MIMO Holds Key to High Bandwidth Wireless Systems

Louis Bélanger, Lyrtech Inc.
Mar 03, 2005 (6:24 AM)

Smart antennas, also known as array antennas or multiple input, multiple output (MIMO) antennas, are proving to be an important technology when designing next-generation voice and/or data wireless communication systems as well as when reengineering existing systems to obtain higher bandwidths and capabilities. Currently, there are a number of MIMO applications, development platforms, and tools that are showing great promise in the quest for wireless systems with higher bandwidth and greater capabilities. The major advantage of smart antenna technology is digital beamforming, which is now making its way out of research laboratories and into real-world applications with great speed.

Digital Beamforming Techniques
It is widely recognized that because they use digital beamforming, smart antennas at one or at both ends of a wireless link are the single best method for resolving congestion problems. In fact, they allow an order of magnitude or more increase in the number of supportable users as compared to traditional methods.

Multibeam and array antenna approaches are depicted in Figures 1a and 1b below. Figure 1a displays the basic concept of digital beamforming, where phase relationships between wireless waves to/from array elements are used to electronically create a beam in a manner equivalent to an antenna dish. The interest in this approach is that multibeams can be virtually created without having to steer the antenna. In fact, such systems are also mentioned as "electronically steerable antennas." Figure 1b shows different approaches of beamforming within a cellular network. The interest in these approaches is that they can be used in existing cellular networks to augment their capabilities, either in the number of users or in available bandwidth.

This beamforming capability is also the root of a number of next-generation communication applications. For instance, Figure 2a shows Japan's next generation system where stratospheric balloon-mounted array antennas are used to point at each mobile user. Another application, displayed in Figure 2b, is an aircraft belly-mounted array antenna that can be electronically steered toward specific ground-based basestations, thus providing increased discrimination between basestations and augmented capabilities in air-to-ground communications. Note however, that these systems are asymmetric in the sense that only one end presents an array of antennas.

Figure 1: Diagram showing adaptive antenna arrays and beamforming: (a) The basic concept of digital beamforming (b) Various forms of beamforming in cellular systems.

Figure 2: Adaptive antenna arrays and beamforming applied to advanced and next-generation systems: (a) Japan's 4G high-atmosphere airborne basestation platforms using adaptive antenna arrays (b) Boeing's concept of belly-mounted array antennas

MIMO System Applications
In wideband/frequency-selective fading propagation environments, the presence of intersymbol interference makes it desirable to combine equalization with an antenna array, thus forming a space-time processor. When used in this way, space-time processors can increase the number of users. However, even further evolution is possible when both ends of the wireless link are equipped with an antenna array. This forms a MIMO link, and in rich scattering channels and with appropriate signal processing, this approach can increase effective data rates an order of magnitude or more.

This approach has been studied extensively, beginning with breakthrough research performed at Bell Labs (referred to as the BLAST [Bell Labs Layered Architecture Space-Time] project) through a large number of US and European projects (I-METRA, SATURN, FLOWS, etc.), and more specifically in the context of broadband wireless and third-generation (3G) Universal Mobile Telecommunications System (UMTS)/ wideband code-division multiple access (W-CDMA) cellular systems.

As a result of this research, engineers found that, when using a combination of digital beamforming and MIMO principles, systems can be designed with considerably increased user and data capacity. Furthermore, MIMO links can generally provide robustness against multipath fading (diversity) and can enable higher data rates. MIMO links can be designed with the transmitter having channel knowledge (thus allowing joint Tx-Rx optimization) or not. In the latter case, the Tx antennas can either send independent bitstreams, thus appearing at the receiver as individual users, or they can transmit correlated bitstreams according to a space-time code.

While most of the research has concentrated on 3G systems, wireless local-area network (WLAN) designers eagerly used this technology to boost capacity. Currently, the latest MIMO-oriented version of the 802.11n standard has seen two competing approaches proposed: one with a 2-2 MIMO matrix (backed by Intel, Agere, Atheros, and others), and another with a 4-4 matrix (backed by Broadcom, Conexant, Airgo, and others). So far, it seems that the simpler 2 x 2 version will prevail. (One can imagine the impact on cost that comes from adding a number of antennas on end-user systems.) What's more, the 2-2 proposal has room to grow to 4-4, while providing an interesting bandwidth boost at the basic 2-2 element level. MIMO concepts are also part of the 802.16 standard for WiMax wireless metropolitan-area networks (MANs).

Advanced Research and Development Platforms
An ongoing applied research project at the Laboratoire de Radiocommunication et de Traitement du Signal (LRTS) at Laval University (Quebec, Canada) and the R2D2 research group at the Ecole Nationale Superieure de Sciences Appliquees et de Technologie (ENSSAT) (Lanion, France) concerns the efficient implementation of novel MIMO algorithms adapted to 3G W-CDMA. In order to speed the development of a prototype, a MATLAB/Simulink model-based design flow is used in conjunction with a Lyrtech SignalMaster platform (Figure 3). This combination provides a powerful prototyping tool featuring a tightly-coupled digital signal processor (DSP)/field-programmable gate array (FPGA) pair and close integration with MATLAB/Simulink and Xilinx's System Generator.

Figure 3: Array antenna prototyping platform

A design and simulation environment, such as Simulink, allows the modeling of both the channels and the systems themselves. For example, Figure 4 depicts a "classic" simulation model where an entire frequency division duplexing (FDD) W-CDMA link (including the channel) is simulated.

Figure 4: Typical Simulink model for a W-CDMA system

In this model, one can find a mix of simulation primitives for DSP functions and C-code functions that come from applications that were previously developed independently. From these well-known simulation capabilities, the possibility of generating code for synthesizing any or all of the components in the model can extend the simulation activity towards target implementation in an approach often referred to as rapid prototyping. Obviously, the implementation code that is generated that way is not optimal, but the considerably shortened time to a working implementation model is of great interest.

The SignalMaster platform is currently being used for the implementation of a simple 2-2 MIMO/W-CDMA link. Figure 5 shows the block diagram of the two-antenna transmitter. The two antennas can transmit independent bitstreams, each one being spread by a different channelization code. In order to stay within the parameters of the standard as much as possible, the same dedicated physical control channel (DPCCH) information is transmitted on both antennas. Finally both antennas belong to the same source and, therefore, used the same scrambling code.

Figure 5: Block diagram of MIMO/W-CDMA transmitter

Future of MIMO Complete MIMO systems are going to hit the market in the near future. This will likely happen with the introduction of array-antenna capable basestations and two to four antenna-element end-user systems. However, researchers are looking at more innovative ways to use such capabilities. For example, the Laval University researchers are actively investigating concepts of distributed MIMO-capable basestations. Using this premise, lower power and non-tower-based basestations systems can be envisioned, thus creating collaborative and more agile networks. This concept, which could be applied to cellular networks and WLANs as well, is illustrated by Figure 6. As an example, a 64-element MIMO system is analyzed as an alternative to a rather large number of 802.11 basestations, which are spread around in campuses such as Laval University. If they succeed in demonstrating the concept, the economic advantages of such a solution become evident.

Figure 6: Standard basestation versus distributed MIMO-capable basestations

In all these cases, however, complexity and processing power needs are ever-increasing. The solution lies in using innovative design flows and taking advantage of the latest advances in FPGA and DSP capabilities. In this context, it is likely that smart antennas and MIMO systems will become a pervasive technology in the years to come.

About the Author
Louis N. Bélanger is a founder of Lyrtech Inc. He is also the product development manager of Lyrtech's SignalMaster DSP/FPGA development platform line of products. He graduated in 1979 from Laval University's Electrical Engineering Department, and in 1985, earned his MSEE in Signal Processing. Louis can be reached at louis.belanger@lyrtech.com.

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