We are currently witnessing an unprecedented increase in the demand for higher speed and better coverage of wireless networks. To meet this enormous demand, one approach is to increase the channel bandwidth over which radio signals are transmitted. However, this approach is not practical because frequency spectrums are expensive and transmitter and channel bandwidths are determined by regulatory standards. Other ways to improve the throughput is to use more complex modulation schemes. This, however, increases the complexity of the radio system and, thus, the cost. This problem requires a better solution.

In the past two years, an emerging technology known as MIMO has been one of the most promising technologies to improve the performance of a wireless link. MIMO refers to a radio link with multiple transmitter antennas and receiver antennas. In wireless links, radio signals from the transmitter travel in space, reflect off objects and reach the receiver over multiple paths. Multipaths can cause interference and signal fading in conventional radios. MIMO takes advantage of multipaths by multiplexing those signals with advanced DSP algorithms to boost wireless bandwidth efficiency and range. Wireless systems using MIMO can significantly improve the spectral efficiency of a system. For example, a wireless local area network (WLAN) system using two transmit antennas to two receive antennas (2 × 2 MIMO) can boost the maximum raw data rate for 802.11a and 802.11g networks from 54 Mbps to more than 100 Mbps.

MIMO orthogonal frequency-division multiplexing (MIMO-OFDM) technology has been adopted by the IEEE 802.11 standards group as the foundation for a high throughput amendment to multimedia wireless fidelity (WiFi) applications. In addition, a consortium of chipset developers has formed task groups such as the TGnSync, WWiSE and, most recently, the Enhanced Wireless Consortium (EWC) are working together to create an IEEE 802.11n specification.

Recently, a number of new products based on MIMO technology were introduced in the market and have delivered significant improvements in data transfer speed and coverage area over products using standard 802.11a/b/g technology. Although currently more expensive, the cost of these MIMO-based devices is expected to drop to levels similar to 802.11 a/b/g devices as the technology gets widely deployed, increasing bandwidth and meeting more user expectations.

MIMO, as a new technology, poses great challenges for silicon chipset vendors, contract manufacturers and brand owners with respect to research and development and production test methods. This article focuses on the physical layer issues and challenges involved with testing MIMO devices. It aims to demystify these challenges as well as offer readers fast, accurate, scalable, and low-cost ways to identify impairments and help improve MIMO system performance.

A standard 802.11a/b/g system uses one transmit antenna and one receive antenna in a radio link as shown in Figure 1. Radio signals from a transmitter traveling in space may reflect off multiple objects and arrive at the receiver through multiple paths. The receiver sees the vector combination of radio signals from these paths. Due to the phase delay difference over these paths, these signals sometimes add up in phase and, sometimes, when they are out of phase, they cancel each other out at the receiver. This causes the received signal strength to fluctuate constantly or fade and can significantly degrade the data throughput of the wireless system.

In wireless systems, radio signals from different users are typically separated by frequency, time or code. With beam-forming technology, also referred as smart antenna technology, each user can also be distinguished by their physical location in space.

Wireless systems use smart antenna technology to reduce the effect of multipath fading and to improve radio link quality and coverage. As shown in Figure 2, smart antenna technology uses adaptive antenna arrays that provide spatial diversity from the propagation channel and signal-processing algorithms in order to detect the direction of the client. A smart antenna is able to steer a transmitted beam by accurately controlling the phase of the signal over each element of the antenna array to the client. Another way to improve the range is to use maximum ratio combining on the receiver side. Here, two independent receivers are used to receive the same signal, and the two received signals are then combined using signal processing to get the desired signal. Antenna arrays are designed using traditional metrics from antenna theory. With beam-forming technology, a single datastream is transmitted over the communication link. Smart antenna technology can be used with existing 802.11a/b/g systems to improve performance. The data packet is compatible with the 802.11a/b/g standard that has the same spectral efficiency.

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