The world has gone wireless. From mobile phones to the latest tablet computers, mobility is a key feature, and there are many different ways to provide wireless data. The demand for higher data rates has increased dramatically in recent years, as have the system requirements. Hence, the performance of the wireless RF subsystem is a vital element of the overall user experience.
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Over the years, this has presented wireless engineers with a number of challenges. Providing more data means higher data rates and higher performance while users also demand more range and longer battery life, whether it’s for Wi-Fi or any other air interfaces used in cellular networks.
Taking these requirements into account, higher bandwidth can provide more data but takes more power. Boosting the output power provides more range, but again drains the battery faster. Coupled with increased demand for application processing and larger screens, the requirements on the battery can severely restrict the opportunities to boost the performance of the wireless link.
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The way to meet this challenge is to make more of what is already out there. Multiple-input multiple-output (MIMO) techniques use multiple antennas and multiple signal streams to boost the data rate, range, and connection reliability without dramatically hitting the battery life. However, it does take more processing power. Up until recently, that power has not been available for the platforms with the most stringent power consumption requirements.
Now, though, all the new wireless standards support MIMO techniques, with 802.11n and 802.1ac Wi-Fi, as well as the LTE 4G cellular standard. Even 3G and some 2G equipment are using the technology. As a result, MIMO is becoming a standard feature in wireless chipsets and for wireless equipment, and its use will grow with the emerging LTE-Advanced (LTE-A) standard.
What Is MIMO?
MIMO makes use of the advantages of having several antennas and a range of signal paths. Usually in a wireless link the strongest signal is chosen to make the connection, and all the other signals are filtered out. These multipath reflections come from reflections from buildings and objects, but they are weaker and arrive at different times. In a system with antenna diversity, different antenna pick up the different signals, filter out the noise, and recombine them to generate a single, stronger signal.
Today’s MIMO employs a slightly different approach, but still uses multiple antennas. In the basic MIMO concept for 11n Wi-Fi, the data to be transmitted is scrambled, encoded, and interleaved and then divided up into parallel data streams, each of which modulates a separate transmitter. Multiple antennas then capture the different streams, which have slightly different phases because they have travelled different routes, and combine them back into one.
Each multipath route can be treated as a separate channel, and the separate antennas take advantage of this to transfer more data. In addition to multiplying throughput, range is increased due to the advantage of antenna diversity, since each receive antenna has a measurement of each transmitted data stream.
With MIMO, the maximum data rate per channel grows linearly with the number of different data streams that are transmitted in the same channel, providing scalability and a more reliable link. This robustness allows the system to scale back the current to achieve the required data rate with minimal power consumption.
For Wi-Fi, the modulation is orthogonal frequency division multiplexing (OFDM) using binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16-phase quadrature amplitude modulation (16QAM), or 64QAM, depending on the data rate. Different data streams then can be transmitted in the same 20 MHz in the same band, improving throughput. Throughput scales linearly with the number of transceivers.
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 4x4 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.
Software Defined Radio
Software defined radio (SDR) is another key wireless technology relevant to MIMO that has been enabled by the increased capabilities of both the design and the process technology. It uses a flexible wide band RF front end with the processing power to decode different protocols. A single device then can be used for a wide range of applications, reducing costs and making system design simpler. The increasing popularity of MIMO in wireless means SDR systems increasingly have to have sufficient processing power to handle this multipath capability.
A field-programmable RF transceiver (FPRF) is one of the key technologies needed to manage multiple bands and multiple standards in MIMO networks. For example, Lime Microsystems’ LMS6002 FPRF targets MIMO applications (see the figure). This flexible low-power chip greatly facilitates the implementation of MIMO systems. Combining four FPRF transceiver chips with a FPGA from Xilinx or Lattice creates a single-band 4x4 MIMO product that can be used for LTE/HSPA+ cellular systems.
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.
Ebrahim Bushehri is the founder and CEO of the field-programmable RF chip start-up Lime Micro. He is also the founder of the non-profit initiative Myriad RF foundation, which seeks to bring open-source RF hardware to a wider audience through the development of low-cost, professional-grade hardware and free design files made available under the creative commons license.