Assessment of low-rate turbo encoding to extend coverage in WCDMA/HSDPA systems

In this paper, we consider the data rate coverage extension available by increasing the encoder performance in a WCDMA/HSDPA system. In the existing system, a rate 1/3 encoder is used together with bit repetition to provide coverage to users at the cell edge. However, this approach suffers from a spectral efficiency loss compared to the use of dedicated low data rate encoding. An alternative scheme with rate 1/5 turbo encoding and rate matching is considered to achieve increased performance at the cell edge, and in a larger dynamic range if we add multi-code support at the new lower rate. In the coverage extension region we find a 1.2 dB link-gain. This translates into an average user throughput increase of up to 20% near the cell edge and a 7-14% increase in system capacity in a data-fair macrocell network. However, in combination with alternative coverage extension methods, such as proportional fair scheduling, the gain disappears.