This post is by Bill Myers, Director of Product Management and Business Development, ISCO.
Much attention has been paid during the design of LTE to the impact of inter-cell interference, with LTE transmissions from macro, micro and femto cells impacting neighboring sites in overlapping coverage. LTE includes measurements of this type of interference and schedules resource blocks and UE usage to minimize this inter-cell interference (e.g., using eICIC). In contrast, external sources of interference (signals that do not originate in the LTE network) can significantly degrade network performance and cannot be avoided as easily, since the LTE network cannot control the source of the interference. External interference continues to be confirmed in real-world deployments and is a catalyst for new FCC initiatives focused on radio spectrum pollution, as presented by Julius Knapp during the keynote at the Silicon Flatirons Conference on Radio Spectrum Pollution in November 2013.
The result of these many sources of interference is reduced signal-to-noise ratio, reduced coverage area and degraded network performance. While the design of LTE is more resilient to interference than 3G, it is still susceptible to interference impacting network performance. In fact, LTE networks offer higher spectral efficiency in bits per second per Hz, but require higher levels of SINR to achieve that performance.
The figure below illustrates the relationship between effective spectral efficiency and SNR, showing the high levels of SNR required to achieve the promise of LTE’s capabilities. External interference decreases SNR, which in turn significantly limits throughput rates.
Interference in the received signal at the base station degrades performance, particularly in terms of coverage area and throughput rate. Coverage area is impacted as interference desensitizes the receiver and effectively raises the noise floor. Throughput rate is impacted due to the reliance of higher modulation coding schemes (MCS) on SINR – lower SINR simply does not enable the system to utilize the higher order modulation and low overhead coding schemes. This problem is compounded by the scheduler as it attempts to efficiently allocate resource blocks to users based on relatively coarse spectrum measurements. Faced with interference, fewer resource blocks are allocated when traffic is light, but as traffic increases, the system no longer has this option. This combination of fewer resource blocks and lower MCS rates can dramatically reduce network performance for LTE networks.
Is LTE safe from interference? Standards efforts focused on inter-cell interference won’t protect the network from external interference. As LTE networks roll out, real-world experience indicates that external sources of interference are prevalent in the 700MHz bands and that this interference impacts network performance. The dynamic nature of the LTE signal can make detection of co-channel interference a challenge, but such interference can not only be detected, it can also be eliminated from the received spectrum, optimizing performance of the impaired channel. Tests in real-world networks show coverage and throughput are protected, resulting in optimal network performance.