896960
Introduction to Algorithmic
Wireless Communications
David Amzallag
Lecture 10:
Worldwide Interoperability for Microwave
Access
July 27, 2008
david.amzallag@bt.com
www.cs.technion.ac.il/~amzallag/awc
[1]
Worldwide Interoperability for Microwave Access (WiMAX)
– It is a trade name for the IEEE802.16 international standards
– WiMAX aims to provide a metropolitan access network which will provide
higher bandwidth and larger coverage than are currently available with
existing wireless technologies such as Wi-Fi and 3G
– Its longer reach will allow hot zones to be developed as opposed to hot
spots which are currently available with Wi-Fi
– WiMAX gets its improved performance through its physical layer technology
which is orthogonal frequency division multiplexing (OFDM) and through the
use of smart antennas
[2]
Previously in ER
– Fixed WiMAX was originally developed for fixed wireless broadband access
under the IEEE802.16a or IEEE802.16-2004 standard
– It was believed that it could act as a possible complement to fixed line DSL
(digital subscriber line) or a low-cost backhaul solution for local area access
technologies such as Wi-Fi
– It was then developed to support mobility under the IEEE802.16e standard
which was finalized in December 2006. This standard provided support for
fast mobility, handovers and roaming
– Over 330 companies back the standard and certification work through the
WiMAX Forum. These backers include equipment vendors such as Intel,
Alcatel, Nokia, Nortel, and Samsung, as well as network providers such as
BT, Deutsche Telekom, BellSouth, and Sprint
www.wimaxforum.org
[3]
Orthogonal Frequency Division Multiplexing (OFDM)
– WiMAX uses OFDM as the physical layer. OFDM allows large amounts of
digital data to be transmitted over a chunk of spectrum with greater
efficiency than existing wireless technologies
– OFDM works by splitting the radio signal into multiple smaller signals that
are then transmitted simultaneously at different frequencies to the receiver
– An OFDM-based system is able to squeeze a 72 Mbit/s data rate out of 20
MHz of channel spectrum under ideal circumstances
– The key to OFDM is that the different frequencies can be transmitted and
received completely independently of each other (this is the orthogonal
property)
– The key advance in OFDM is to divide up the spectrum into a large number
of overlapping channels that have a slow enough data rate to not require
any form of equalization — they are what is termed ‘flat’ fading channels
[4]
[5]
Fixed vs. Mobile
– Fixed WiMAX serves different users by sharing all the channels in time —
time division multiple access (TDMA) — whereby one gets access to the
channel, then another and another and so on
– Fixed WiMAX also uses a different frequency for uplink, and downlink
transmissions, so that they can take place independently — this is known as
frequency division duplex (FDD)
– Mobile WiMAX goes further and uses a technique known as orthogonal
frequency division multiple access (OFDMA) — this allows multiplexing in
both time and frequency, whereby the channels are divided into groups and
users are allocated slots in time for one or more of these groups of channels
– In addition, mobile WiMAX uses a single piece of spectrum for both uplink
and downlink and alternates uplink and downlink frames in a technique
known as time division duplex (TDD)
[6]
[7]
Smart antennas
– The other ‘big thing’ that WiMAX introduces is smart antennas — OFDM is
very well suited to smart antenna techniques, much more so than existing
3G technology
– Smart antennas could increase the coverage, range or throughput of a basestation. In ideal conditions throughput can be increased by a factor of 2 or
more
– Typical cellular base-stations are split into different sectors with each sector
having its own antenna and operating on a different frequency in GSM
– With normal antennas you can adjust the power and the tilt of the antenna
but you cannot adjust the beam itself
[8]
So, what is smart antenna?
– Smart antennas work differently — they consist of multiple transmit
elements and the key to their operation is to adjust the amplitude and phase
of each element so as to create constructive interference where there is a
user, or group of users, and destructive interference where there are no
users
– This results in more of the overall transmit power being received, reduces
multipath and can avoid other sources of interference (such as neighbouring
cells reusing the same frequencies)
– If interference becomes a problem a null beam is sent out to reduce the
interference. The technique of beam forming should increase the coverage
of a WiMAX radio access station
[9]
Spatial multiplexing
– Multipath signals need not be bad news for radio systems — it is actually
possible to send and receive different data streams on each path
– If there are N transmitter and N receiver elements it is possible to beam
form effectively at both the transmitter and receiver, and align one beam at
both ends to a single multipath
– N elements at a receiver or transmitter are able to form N beams
– If there are also N independent multipaths, then the capacity can, in theory,
be N times that of a system with a normal antenna
[10]
More spatial multiplexing
– In practice, however, some multipaths are weak and can only be used with
low data rates; multipaths also change rapidly with terminal movement and
they are only really present in urban environments
– Such systems are typically known as MIMO (multiple in multiple out)
– There is a trade-off with using smart antennas, with beam forming and
spatial multiplexing, between the benefit gained and the signalling overhead.
Therefore it is difficult to say at this stage how much of the potential
benefits, in terms of coverage and throughput, will be realized
[11]
WiMAX spectrum
– There are three frequency bands defined in the standard, the 2.5 GHz and
3.5 GHz licensed spectrum, and the unlicensed 5 GHz spectrum
– The mobile standard (IEEE802.16e) only supports the licensed frequency
bands
– It also supports channel sizes between 1.5 MHz and 20 MHz which is an
important feature of WiMAX — allowing it to operate in small segments of
spectrum that might become available, although this greatly reduces the
efficiency of the system
– Ideally, for a mobile WiMAX system, 10—20 MHz of spectrum would be
needed, since mobile WiMAX uses only a single segment of spectrum for
both uplinks and downlinks
[12]
Any effect on the number of base stations?
– Yes. The frequency band used also affects the spacing of base stations
– As a general rule, the cell radius is proportional to the carrier frequency
used. Thus a WiMAX network at 3.5 GHz could require roughly 60—80%
more base-stations than a 2.1 GHz 3G network
[13]
Fixed WiMAX applications
1. Digital subscriber line (DSL) alternative or fill-in
One of the main uses of WiMAX will be as a fill-in or an alternative to cable
and DSL. Fixed WiMAX offers a good solution for broadband access in
greenfield sites. At least 15% of the US market, and huge portions of the
rest of the world, do not have a broadband infrastructure. WiMAX is
therefore a good solution as it can be rolled out quickly with less expense
than a wired network
2. Mobile backhaul
The cost of backhaul for celluar and Wi-Fi networks represents a significant
portion of their recurring costs. WiMAX can provide point-to-point links of up
to 30 miles and can provide data rates capable of supporting multiple E1s
(standard 2 Mbit/s links). Operators can therefore use WiMAX equipment to
backhaul base-station traffic to their network
[14]
3. Temporary broadband
Due to the ease of deploying WiMAX, the technology could be used for
temporary situations such as sporting events, construction sites and trade
shows.
Examples of where this has been done include the Tour de France, the
Winter Olympics in Turin, the Ironman Triathlon World Championship and the
Sundance Film Festival
4. Public safety
Police departments are considering equipping patrol cars with computers
with wireless access. WiMAX offers a good solution for this due to its more
robust security. WiMAX could also provide video surveillance cameras with
broadband connectivity to control centres and support vehicles enabling
officers to view situations and make decisions in real time
[15]
Mobile WiMAX applications
1. 4G networks
Sprint Nextel has announced that they have chosen WiMAX for their next
generation — 4G — mobile network build. Korean Telecom has also been
undertaking trials of WiBro as a 4G technology
2. Urban mobile broadband access
Given the high cost of a nationwide network build, it is likely that mobile
WiMAX would be deployed firstly in urban areas as a means of offering
broadband-like data rates on the move (500 kbit/s+)
[16]
Epilogue
– It is clear that, to compete with existing delivery mechanisms, either WiMAX
has to offer a lower cost per MB than existing technologies, or new services
(not possible on current systems) must be introduced
– What is needed is a service that both requires high data rates, which only
WiMAX can provide in the medium term, and generates a high revenue per
MB to make the service profitable. Mobile TV or mobile HDTV are potential
candidates for this class of service
[17]