Rake receiver

Rake receiver:
The dictionary meaning of rake is to gather or collect together
something and actually is a garden tool to collect leaves. But in the terms of
computer network, it is for the purpose of collecting signals from multiple
paths arriving at the receiver end and used specially in CDMA cellular
systems. A Rake Receiver is a radio receiver which is designed for the
purpose to counter the effects of multipath fading. Due to reflections from
multiple obstacles in the environment, the radio channel can consist of
multiple copies of the transmitted signal having different amplitude, phases
or delays. A rake receiver can resolve this issue and combine them. For this
purpose, several sub-receivers are used which are known as “fingers”. The
idea of a basic rake receiver was first proposed by Price and Green.
• When the transmitter transmits the signal then it travels through the
environment which consists of various obstacles and the transmitted
signal is reflected by them and is received by the rake receiver from
multiple paths. Rake receiver then feeds them to different fingers
(correlators). The delays in each received signal are compensated and
are feeded to the Combiner, Integrator and Comparator which
combines them suitably with different appropriate time delays.
Working of Rake receiver:
• The received signal from multiple paths arrives at the antenna with
different delays and phases.
• The signal is first passed through a bank of matched filters, each of
which corresponds to a specific path.
• The output of each matched filter is sampled at the symbol rate and the
resulting samples are combined.
• The combining process is done using a technique called maximum ratio
combining (MRC) which gives more weight to the signals that have
higher signal-to-noise ratio (SNR).
• The combined signal is then demodulated to obtain the transmitted
symbols.
• The rake receiver also performs channel estimation by estimating the
complex gains of each path using a technique called pilot symbols.
• The channel estimates are used to adjust the weights in the combining
process to ensure optimal performance.
• The rake receiver also uses a technique called diversity combining to
reduce the effect of fading by combining the signals from multiple
antennas.
• The combined signal from each antenna is then passed through the
matched filter and combined using maximum ratio combining.
• The rake receiver is able to recover the transmitted signal even in the
presence of severe multipath fading and interference.
• Some key applications of Rake receiver are:
• CDMA Systems: Rake receivers are extensively used in CDMA
systems, where they are used to combat the effects of multipath
fading.
• Wireless Networks: Rake receivers are also used in wireless networks
to improve the performance of the system, especially in environments
where the signal is weak.
• Satellite Communications: In satellite communication systems, Rake
receivers are used to detect and extract weak signals that have traveled
long distances.
• Mobile Communications: Rake receivers are used in mobile
communication systems to improve the quality of the received signal
by minimizing the effects of multipath fading.
• High-Speed Data Transmission: Rake receivers are used in high-
speed data transmission systems to reduce errors caused by the
transmission of signals over long distances.
Advantages:
• Multipath mitigation: The rake receiver mitigates the effect of multipath
propagation by combining multiple replicas of the same signal, which have
been transmitted over different paths.
• Diversity gain: By combining multiple replicas of the same signal, the rake
receiver provides diversity gain, which helps to improve the signal-to-noise
ratio (SNR) and reduce the bit error rate (BER) of the received signal.
• Interference rejection: The rake receiver can also be used to reject
interference from other signals that are transmitted over the same frequency
band.
• Simple implementation: The rake receiver is a simple and effective technique
that can be implemented using digital signal processing (DSP) algorithms.
• Compatibility: The rake receiver is compatible with different
modulation schemes and can be used in various wireless
communication systems, such as code division multiple access
(CDMA), time division multiple access (TDMA), and frequency
division multiple access (FDMA).
• Improvement in system capacity: The use of a rake receiver in a
wireless communication system can increase the system capacity by
allowing the use of more frequency bands or by increasing the number
of users that can be supported.
Disadvantages:
• Complex implementation: The Rake receiver is a complex receiver with a large number of
matched filters, which makes it difficult to implement.
• High power consumption: The Rake receiver requires a lot of power to operate, which can be a
problem in battery-powered devices.
• Limited performance in deep fades: The Rake receiver is not very effective in deep fades, where
the signal is severely attenuated. In such cases, other techniques such as diversity combining may
be more effective.
• Sensitivity to multipath delay spread: The Rake receiver is designed to work in the presence
of multipath, but it is sensitive to the delay spread of the channel. In channels with large delay
spreads, the performance of the Rake receiver may degrade.
• Limited applicability to narrowband systems: The Rake receiver is designed to work with
wideband systems that have significant multipath propagation. In narrowband systems with little
or no multipath, the Rake receiver may not be necessary or effective.
• The Rake receiver is an important solution to the problem of multipath
fading in wireless communication. It is designed to improve the signal
quality and reduce the effects of fading by using multiple copies of the
transmitted signal that arrive at the receiver at different times.
Although the Rake receiver is more complex, more power-consuming,
and more expensive than other types of receivers, it provides better
coverage and improved data rates, making it an attractive solution for
high-speed wireless communication.