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Multiple Input, Multiple Output wireless technologies [1, 2, 3, 4, 5] are getting a lot of attention these days.
MIMO has the ability to significantly increase raw data throughput in spectrally limited environments, while at the same time providing immunity to the multipath effects common in urban settings. It has been suggested for use in beyond 3G (B3G) and 4G cellular communications, automobile communications, wireless local and wide area networks, and military communications.
Given the benefits inherent in a MIMO based architecture but something of a lack of maturity in MIMO technology, a question is raised as to how MIMO capabilities can be inserted into radio systems in a cost effective manner after those radios are already in service.
This article proposes a model using software defined radio technology as a key enabler in allowing today's emerging wireless systems to support the future insertion of MIMO technology while minimizing overall capital investment.
MIMO overview
The capacity of a wireless link is generally measured in bits per second per Hertz (b/s/Hz). The methods available to increase this capacity in a traditional Single Input, Single Output (SISO) wireless system are fairly limited: increase the bandwidth, allowing a corresponding increase in the bits per second, or increase the transmit power, allowing a higher level modulation scheme to be utilized for a given bit error rate, effectively increasing the bits per second within the same bandwidth.
The problem with both of these techniques is that any increase in power or bandwidth can negatively impact other communications systems operating in adjacent spectral channels or within a given geographic area. As such, bandwidth and power for a given communications system are generally well regulated, limiting the ability of the system to support any increase in capacity or performance.
MIMO technologies overcome the deficiencies of these traditional methods through the use of spatial diversity [3, 4]. Data in a MIMO system is transmitted over T transmit antennas through what is referred to as a "MIMO channel" to R receive antennas supported by the receiver terminal (see Figure 1).
If the antennas within the transmit array and the antennas within the receive array are spaced sufficiently far apart, the signals traveling between the various transmit and receive antennas through the MIMO channel will fluctuate or fade in an independent manner. The transmitted data can therefore be encoded, using a so-called space-time code, to make use of this spatial diversity and allow processing at the receiver to extract the underlying data.
The specific coding scheme utilized in the MIMO system is selected based on the target performance, the acceptable level of computational complexity in the receiver’s signal processing subsystem, and the level of a priori knowledge of the transmission channel.
Some schemes, referred to as space-time diversity codes, optimize for "diversity order", which defines the performance gain that can be obtained through the number of decorrelated spatial branches that can be achieved through the MIMO channel.
Other schemes, referred to as Spatial Multiplexing, optimize for channel capacity. Both of these types of schemes are discussed with additional detail below. These schemes can be used in combination to obtain the benefits accrued by both.
Ultimately, the space-time coding scheme operating in conjunction with the MIMO channel allows the MIMO based system to support a significant increase in both performance and capacity over an equivalent SISO system while maintaining the same bandwidth and power.
Click here for Figure 1
Figure 1: Conceptual diagram of a MIMO system.
Space-Time diversity coding
In space-time diversity coding, each modulated symbol is encoded and transmitted from each of the transmit antennas [7]. This maximizes the total available spatial diversity from the MIMO channel, on a per symbol basis, offering a significant increase in bit error rate performance over an equivalent SISO channel operating at the same transmit power. Space-time diversity coding works with any number of transmit or receive antennas, with the total diversity order equal to T*R [3].
Various space-time coding schemes have been developed for use in space-time diversity coding. In one of the earlier schemes, referred to as Delay Diversity, each symbol sent on one antenna is delayed by a symbol period and then sent on another antenna [8].
This scheme is a simple example of a space-time trellis code (STTC), and is typically decoded through the use of a fairly complex maximum likelihood sequence estimator in the front-end of the receiver. One of the more popular schemes for space-time diversity coding is the Alamouti scheme [9]. This scheme utilizes a simple space-time block code (STBC) that encodes two modulated symbols into a matrix that is two rows by two columns in size.
During each symbol period, the contents of a row are transmitted via the corresponding antennas. Decoding of a space-time blocking code can also be done using a maximum likelihood detector, but other techniques can also be employed [3,4].
Space-time diversity codes support a symbol rate of at most one symbol per symbol period [7]. However, the improvement in signal to noise ratio at the receiver using space-time diversity coding can be quite high, with one paper reporting up to 16dB improvement for a two transmit and two receive antenna system [10].
This improvement allows an increase in the number of bits transmitted per symbol period while maintaining the same bandwidth, transmit power and bit error ratio, thus improving the capacity of the wireless link. It can also be used to extend distance over which a symbol can be transmitted, again while maintaining bandwidth, transmit power and bit error rate performance. This can improve the transmitter to receiver ratios, lowering site count and associated periodic costs.
Spatial Multiplexing
Spatial multiplexing maximizes the link capacity that is sent over a given bandwidth by transmitting a different symbol on each antenna during each symbol period [3, 4]. Thus the number of symbols transmitted per symbol period is equal to the number of transmit antennas.
For spatial multiplexing to work, the number of receive antennas must be greater than or equal to the number of transmit antennas. The space-time code in a spatial multiplexing scheme is inherent in the multiplexing function [8].
The predominant encoding schemes associated with spatial multiplexing break into two types: horizontal encoding and vertical encoding (see Figure 2) [4, 8]. In horizontal encoding, the bit stream to be transmitted is demultiplexed into T separate data streams.
Each of these data streams is then temporally encoded, interleaved and converted to transmission symbols, with different modulation schemes allowed on each transmit channel. In contrast, in vertical encoding, the bit stream to be transmitted is encoded using a space-time block code and then converted into transmission symbols. The transmission symbols are then demultiplexed into T bit streams and transmitted.
Vertical encoding offers improved diversity gain over horizontal encoding because each data bit can be spread across all of the transmit antennas. However horizontal encoding accrues an advantage in receiver complexity in that the individual data streams are decoded separately, typically using a relatively simple linear receiver, such as the Zero Forcing receiver or Minimum Mean Squared Error receiver outlined in [3]. Vertical encoding, on the other hand requires joint decoding at the receiver, which significantly increases receiver complexity [11].
Click here for Figure 2
Figure 2: Spatial multiplexing schemes.
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