Advanced Cellular Technologies Episode one: GRAKE

In the next few posts I will provide a high level description of a number of new cellular technologies that are either being implemented or are strong candidates for future generation cellular systems.

Generalised-RAKE, or G-RAKE, has attracted some attention recently, because of the gain it provides over traditional RAKE receivers, while being very similar in architecture and operation.

An important property of CDMA systems on the downlink is that signals within the same cell are transmitted using orthogonal waveforms so that they will not interfere with each another. The so-called “orthogonal codes” separate users from each other and they work best when they are perfectly synchronised and aligned in time.

The multipath effect causes the received signal to be spread in time, therefore the signal is received with duplicates or “echoes” due to channel reflections. This scattering effect causes the signal to be dispersed in time, which causes the orthogonal codes to partially loose their perfect time alignment (orthogonality). Loss of orthogonality leads to what is called: Intra-cell or same-cell interference, which is the interference that users of one cell cause to each other. (This is in distinction to other-cell or inter-cell interference, which is interference coming from neighbouring cells)

A traditional RAKE receiver works by combining the signal duplicates or echoes constructively to achieve a good SNR (signal to Noise ratio). Interference, whether from the same-cell or neighbouring cells, is treated as white noise.

The Generalised-RAKE (G-RAKE) effectively distinguishes between inter-cell interference, which is a pure white noise, and intra-cell interference, which can be mitigated by equalizers. The combining of signals in a Generalized RAKE receiver tries to reverse the adverse effect of intra-cell interference. Unlike the conventional RAKE receiver, G-RAKE tries to match to the channel as well as "whiten" the interference.

The G-RAKE gives a typical improvement of 1-3 dB in SNR for a moderate increase in complexity. G-RAKE provides significant improvement in BER performance, which translates to higher data throughput, as well as increased capacity.

There is a debate about the benefit of G-RAKE in comparison with pure channel equalisation approach. As usual, there is always a trade-off between complexity and performance. The performance improvement also depends on various channel parameters (e.g. speed, reflections, multi-path ...etc).