The G-Rake receiver, which resides in the demodulator, produces estimates of the
modulation symbols from the received signal. Because the G-Rake receiver is similar
to a traditional Rake receiver, let us also briefly review Rake reception.

Signal energy is collected from different delayed versions of a transmitted signal. As seen in Figure 2, the channel response creates multiple images of the transmitted signal (that is, the dispersive, multipath channel gives rise to different versions). The “fingers” of the Rake receiver extract signal energy from delayed signal images by despreading and combining them – the Rake receiver coherently combines the finger outputs using complex conjugates of estimated channel coefficients to estimate the modulation symbol. Figure 3 shows a simple example
in which two despread values are combined (x1 and x2, which correspond to
two signal paths). Each despread value consists of a signal component (s), an interference component (i), and a noise component (n). When combining the values, the Rake

WCDMA technology is being deployed worldwide to provide third-generationmobile systems and services. The WCDMA standard continues to evolve with HSDPA and EUL.

Receiver technology is also evolving. Today's terminals and base stations employ Rake receivers, which collect signal energy that has been dispersed in time by the multi-path radio channel. In the future, however, besides collecting signal energy, advanced receivers will be used to suppress interference. In this regard, G-Rake receivers show great promise.

To understand the G-Rake receiver, let us briefly review WCDMA transmission. Figure 1 shows the transmission and reception of a single stream of information. Information bits are encoded with a forward error correction (FEC) encoder, such as a convolutional encoder or turbo encoder. The encoded bits are used to create modulation symbols, such as quadrature phase-shift keying (QPSK) symbols. These are then spread so that each symbol is represented by a sequence of "chips." Next, the spread-spectrum signal is mixed up to a radio frequency and transmitted. At the receiver, the radio signal is mixed down to baseband for demodulation and decoding.

WCDMA continues to evolve to support high-bit-rate applications. High-speed downlink packet access (HSDPA) technology, for example, substantially increases data rates in the downlink. As data rates increase, however, greater self-interference from the dispersive radio channel limits performance. As a consequence, Ericsson has developed advanced receivers for WCDMA terminal platforms and base stations.



The authors describe an advanced receiver approach known as generalized Rake (G-Rake) reception. The G-Rake receiver functions like an equalizer, suppressing self-interference. To minimize cost and time-to-market, the receiver architecture builds on the traditional Rake receiver architecture. Tests show that the G-Rake receiver can significantly improve throughput and system capacity for high-bit-rate applications. Ericsson will include G-Rake for HSDPA services starting with its U350 and U360 WCDMA platforms.

G-Rake reception is also being considered for other applications. For voice service, for instance, it can increase downlink capacity by 30%. Data and voice applications improve when G-Rake reception is used in conjunction with the two-antenna terminal platforms that are part of EMP's technology roadmap. G-Rake will also significantly improve performance on the uplink when data rates exceed 2Mbps in later phases of the enhanced uplink (EUL).