher C/I. The basic RX diversity method uses an algorithm called Maximum Ratio Combining (MRC) and is ideal for the situation where the interference is uncorrelated between the diversity branches. By combining the received signal from diverse antennas, the desired signal is enhanced and the performance is improved.

3.3 RelatedStatistics For object types, counters and formulas see Reference [3].

3.4 Main Changesin EricssonGSMSystemR12 / BSSR12 The capability to suppress co-channel interference in synchronized networks has been improved. The training sequences exhibit poor cross-correlation properties. In the Ericsson BSS R12, the unwanted effects of these cross-correlations are compensated, thus increasing the performance of the interference suppression algorithms. This enhancement of IRC is useful as long as the TSC of the co-channel interference is different from that of the desired signal.

4 Engineering Guidelines 4.1 General

IRC is an optional feature. If IRC exists in the network it is recommended to set the IRC parameter to On for all cells. The system is optimized to use the most efficient available algorithm, IRC or MRC.

4.2 Measurements Measurements of FER (if available) and RXQUAL can be collected before and after enabling IRC. Both FER and RXQUAL should improve when using IRC. Such measurements may be gathered by means of a reporting and monitoring tool such as MRR, which also produces an MRR Comparison Report. For further information, please refer to Reference [2] It is important to secure that the interference is equivalent between the different measurement periods while measuring FER or RXQUAL. This is verified with FAS Cell Report and FAS Cell Comparison Report. These reports give detailed information on the uplink interference on cell level. For further information, please refer to Reference [1]

4.3 IRC Gain The gain obtained by using the IRC algorithm is dependent on several factors such as:       

Number of simultaneous interfering mobiles. Synchronization of cells in the network. Type of interference. Propagation environment. Type of diversity (polarization or spatial-diversity). Type of service. TSC plan

The situation where IRC is expected to be most useful is in typical urban environments where the interference originates from one dominant interferer. In this situation the typical gain by using the IRC algorithm would be approximately 6 dB if  the cells are synchronized and the interfering bursts have different TSC than the own burst. TSC planning is important to be able to fully utilize IRC and get an optimal gain from IRC. See Reference [4] for more information on synchronization of cells and TSC planning. If the cells are not synchronized, the typical gain would drop to approximately 2-3 dB on average. In general these numbers are what could be expected in average in a network, but there are cases where deviations might occur: 

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Interference from several simultaneous sources with similar power would decrease the gain. The use of spatial diversity (instead of polarization diversity as assumed in the numbers above) would often improve the performance. The presence of synchronized co-channel interference with the same TSC as the desired signal will decrease the gain.

5 Parameters 5.1 Main ControllingParameters IRC indicates if the IRC algorithm is available or not for a cell that is configured for RX Diversity. The IRC parameter is configured per cell. IRC = On indicates that the system may select the most optimal algorithm of IRC and MRC for each channel. IRC = Off indicates that the system may only select the MRC algorithm.

5.2 Parametersfor Special Adjustments TSC is the training sequence code that may be planned to improve the gain of IRC.

5.3 ValueRangesand DefaultsValues Table 1

Value Ranges and Defaults Values

Parameter name

Default value

Recommended Value range value

IRC

Off

-

On/Off

TSC

BCC part of  BSIC

-

0 to 7

Unit

 

6 Concepts C/I

The carrier to interference ratio (C/I) is defined as the ratio between the level of the received signal to the level of the interfering signal.

Multipath fading Multipath fading occurs when signals arrive at the receiver both directly from the transmitter, and, indirectly, due to propagation through objects or reflection. These signals arrive at slightly different times, with different amplitudes and phases. They sum together constructively and also destructively (fading dips). The fading dips appear at different spatial locations for different frequencies, i.e. they are frequency and location dependent. This phenomenon is called multipath fading. Fading dips are separated by approximately 17 cm for GSM 800 and GSM 900, and approximately 8 cm for GSM 1800 and G SM 1900 Receiver Antenna

Receiver Antenna Diversity is the technique used to implement RX diversity and IRC. It capitalizes on the low probability that two

Diversity

independent reception channels are being affected by a deep fading dip at the same time. It requires two RX antennas at the base station.

Glossary BCC Base Station Color Code BSIC Base Station Identity Code FER  Frame Erasure Rate FOX Frequency Optimization eXpert IRC Interference Rejection Combining MRC Maximum Ratio Combining RXQUAL Received Quality RX Receiver TSC Training Sequence Code TRX Transceiver