Rake receiver

A rake receiver exploits multipath propagation by using multiple correlators, each locked to a different path delay. The received signal is a sum of attenuated, delayed, and phase-shifted copies of the transmitted signal; the receiver assigns a 'finger' to each significant path, despreads each, and combines them with appropriate weights to maximize signal-to-noise ratio.¹

The name derives from the visual similarity of the receiver's block diagram to a garden rake, with tines representing the correlators. The combining can be done using maximal-ratio combining (MRC), equal-gain combining, or selection combining, with MRC being optimal for additive white Gaussian noise.²

The concept was first described by Robert Price and Paul E. Green Jr. in 1958, but the term 'rake' was coined later. The technique was initially developed for military spread-spectrum communications, particularly in the Rake system at MIT Lincoln Laboratory.³

It became commercially significant with the advent of CDMA cellular systems in the 1990s, where it was adopted for IS-95 and later for WCDMA (UMTS). The rake receiver remains a fundamental component in many 3G and 4G receivers, though modern systems also employ advanced equalization and MIMO techniques.⁴

In practice, a rake receiver includes a searcher that continuously scans for multipath components, a set of fingers (typically 3 to 6 in mobile devices), and a combiner. Each finger performs despreading and channel estimation, and the combiner applies weights based on the estimated channel coefficients.⁵

Challenges include finger assignment, tracking time-varying delays, and managing interference between paths. In WCDMA, the rake receiver is used in the downlink, while the uplink may use a similar structure at the base station. The number of fingers is a trade-off between performance and complexity.⁶

Rake receivers are used in CDMA-based systems (IS-95, CDMA2000, WCDMA) and in GPS receivers to mitigate multipath. They are also used in some wireless LAN and ultra-wideband systems.⁷

Limitations include performance degradation in dense multipath environments where paths are not resolvable, and increased complexity with many fingers. In modern systems like LTE and 5G, rake receivers are often replaced by more advanced equalizers and MIMO processing, but the principle remains relevant in certain contexts.⁸

One lesser-known fact is that the rake receiver concept was patented by Price and Green in 1958, but the term 'rake' was coined by a colleague, possibly due to the diagram's resemblance to a garden tool.³

Another edge case is the use of rake receivers in underwater acoustic communications, where multipath is severe and the technique helps in symbol detection. Additionally, some implementations use a 'rake with interference cancellation' to improve performance in multi-user scenarios.⁷

Multipath fading

The phenomenon where a signal arrives at a receiver via multiple paths, causing constructive and destructive interference.

Correlator

A device that measures the similarity between a received signal and a known code sequence.

Maximal-ratio combining (MRC)

A method of combining signals from multiple paths by weighting each proportionally to its signal-to-noise ratio.

Finger

A single correlator in a rake receiver assigned to a specific multipath component.