MIMO stands for “Multiple Input Multiple Output”. A MIMO system consists of multiple antennas at both the transmitter and the receiver.
A scattering channel can be problematic because it causes interference and hence sub-optimal reception. Many methods have been introduced to counter the effect of interference, such as interference cancellation, phased array antennas and many of the diversity techniques mentioned in this document. However, it is possible to exploit the spatial diversity aspects of the channel, especially when it is highly dispersive and causes a reasonable degree of scattering. The MIMO concept is designed to take advantage of the channel dispersive nature instead of counteracting it.
There are various techniques that work with a MIMO system. Spatial multiplexing for example works by splitting the data stream to multiple streams (equal to number of transmitting antennas). If the channels are totally independent and uncorrelated it is possible to increase the overall data rate by multiple folds.
Another method is to employ space-time coding where the transmitted data is sent over multiple antennas at different times. With a MIMO system, the combined effect is a reduced power requirement (signal to noise ratio), or an increased throughput if traded off with power.
The scope of MIMO techniques varies from one implementation to another, and a combination of: spatial multiplexing, beamforming, space time coding, array antennas … etc. can all be considered part of a MIMO system implementation.
Theoretically, a NxN MIMO system can increase the throughput in the system by N folds compared to a simple “single input single output” system.
Showing posts with label Advanced Receivers. Show all posts
Showing posts with label Advanced Receivers. Show all posts
10 Jan 2007
2 Jan 2007
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).
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).
Subscribe to:
Posts (Atom)