WIMAX: Gearing Up

Although IEEE 802.16 defines multiple air interfaces, the WiMax Forum and
most vendors are currently focused on the 256 carrier orthogonal frequency
division multiplexing (OFDM) air interface, which uses bandwidths between 1.25
and 20 MHz and is designed for NLOS frequencies <11 GHz. There is very little
spectrum available above 5.8 GHz, so most vendors are targeting frequency bands
from 700 MHz to 5.8 GHz. Of the 256 carriers, only 200 are actually used,
leaving unused carrier for guard bands and a null carrier at the channel center
frequency. Used carriers include pilots and actual data.

The next generation of broadband wireless access (BWA) is almost upon us.
IEEE 802.16 standard and the WiMax Forum will enable interoperability and lower
price-points that were previously unattainable with proprietary point-to-point (PTP)
and point-to-multipoint (PMP) communication schemes.

802.16 and WiMax

In July 1999, the first formal gathering took place for the IEEE 802.16
Working Group on BWA. The original 802.16 standard described a 10-66 GHz
point-to-point system using a traditional single carrier modulated with QAM.
Since then, two different task groups have been chartered to define an interface
targeted at frequencies less than 11 GHz for fixed, non-line-of-sight (NLOS)
applications. This interface uses 256-carrier OFDM modulation and was originally
802.16d. In June 2204, it was formally released by the IEEE as 802.16-2004.
While the original 802.16 standard for 10-66 GHz operation had very little
industry adoption, the sub 11 GHz standard is exactly the opposite. Many vendors
are working to introduce solutions that will support the new 256-carrier OFDM
interface standard and will operate in bands ranging from 700 MHz to 5.8 GHz.

By
itself, the 802.16 standard is not sufficiently robust to guarantee good
interoperability between devices from multiple vendors. Interoperability is seen
as an enabler to help grow the market and reduce overall costs. To address these
issues, a group of industry leaders including Intel and Nokia formed an
organization called the WiMax Forum. Although the WiMax Forum is not formally
connected with the IEEE, the purpose of the group is to help ensure
interoperability, promote the 802.16 standard, and help grow the overall market
acceptance for 802.16 as a BWA standard. The WiMax forum is growing rapidly and
currently has over 200 members.

One of the WiMax forums' first deliverables is to establish specific
conformance procedures and testing labs to administer the testing. This process
will result in WiMax-certified products that have guaranteed interoperability
with other WiMax certified solutions. Initially, the first WiMax profiles are
targeted at applications at 3.5 GHz and 5.8 GHz. WiMax certification is expected
to start by mid-2005.

Key Applications

Figure 1. Typical OFDM spectrum with 200 carriers in use

WiMax certified products are initially targeting applications such as
wireless backhaul for telcos, high bandwidth/ high reliability remote
connectivity, E1/T1 replacements for small and medium size businesses, and
residential 'wireless DSL' that provide 'last mile' connectivity for
broadband Internet at home.

To address these diverse needs, vendors' solutions provide various level of
price / performance. Premium devices provide carrier grade performance. These
devices are engineered to provide guaranteed performance and reliability that is
comparable to dedicated copper or fiber connections. An example application is a
cellular service provider that needs backhaul for connecting remote cell towers
or a remote oil platform that requires high bandwidth and guaranteed
performance. The price / performance of these systems should be significantly
better than existing proprietary solutions and will cause many existing copper
or fiber links to convert to wireless.

Figure 2. Typical downlink and uplink frames

Business class is one of the largest segments of potential WiMax
applications. Typical applications will be connecting Wi-Fi hotspots to the
Internet or replacing leased E1/T1 lines in commercial backhaul. Because 802.16
allows network operators to configure systems with different levels of bandwidth
and quality of service, it is possible for operators to configure performance
that matches an individual customer's needs. Some customers will require
bandwidth and latency that is T1 equivalent and may be willing to pay
$500/month, others need much less capability and expect to pay significantly
less. WiMax devices can be configured to meet both these needs with
price/performance that is optimized for each customer.

A typical consumer class WiMax device would be used for wireless DSL at home.
Always-on connectivity and 1 to 2 Mbps transfer rate is sufficient for most home
and small business needs. Because WiMax is designed for NLOS and the base
transceiver station (BTS) and consumer premise equipment (CPE) will be
relatively inexpensive, wireless ISPs can deploy networks supporting individual
neighborhoods that previously had very limited or no choices for broadband
access. As volumes ramp up, expect CPE to cost $200-300 for unlicensed band
operation.

Air Interface

Although IEEE 802.16 defines multiple air interfaces, the WiMax Forum and
most vendors are currently focused on the 256 carrier orthogonal frequency
division multiplexing (OFDM) air interface, which uses bandwidths between 1.25
and 20 MHz and is designed for NLOS frequencies <11 GHz. There is very little
spectrum available above 5.8 GHz, so most vendors are targeting frequency bands
from 700 MHz to 5.8 GHz. Of the 256 carriers, only 200 are actually used,
leaving unused carrier for guard bands and a null carrier at the channel center
frequency. Used carriers include pilots and actual data.

Figure 3. I/Q Constellation from a signal downlink frame with BPSK, QPSK and 16 QAM modulation

Of these used carriers, subsets can be assigned to individual users or
groups. This is known as sub-channelization and allows more granularity to
dynamically allocate bandwidth and services. Figure 1 shows a typical OFDM
spectrum with 200 carriers in use.

Framing is a traditional packet based structure that uses a preamble and
header followed by data bursts. In the most simply configuration of a downlink
frame, the BTS transmits a preamble, header, and multiple downlink bursts, with
each burst assigned to a different user. In an uplink frame, each CPE transmits
in TDM fashion, such that the burst from user 1 arrives at the BTS, followed by
the burst from user 2, and so forth. Figure 2 shows typical downlink and uplink
frames.

These links can be configured as TDD, FDD, or half-duplex FDD. In TDD, the
downlink and uplink transmission are on the same RF frequency, but not at the
same time. In FDD, the downlink and uplinks are transmitted simultaneously, but
on different RF frequencies.

Each preamble, header, and burst is made up of one or more OFDM symbols.
Modulation on the OFDM carriers is BPSK, QPSK, 16 QAM, or 64 QAM. Signal
conditions and bandwidth requirements will determine which modulation is
selected for individual data bursts. Particularly interesting is the ability of
the BTS to use different modulation formats on each data burst. Figure 3 shows a
constellation diagram taken from an actual measured waveform. This constellation
shows a frame control header with BPSK modulation, Downlink Burst 1 with QPSK
modulation and Downlink Burst 2 with 16 QAM. Because of amplitude scaling, the
BPSK, QPSK, and 16 QAM decision points do not fall on top of each other.

WiMax and the IEEE 802.16 standard will revolutionize the broadband wireless
access industry and open many opportunities to deploy systems in applications
that was previously cost prohibitive.

Allen Henley, strategic planner, wireless business unit, Agilent Technologies