A rake receiver is a radio receiver designed to counter the effects of

multipath fading In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflect ...

. It does this by using several "sub-receivers" called ''fingers'', that is, several correlators each assigned to a different multipath component. Each finger independently decodes a single multipath component; at a later stage the contribution of all fingers are combined in order to make the most use of the different

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...

characteristics of each transmission path. This could very well result in higher

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in de ...

(or E_b/N₀) in a multipath environment than in a "clean" environment. The multipath channel through which a radio wave transmits can be viewed as transmitting the original (

line of sight The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observer/ spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken ...

) wave pulse through a number of multipath components. Multipath components are delayed copies of the original transmitted wave traveling through a different echo path, each with a different magnitude and time-of-arrival at the receiver. Since each component contains the original information, if the magnitude and time-of-arrival (phase) of each component is computed at the receiver (through a process called channel estimation), then all the components can be added coherently to improve the information reliability.

Mathematical definition

A rake receiver utilizes multiple correlators to separately detect the ''M'' strongest multipath components. Each correlator output may be quantized using several bits. Demodulation and bit decisions are then based on the weighted outputs of the ''M'' correlators, which provide a better estimate of the transmitted signal than is provided by a single component.

History

Rake receivers must have either a general-purpose

CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...

or some other form of

digital signal processing Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...

hardware in them to process and correlate the intended signal. Rake receivers only became common after 16-bit CPUs capable of signal processing became widely available. The rake receiver was patented in the US by Robert Price and Paul E. Green in July 1956, (U.S. Pat. No. 2,982,853) but it took until the 1970s to design practical implementations of the receiver. Radio astronomers were the first substantial users of rake receivers in the late 1960s to mid-1980s as this kind of receiver could scan large sky regions yet not create large volumes of data beyond what most data recorders could handle at the time.

Astropulse Astropulse is a volunteer computing project to search for primordial black holes, pulsars, and extraterrestrial intelligence (ETI). Volunteer resources are harnessed through Berkeley Open Infrastructure for Network Computing (BOINC) platform. I ...

, which is part of the SETI@Home project, uses a variant of a rake receiver as part of its sky searches—so this kind of receiver is still current for the needs of radio astronomy.

Use

Rake receivers are common in a wide variety of

CDMA Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communicatio ...

and

W-CDMA The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the Int ...

radio devices such as mobile phones and

wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office buildi ...

equipment. Rake receivers are also used in

radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...

. The CSIRO Parkes radio telescope and Jodrell Bank telescope have filter-bank recording formats that can be processed in real time by software-based rake receivers. In a Flexible rake Receiver,

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

reception is performed with a single correlator engine and a

stream A stream is a continuous body of surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long large streams ...

buffer storing the entire delay spread of

baseband In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable i ...

input/output In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...

(I/O)

sample Sample or samples may refer to: Base meaning * Sample (statistics), a subset of a population – complete data set * Sample (signal), a digital discrete sample of a continuous analog signal * Sample (material), a specimen or small quantity of ...

s. The primary advantage of the rake receiver is flexible multipath allocation supporting enhanced

modularity Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a sy ...

of the receiver and resource sharing among multiple channel decoders.

References

{{DEFAULTSORT:Rake Receiver Telecommunications equipment Radiofrequency receivers Code division multiple access