Focus on 11GHZ is questionable, IMHO.
Meanwhile, Fixed Wi-Max certification test procedures are taking place in Spain, prior to the "First Wave" of actual
certification and interoperability tests..
augustus 25, 2005
WiMAX advantages bring about new challenges
While many applaud the advantages of WiMAX systems, designers need a
clear understanding of the 802.16 spec and the resulting RF
requirements.
By Darcy Poulin, SiGe Semiconductor Inc.
WiMAX, which stands for Worldwide Interoperability for Microwave
Access and is a form of broadband wireless access, is based on the
IEEE 802.16 standard for wireless metropolitan-area networks (MANs).
Widespread deployment of the technology is expected over the next
three to five years, driven by WiMAX's ability to deliver
affordable "Last Mile" broadband Internet services. Many of the
companies entering the WiMAX market include those that have
dominated the WLAN arena. As engineers schooled in WLAN tackle this
emerging standard, they face many different system considerations,
especially in terms of RF requirements and architectures.
In a typical 20-MHz channel bandwidth deployment scenario, WiMAX
Forum certified products will support downlink data rates of 65
Mbits/s at close range to 16 Mbits/s at distances of 9 to 10 km,
which is enough bandwidth and transmission range to deliver high-
speed simultaneous access to voice, data, and video services to
hundreds of businesses or thousands of residences. WiMAX's extended
range is driving a significant market opportunity. In addition, it's
proving useful in delivering broadband services to rural areas where
it's cost-prohibitive to install landline infrastructure.
Understanding the standards
How does the 802.16 WiMAX standard compare to the 802.11 WLAN
standard? To start, both are based on orthogonal frequency division
multiplexing (OFDM), use multiple pilot tones, and support
modulations ranging from BPSK to 64 QAM.
But there are some major differences as well. For instance, rather
than a fixed 20-MHz bandwidth with 52 subcarriers as in 802.11,
WiMAX systems can use variable bandwidths from 1 to 28 MHz with 256
subcarriers (192 data subcarriers) in either licensed or unlicensed
spectrum. The first WiMAX rollouts are expected to use 3.5- and 7-
MHz channel bandwidths.
WiMAX supports subchannelization, meaning that instead of
transmitting on all 192 data subcarriers, you can transmit on just a
subset. In this scenario, by using the same amount of power over
fewer carriers, the system achieves greater range. As WiMAX CPE
evolves into in-building devices, it'll be necessary to make up for
the power loss incurred when transmitting the signal outside the
building. Because CPE is typically limited in power, concentrating
the power over fewer subcarriers in the uplink can balance the power
in the uplink and downlink, and enable greater range.
While the larger number of subcarriers gives WiMAX an advantage over
802.11, the resulting challenge to the system design is that the
subcarriers are spaced more closely together, so there are tighter
requirements for phase noise and timing jitter. This translates to a
need for higher-performance synthesizers.
WiMAX also uses a variable-length guard interval to improve
performance in multi-path environments. The guard interval is a time
delay at the beginning of the packet to compensate for multi-path
interference. With a very clear channel, the guard interval can be
shortened, increasing the throughput. With more subcarriers, and
with a variable-length guard interval, a WiMAX system's overall
spectral efficiency will be 15 to 40% higher than a WLAN system. For
instance, WiMAX achieves a spectral efficiency ranging from 3.1 to
3.8 Mbits/s/MHz, compared to only 2.7 Mbits/s/MHz for 802.11a/b/g
(see the table).
Error-vector magnitude (EVM) requirements for 802.11 are specified
at -25 dB, which is required to achieve a 10% packet error rate. For
802.16, EVM is held to -31 dB, which is based on a 1% packet error
rate. This lower error rate helps contribute to WiMAX's longer
range. Also contributing to the longer range is the receiver noise
figure, which is more stringent for 802.16. Specifically, 802.11's
maximum noise figure is 10 dB, while 802.16 operates at 7 dB.
802.11 only supports time division duplexing (TDD), where transmit
and receive (Tx/Rx) functions occur on the same channel, but at
different times. In comparison, the 802.16 spec offers more
flexibility, supporting TDD, frequency division duplexing (FDD), and
half-duplex FDD (H-FDD). FDD uses simultaneous Tx/Rx on different
frequencies; H-FDD transmits on different channels at different
times. The approach that designers select affects cost, footprint,
and design time. For example, an FDD system will cost more because
simultaneous Tx/Rx requires two complete radios. However, FDD will
allow greater throughput, as bandwidth is dedicated for receive and
transmit, and this bandwidth is used simultaneously.
Another significant difference between WiMAX and 802.11 is ranging
and transmit dynamic range. In 802.11, the output power is virtually
fixed, and systems typically transmit at the same power all the
time. However, for WiMAX, a ranging process determines the correct
timing offset and power settings. This process ensures that
transmissions from each subscriber station arrive at the base
station at the proper time and at the same power level. As a result,
the 802.16 standard requires that subscriber stations have a 50-dB
transmit dynamic range. This allows systems that are close to the
base station to back off their transmit power, while those far away
can transmit at maximum power. This is significant because WiMAX
supports transmit ranges of several kilometers, and transmitting at
maximum power near the base station would be disastrous.
Overall system design challenges
When designing a new WiMAX system, the first question is whether the
system will be TDD, FDD, or H-FDD. Many countries, such as Canada
and much of Europe, are generally adopting an FDD structure. In the
U.S., if the system will be used in licensed spectrum, then the
duplexing will already be specified. If the system will be FDD, two
complete radios (including synthesizers) operating simultaneously on
different frequencies will be required.
This type of system will need extensive external filtering to
prevent the transmit power from leaking into and interfering with
the receiver. In addition to cost, the dual radios and filtering
required become a significant concern for board space. Many industry
leaders expect that base stations will use full FDD mode due to its
higher throughput, while the subscriber stations will use lower cost
H-FDD or TDD.
Given a choice, H-FDD can be an attractive alternative because it
has a single radio (and single synthesizer), and similar costs to
TDD. The key concern with H-FDD is that the synthesizer must be able
to switch between the transmitter and receiver within 100 Ìs.
Because the system isn't simultaneously transmitting and receiving,
the filtering requirements are relaxed significantly compared to
FDD.
Perhaps the 802.16 specification that has the greatest impact on
system design is EVM, because the EVM must be 6 dB higher for 802.16
than for 802.11. This has a number of implications. First, all the
system blocks must be more linear. Second, phase noise must be
considerably better than in an 802.11 design. Tighter phase noise
requirements have implications for the synthesizer, which result in
a longer settling time. Third, if an I/Q interface is chosen, then
I/Q balance must be tighter as well, and will likely require I/Q
calibration.
The biggest impact of a tighter EVM requirement is on the power
amplifier (PA). In addition to having to meet a -31 dB EVM, target,
there are two other significant factors that conspire to make PA
design challenging, First, the peak-to-average power ratio (PAPR) of
802.16 is higher than 802.11. Because 802.16 has more subcarriers,
the PAPR is about 10 dB, which is 2 dB higher than 802.11's 8-dB
PAPR. Second, an 802.16 system typically transmits at a higher power
than an 802.11 system. Hence, the PAs in WiMAX systems must deliver
more power, they must be more linear, and they must be able to
handle a higher PAPR than 802.11 PAs.
The end result is that the PAs will consume more power, and they'll
be less efficient. Considerable effort must be made to develop
higher efficiency, more linear PAs, especially for mobile
applications where power consumption is critical. It's also possible
that adaptive predistortion will be needed to achieve high linearity
with high efficiency.
RF architectures
When selecting an RF architecture for a WiMAX design, the basic
choice is between a superheterodyne or direct-conversion
architecture. In terms of satisfying the stricter transmitter
regulatory requirements, a superheterodyne architecture is
advantageous because of off-chip filtering of unwanted emissions.
There are two different kinds of superheterodyne baseband
interfaces: IF and I/Q. With an IF interface, the signal at the
baseband processor is at a low (but not zero) frequency. Typical IF
frequencies range from 10 to 50 MHz. With an I/Q interface, the
signal at the baseband processor extends to dc. In this case, any
I/Q imbalance will result in images that fall directly on top of the
desired signal and appear as noise. Therefore, I/Q balance is
critical for an I/Q interface, and it's likely that external I/Q
calibration will be required. For this reason, the IF interface is
preferable because it doesn't require any external calibration.
A direct-conversion transmitter architecture, on the other hand,
takes the two I/Q inputs at baseband and directly modulates them up
to the RF. This architecture is attractive because it leads to a
smaller and less expensive radio design. It removes the need for an
IF local oscillator, and it eliminates the surface acoustic wave
(SAW) filter. The challenge with this approach is that performance
is harder to maintain. For instance, any small dc offsets that occur
will degrade system performance. I/Q balance is also critical.
Therefore, both dc and I/Q calibration will be required. In
addition, without SAW filtering, spurious transmissions may result
in spectral mask emission failures.
IC options
In most current 802.11 designs, the RF, physical layer (PHY), and
media access controller (MAC) are all integrated on one IC. Because
WiMAX is new and production volume levels are relatively low, WiMAX
ICs aren't yet as integrated. Hence, IC designers must decide how to
partition functionality. While it's possible to integrate the
transmitter and receiver, with both the RF and IF sections on one
chip, this approach is not common today.
For a superheterodyne architecture, it's common to partition the
chip. The partitioning can be done either at RF/IF (where transmit
and receive are on the same chip, but with separate chips for RF and
IF), or at Tx/Rx (there's a separate Tx and Rx chip, but both
include RF and IF chains).
An RF/IF partitioning is a better alternative, as one synthesizer
can be shared between both ICs. This technique uses one fully-
programmable synthesizer located on the IF chip that produces all
the required local-oscillator signals to drive both the transmit and
receive paths. To achieve the best performance at the lowest cost,
IC makers can use different process technologies for the two ICs.
For instance, it's possible to use a silicon CMOS process for the IF
chip, and SiGe or GaAs for the RF device.
References
Mannion, Patrick. "WiMax upheaval on course," EE Times 03/14/2005
Big telcos push WiMax networks closer to reality, 6/13/05
WiMax opens wide range of design options, 3/14/05
About the author
Darcy Poulin, of SiGe Semiconductor Inc., can be reached at
dp@.... |
