Monday, 12 May 2014

Improvement in Interference Rejection and Suppression Technology

In the last post where I talked about FeICIC I mentioned about the advanced Interference rejecting receivers, here is one very good article from NTT Docomo technical journal. The following is from this article:

Rel. 11 LTE has introduced MMSE-Interference Rejection Combining (MMSE-IRC) receivers as a mobile terminal interference rejection and suppression technology to mitigate the effects of these interference signals and increase user throughput even in areas that are recently experiencing high interference. Rel. 8 LTE receivers support MIMO transmission technology, so receivers were equipped with at least two antennas since it was first introduced. The MMSE-IRC receivers in Rel. 11 LTE, are able to use the multiple receiver antennas to create points, in the arrival direction of the interference signal, where the antenna gain drops (“nulls”) and use them to suppress the interference signal (Figure 1). The terminal orients a null toward the main interference signal, which is the signal that particularly affects the degradation of throughput, thereby improving the Signal-to-Interferenceplus-Noise power Ratio (SINR) and improving throughput performance.

However, with the original MIMO multiplexed transmission, which realized high throughput using multiple transmit and receiver antennas, the receiver antennas are used to separate the signals between layers, so interference from adjacent cells cannot be suppressed and throughput cannot be improved, particularly for mobile terminals with two receiver antennas.

On the other hand, the 3GPP has already included interference rejection and suppression technology in performance specifications for mobile terminals equipped with W-CDMA/High-Speed Downlink Packet Access (HSDPA) in Rel. 7 of the Universal Mobile Telecommunications System (UMTS). With W-CDMA, receivers normally use one receiver antenna and perform Rake reception, but the effects of multipath interference degrading reception performance was an issue.

Thus, the following three receiver extensions were studied and introduced.
• Type 1/1i extends the Rake receiver to use two antennas.
• Type 2/2i extends the Rake receiver to an MMSE receiver that suppresses multipath and adjacent-cell interference.
• Type 3/3i extends the MMSE interference-suppressing receiver defined in Type 2/2i to use two receiver antennas.

The functional extensions in receivers in Rel.7 UMTS and Rel. 11 LTE are summarized in Table 1. The MMSE-IRC receivers in Rel. 11 LTE incorporate receiver algorithms that are generally equivalent to those in the Type 3/3i receivers introduced in WCDMA/HSDPA. However, in the WCDMA/HSDPA receivers they also operate to suppress inter-coding interference within a cell. There is no interference within a cell in LTE systems, so in the MMSE-IRC receivers introduced in Rel. 11 LTE, they operate to suppress interference arriving from adjacent cells.

From my understanding, a similar approach is being proposed for the Mobile Relay Node (MRN)

Anyway, the complete article is as follows:

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