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Saturday, May 14, 2011

Benchmarking LTE to HSPA

Radio performance has a direct impact on the cost of deploying the network in terms of the required number of base station sites and in terms of the transceivers required. The operator is interested in the network efficiency: how many customers can be served, how much data can be provided and how many base station sites are required. In this article comparison between LTE and HSPA with regard ti radio performance.

The spectral efficiency gain of LTE over HSDPA can be explained by a few characteristics in the LTE system design. Those factors are shown in the illustration below for the downlink. LTE uses orthogonal modulation to avoid intra-cell interference providing a major boost in capacity compared to HSDPA with Rake receiver. The CDMA transmission in HSDPA suffers from intra-cell interference caused by multi-path propagation. The CDMA codes are orthogonal but only in the single path channel. The LTE benefi t over HSDPA depends on the amount of multi-path propagation.

Another major benefit in LTE is the frequency domain scheduling, which is not possible in a CDMA based system. The CDMA systems transmit the signal using the full bandwidth. MIMO provides some efficiency benefit in LTE since MIMO was not included in HSDPA Release 6. The inter-cell interference rejection combining is used in LTE simulation results and it gives some further capacity boost for LTE. Adding these factors together gives the result that LTE is indeed expected to provide approximately three times higher efficiency than HSDPA Release 6. The LTE performance gain over HSDPA Release 6 will be larger in small micro or indoor cells where the multistream MIMO transmission can be used more often for taking advantage of high LTE data rates.

The LTE uplink efficiency gain over HSUPA mainly comes from the orthogonal uplink while HSUPA suffers from intra-cell interference.

HSPA evolution brings some performance enhancements to HSPA capacity. MIMO is part of HSPA Release 7; terminal equalizers can remove intra-cell interference in downlink and base station interference cancellation can remove intra-cell interference in uplink. Dual carrier HSDPA transmission is included in Release 8, which enables some frequency domain scheduling gain. Even if all the latest enhancements from HSPA evolution are included, LTE still shows an efficiency benefit over HSPA.

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