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Mobile Network Excurion

Interference coordination in mobile networks has been an active study area since a long time.
It has become a hot topic in research in the first decade of our century with the emergence of LTE.
Hi there, I have spent quite a while studying intereference coordination in cellular networks. I hope you enjoy my brief introduction on this page.


Small cells of mobile networks in urban areas increase the problem of inter-cell interference, which occurs when close-by transceivers use the same radio resources for transmission and/or reception. This interference may lead to severe performance degradation or connection loss especially in the border areas of cells. GSM and WLAN networks solve this problem by avoiding the reuse of the same frequency bands in neighboring cells. This comes at the drawback of wasting precious frequency resources in every cell, and hence it drastically reduces the utilization of the scarce frequency spectrum. Instead, it is desirable to reduce the interference by other means. A promising approach to solve the problem of inter-cell interference is inter-cell Interference Coordination (IFCO), where neighboring base stations or access points coordinate their transmissions in order to minimize interference. This can be particularly effective when combined with beamforming antennas, which additionally allow the exploitation of spatial multiplexing and thus the transmission to spatially separated terminals on the same frequency/time resources.
Typical hexagonal model of a cellular network
Typical three-sectorized cell

Interference Mitigation Techniques

Frequency reuse schemes

Conventional FDMA systems combat inter-cell interference by avoiding the reuse of the same frequency bands in adjacent cells. A certain frequency band is therefore only used in every n-th cell. For example, in a scenario with three cell sectors per base station, a reuse factor of 3 is obtained by assigning disjoint frequency bands to all three cell sectors. This concept is also known as hard frequency reuse. The overall bandwidth is divided into three reuse partitions, and every partition is served with a maximum transmit power. An increase of the frequency reuse factor commonly increases the cell edge performance while at the same time reducing the overall spectral efficiency. Typically, the spectral efficiency at the cell edge increases by a factor of two when moving from reuse 1 to reuse 3 in a hexagonal cellular scenario with omnidirectional antennas.
Hard Frequency Reuse
Soft Frequency Reuse

Frequency reuse schemes

In contrast to frequency reuse, which tries to avoid interference in the first place, interference cancelation aims at removing interference from the received signal. Two classes of co-channel interference cancelers exist. Filter-based approaches try to mitigate co-channel interference by means of linear filters and interference models. In contrast, Multi-User Detection (MUD) directly includes the interfering signals in the decoding process. This is done by jointly decoding the signal of interest and the interfering signals, or by decoding and subtracting the interfering signals from the signal of interest. The implementation of interference cancelation does not require any modifications of the system standard, making it an attractive technique. It has successfully been applied to both CDMA systems and TDMA systems.

Interference randomization

Frequency reuse aims at reducing the power of the interfering signals. Alike, interference cancelation tries to eliminate interfering signals in the received signal. In contrast, interference randomization, which is also known as interference whitening, tries to make interference appear like background noise, i.e., it averages the interference across the data symbols of a data block or the whole frequency band. This is widely applied in CDMA systems, where neighboring cells are separated by pseudo noise sequences. In FDMA/TDMA systems like GSM, frequency hopping in combination with interleaving can achieve a whitening effect. In OFDMA-based systems, interference randomization can be achieved in a number of different ways. First, if a frequency-diverse resource allocation is used, different spreading patterns can be used in adjacent cells. Second, combinations of CDMA and IDMA with OFDM have been proposed by research. IDMA-based schemes have also been proposed within the 3GPP LTE standardization.

Interference coordination

Interference Coordination (IFCO), also known as Inter-Cell Interference Coordination (ICIC), has gained attention to mitigate interference in cellular networks. In order to reduce interference, IFCO coordinates the usage of resources in neighboring cell sectors, either in a static, a semi-static, or in a dynamic fashion. Resources for coordination are space (with beamforming antennas), time slots, frequency ranges, code resources, or transmit power. For example, in an OFDMA system, IFCO can prevent two cell edge terminals in neighboring cell sectors to be served on the same time/frequency resource. While the main goal is the reduction of the interference, secondary goals determine exactly how the interference coordination is done. Secondary goals can for example be the maximization of the overall network capacity, or the maximization of the cell edge throughput.