802.11
MIMO tech for next-gen
cellular systems
By Gurinder Dhillon
Director of Marketing
Airgo Networks Inc.
E-mail: gdhillon@airgonetworks.com
All wireless technologies face the
challenges of signal fading, multipath, increasing interference and
limited spectrum. MIMO technology exploits multipath to provide
higher data throughput and simultaneous increase in range and
reliability—all without consuming
extra radio frequency. It solves two
of the toughest problems facing
any wireless technology today:
speed and range.
Digital communication using
MIMO processing has emerged as
a breakthrough for revolutionary
wireless systems. This modulation
format has been shown to be
spectrally compliant. It achieves
higher data rates and spectral efficiencies, allows longer range and
supports backward-compatibility
with existing OFDM standards.
MIMO basics
For every technology, it is often
useful to give a short definition
that sums up its salient characteristics. For MIMO, this would be two
or more data signals transmitted
in the same radio channel at the
same time.
In Figure 1, more than one
coherent radio upconverter and
antenna are used to transmit the
multiple signals, and more than
one coherent radio downconverter and antenna are used to
Figure 2: MIMO technology has penetrated other important commercial wireless markets besides WLAN.
receive the multiple signals.
With MIMO, the maximum
data rate per channel grows linearly with the number of different
data streams that are transmitted
in the same channel.
MIMO’s approach is to transmit and receive two or more data
streams through a single radio
channel. This means the system
can deliver two or more times the
data rate per channel. By allowing
for simultaneous transmission of
multiple data streams (Figure 1),
MIMO multiplies wireless data
capacity without using additional
frequency spectrum.
Peak throughput in MIMO systems increases by a factor equal
to the number of signal streams
transmitted in the radio channel.
Figure 1: MIMO transmits signals simultaneously on the same channel.
Because there are multiple signals,
each of which is transmitted from
a different radio and antenna,
MIMO signals are sometimes
called “multidimensional” signals.
Conventional radio signals are
referred to as “one-dimensional
signals” because they only transmit
one data stream over the radio
channel even if multiple antennas
are used.
The appetite for higher data
rate continues to increase as
consumer demand for bandwidth-hungry applications such
as Internet and e-mail access
grows to encompass gaming, and
A/V streaming.
Advancements in handset
processors and further integration of technologies such as
higher-megapixel cameras and
videos into handsets will only
exacerbate the challenges of
enabling more bandwidth-consuming applications at longer
ranges and more efficient use of
the limited spectrum available to
network operators.
Research, innovation, development and application of MIMO
to OFDM in the WLAN space and
the subsequent adoption of
MIMO by the 802.11n standard
body have shown MIMO’s capability to provide significantly
higher data rates and extend the
range of Wi-Fi to reach more mobile customer segments. Noting
this capability, other standards
organizations have realized what
MIMO can do for other technologies, both fixed mobile and cellular. Standards bodies such as
3G, WiBro, WiMAX, 802.20 and
4G have been exploring the use
of MIMO—some have already
adopted it in their respective
technology areas.
Path to 4G
CDMA has been considered a
revolutionary technology since
its inception. Due to its improvements over other standards like
Electronic Engineering Times-Asia | September 1-15, 2006 | eetasia.com
Figure 3: Receive-side components of a MIMO OFDM system consists of MIMO parts such as modules for synchronization and channel estimation, as well as
OFDM parts like FFT, deinterleaving and decoding.
GSM, Qualcomm not only drove
CDMA for 3GPP2, but also injected
it into the 3GPP W-CDMA standards. CDMA’s huge success has
helped Qualcomm extract royalties from handset OEMs.
Of late, however, OFDM has
been gaining ground and emerging as the new disruptive technology of the future. OFDM has
earned this distinction by its
ability to easily scale in the spectrum domain and handle common radio frequency distortions
without the need for complex
equalization algorithms.
In the last few years, OFDM
technology has been successfully
applied to wireless applications
such as WLAN, broadcasting
(DVB) and WiBro/WiMAX. Recently, startup Flarion delivered
OFDM’s promise to provide cellular applications with downlink
burst rates of up to 3.2Mbps.
Meanwhile, Qualcomm acquired Flarion, specifically after
realizing OFDM’s potential to
become the technology of choice
for 4G services.
MIMO technology is at different stages of adoption or
deployment in 3G cellular systems, broadband fixed wireless
systems, high speed WLAN and
mobile ad-hoc networks.
Economical MIMO OFDMbased WLAN systems were imple-
mented in 2003, setting a new
course for the wireless industry.
MIMO OFDM will be the basic
modulation format for 802.11n
WLAN, as every proposal submitted for this standard uses
MIM OFDM as the primary method of achieving greater range
and throughput compared to
802.11a/g standards.
Wide acceptance
Over the past few years, MIMO
technology has penetrated other
important commercial wireless
markets besides WLAN. Multiple
MIMO-based standards are envisioned for the cellular frontier.
MIMO is already being standardized as part of the 3GPP road map
for HSDPA and W-CDMA.
WiMAX broadband wireless
access networks and 3G cellular
mobile networks have already
adopted MIMO as an optional
mode of operation for performance enhancement. MIMO
OFDM is being proposed and
implemented as part of WiBro
and is already part of the feasibility study for the 3GPP long-term
evolution standard.
Some handset OEMs and
network providers are already
developing and testing MIMO
OFDM-based cellular systems
for next-generation networks
and MIMO implementations in
Electronic Engineering Times-Asia | September 1-15, 2006 | eetasia.com
limited test environments. Tests
conducted by Nortel, Ericsson,
Motorola and Siemens have
shown a lot of promise.
Implementing MIMO
Airgo’s success in developing and
launching MIMO-based WLAN
chipsets has proven that MIMO is
more effective in enhancing data
rates and coverage than standard
OFDM-based products, and in providing greater spectral efficiency.
The performance gains of MIMO
can be achieved by applying it to
another standard without changing the underlying wireless protocols. But, from an implementation
perspective, MIMO systems are architecturally and computationally
complex, requiring much more sophisticated algorithms and higher
data-processing capability.
Figure 3 shows the typical
components of the Rx side of a
MIMO OFDM transceiver chain.
The chain consists of MIMO
parts, including modules for
synchronization, channel estimation, interference processing and
MIMO detection, as well as OFDM
parts like FFT, deinterleaving and
decoding.
Airgo has amassed significant
IPs and fine-tuned its technologies, including areas of low-complexity receiver algorithms, MIMO
channel estimation algorithms,
mixing of MIMO and non-MIMO
transceivers, MIMO architectures,
optimal receiver systems and
algorithms for wireless.
In addition, significant time
and effort have been spent cooptimizing MIMO modules with
OFDM components, including
MIMO OFDM decoding, MIMO
decomposition and optimal
methods of reducing algorithmic
and architectural complexity, and
power consumption.
System components
Figure 3 also shows how the
MIMO OFDM system for WLAN
can be easily leveraged to another
standard like 3G, WiBro, WiMAX,
802.20 or 4G and what components need adaptation to the new
standard. As shown, most of the
components are common to any
MIMO system and can be readily
leveraged. Components that need
changes, on the other hand, are
limited to the OFDM and coding
parts of the system.
MIMO is the disruptive technology that will change the wireless industry’s playing field over
the next decade. It is penetrating
every standard of wireless technology. Today, some companies
are already maximizing the data
rate, coverage and spectral efficiency benefits that MIMO OFDM
provides.