20 www.rfdesign.com November 2005
Next-Generation Wireless
W
e 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. MIM
O 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 x 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
introdu
ced 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.
How a MIMO system works
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 technol
-
ogy, 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
The challenges of testing MIMO
To meet the demand for higher data rates and better coverage of wireless
networks without increasing bandwidth or acquiring expensive frequency bands,
an emerging technology called multiple input, multiple output (MIMO) has
appeared. MIMO is capable of significantly increasing wireless data throughput.
Because this technology presents technical hurdles to chipset vendors, this
article will focus on demystifying physical layer issues with MIMO and present
ways of improving MIMO performance.
By Fan Liang
Figure 1. Standard WLAN system.
Figure 2. Wireless system using beam-forming (smart antenna)
technology.
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