Cutting edge at the cell edge: Co-channel interference mitigation in emerging broadband wireless systems

Emerging Broadband Wireless systems, such as those based on IEEE 802.16m or 3GPP LTE-A, will re-use spectrum in every sector (1 : 1 reuse) and maximize system spectrum efficiency. In so doing, the spectrum efficiency for cell-edge users is very poor in comparison with the system-wide average, and nomadic users located near the cell edge experience very low throughput. This is an important concern for India, where nomadic users constitute 85% of mobile phone users, urban cells are relatively small, and the fraction of cell-edge users is as high as 40%. The basic approach to improving cell-edge performance is the so-called fractional frequency reuse (FFR), where spectrum reuse is less than 1 : 1 for cell-edge users. However, techniques that mitigate/cancel interference can improve the spectrum efficiency of FFR. This paper reviews an integrated package of three techniques, proposed by CEWiT for inclusion in the international standards, which mitigate the impact of cell-edge co-channel interference in the downlink: two-dimensional phase offset diversity, inter-cell interference cancellation, and conjugate data repetition. The first two techniques employ multiple transmit antennas. The first and third are open-loop techniques, which will work for nomadic as well as high-speed users, while the second is closed-loop and involves base-station co-operation, and is suitable for nomadic users. All three techniques share the important feature that the transmit signal from each basestation appears to be emanating from a single transmit antenna, enabling the use of low-complexity multi-antenna interference suppression algorithms at the receiver. The paper will discuss the performance of each technique, and also show that they can be employed together in a complementary manner to improve cell-edge performance.