Interference rejection combining in LTE networks

Long Term Evolution (LTE) provides high spectral efficiency within one cell, but it is shown to be highly vulnerable to inter-cell interference. While LTE as well as Wideband Code Division Multiple Access (WCDMA) suffers uplink interference especially with a frequency reuse factor of one, WCDMA has some inherent layer 1 mechanisms to alleviate these effects, namely macro-diversity and spreading codes. In contrast, LTE user data are more localized in the time/frequency domain, calling for different mechanisms for interference reduction in the receiver. For the higher layers, techniques such as fractional power control or fractional frequency reuse can be used to mitigate the interference but are shown to be sub-optimal. For layer 1, interference rejection combining (IRC) is described as an efficient alternative to increase uplink bit rates in areas where cells overlap. IRC principles in the presence of radio fading channels are detailed, while strategies for both interference suppression and signal-to-interference-plus-noise ratio (SINR) maximization are considered. Taking a didactic approach, we compare multiple antenna receiver techniques (maximal ratio combining (MRC), IRC, multiple input multiple output (MIMO)), analyze the key factors of IRC efficiency (the number of receiving antennas and interferers, time synchronization, interference over thermal, and scheduling decisions), and provide link level performance comparisons. We also provide implementation considerations for integrating the IRC in the current eNodeB receiver. As the purpose of this paper is to give a basic level introduction to IRC applied to the LTE uplink and highlight the respective surroundings, the IRC approach will be covered rather intuitively by illustrations without a great deal of theoretical detail. © 2012 Alcatel-Lucent.