Interference Cancellation for Cellular Systems

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Cellular networks today are interference-limited and will only become increasingly so in the future,due to the many users that need to share the spectrum to achieve high-rate multimedia communication. Despite the enormous amount of academic and industrial research in the past twenty years on interference-aware receivers and the large performance improvements promised by these multiuser techniques, today’s receivers still generally treat interference as background noise. In this paper, we enumerate the reasons for this widespread scepticism, and discuss how current and future trends will increase the need and viability of multiuser receivers for both the uplink, where many asynchronous users will be simultaneously detected, and the downlink, where users will be scheduled and largely orthogonalized, but the mobile handset will still need to cope with a few dominant interfering base stations.New results for interference cancelling receivers that use conventional front ends are shown to alleviate many of the shortcomings of prior techniques, particularly for the challenging uplink. This paper gives an overview of key recent research breakthroughs on interference cancellation, and highlights system-level considerations for future multiuser receivers.

The performance of today’s cellular networks is limited by interference, more than by any other single effect. Interference is distinguished from noise in that it is caused by other human-designed devices, often most of it from devices designed to use the same network, which makes it particularly interesting and aggravating. Whereas conventional noise can be overcome by increasing the transmit power, overall interference is increased by this simple-minded approach, since 

neighbouring devices now have to contend with even more interference than before. In both centralized and ad hoc networks, overall system capacity can be maximized by having each device use the minimum required transmission power, so that the interference caused to other devices in the network is also minimized.

In the downlink each receiver only needs to decode a single desired signal from K intracell signals, while suppressing other cell interference from a few dominant sources as shown in Figure 1. On the other hand, in the uplink the base station receiver must decode all K desired users while suppressing other cell interference from many independent sources, as shown in Figure 2.

Interference Cancellation for Cellular Systems

The uplink, since all users are at different distances from the base station Future cellular systems will employ sophisticated scheduling algorithms in the downlink so the primary function of the mobile unit will be to decode the desired signal in the presence of interference from the neighbouring cells. This is fortunate, since the mobile units will still be highly power limited and hence have limited processing power. It is difficult to coordinate and accurately synchronize scheduling algorithms for ND have rapidly changing multipath channels. Although this is a more challenging task, the base station receivers will generally have a much higher complexity allowance than their mobile counterparts.

Figure 1: In the downlink scenario, each receiver only needs to decode its own signal, while suppressing other cell interference from just a few dominant neighbouring cells. Because all K users’ signals originate at the base station, the link is synchronous and the K ¡ 1 intra cell interferers can be orthogonalized at the base station transmitter. Typically though, some orthogonality is lost in the channel. For these reasons, downlink receivers at the user terminals will employ relatively simple multi-user receivers that attempt to restore the orthogonality of the intra cell users via either a chip-level equalizer (CDMA) or inter-carrier interference suppression (multi-user OFDM), while handling at most a few dominant and unknown other-cell interferers. And although multi-user scheduling may increase throughput and decrease the number of interfering users, at lower spreading factors interference suppression will become even more crucial.

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