The multiple signals arrive at the receivers at different times in different phases, depending on the different paths they take. Some signals will be direct, others via multiple different paths. With this special multiplexing, each signal is unique as defined by the characteristics of the path it takes.
The unique signatures produced by each signal over the multiple paths allow the receivers to sort out the individual signals using algorithms implemented by DSP techniques. The same signals from different antennas then can be combined to reinforce one another, improving signal-to-noise ratio and, therefore, the reliability and range.
Perhaps the greater benefit of MIMO is the transmission’s increased range and robustness, as it permits multiple streams. It also helps improve the signal-to-noise ratio and reliability significantly over other implementations.
Types Of MIMO
MIMO is not like the typical smart antenna systems used in cellular networks. A smart antenna uses beam forming to focus the transmitted signal energy toward the receiver to strengthen the signal. Beam forming may provide better range in certain applications, but there are problems with hidden nodes that the basestation can’t “see,” reducing the number of clients that can be supported. Also, the power consumption requirements limit the number of transmit chains. While MIMO can be used with beam-forming systems, there is little point as it relies on the phase shift of the reflected multipath signals.
Many MIMO systems use two transmitters and receivers, but the various standards allow other versions using different numbers of transmitters and receivers. Other possibilities include 2 by 3 (transmitters and receivers respectively), 3 by 2, 3 by 3, 3 by 4, 4 by 3, and 4 by 4. Beyond the 4-by-4 configuration, very little additional gain is normally achieved. The use of two transmitters and three receivers seems to be the most popular, although chipsets are now being implemented to support 4×4 MIMO for the highest-data-rate links.
Transmitting two or more data streams in the same bandwidth multiplies the data rate by the number of streams used. MIMO in the 11n standard also allows two 20-MHz channels to be bonded together into a single 40-MHz channel, which can provide even higher data rates.
With this channel bonding and four streams running on MIMO, a maximum potential data rate of 600 Mbits/s is achievable. Data rates surpassing 100 Mbits/s then can be supported over a 100-m range in hostile RF environments.
Implementation Of MIMO
Improvements in process technology, both for analog and digital devices, have opened up the use of MIMO in many applications. Until recently, very little dedicated hardware was unique to MIMO other than the separate transmit and receive chains. However, the improvements in RF design coupled with process technology mean that multiple transceiver chains can be integrated onto a single chip.
On the digital side, the processing power required to process the data was previously prohibitive in terms of silicon area and power consumption to be handled by the generally available field-programmable hardware. With the latest digital CMOS process technologies, this processing power is available in dedicated FPGA and DSP processors on chip, allowing system performance to be achieved and enhanced through software.
Rather than using the MIMO protocols hardwired into an FPGA, the performance of a DSP, often with dedicated accelerator blocks, is now sufficient to handle MIMO. The MIMO algorithms then can be constantly improved, increasing the reliability of the connection and reducing the power to extend the battery life. While this approach cannot change the number of channels that are used, or the number of antennas, it can enhance the signal processing to boost the performance of the link. So, the link can operate at lower current for the same power budget.
The wireless world needs robust data links. The benefits of MIMO are driving more and more standards from 802.11n to LTE-A to adopt the technology. Using antenna diversity with both multiple paths and multiple channels, coupled with the processing power in the back end, allows a dramatic improvement in the link budget and reduction in the power consumption. By adding a few antenna and more processing power, the next generation of smart wireless systems will be able to combine flexibility while delivering enhanced system performance.
The author is currently pursuing my final year btech in ECE at RSET, Cochin