WiMAX
CTP-ITU School on New Perspectives on
Wireless Networking
Abdus Salam ICTP, Triest, February 2008
Ermanno Pietrosemoli
Latin American Networking School
(Fundación EsLaRed) – ULA
Mérida Venezuela www.eslared.org.ve
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Agenda
Motivation
802.16 Standard
WiMAX
Comparison with WiFi
Examples
Concluding remarks
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Motivation
Back in 2000, the Venezuelan spectrum regulator
conducted a public auction of the frequency band
between 3400 and 3500 MHz with the aim of
providing voice and data services via wireless
technology, in what was known as WLL (Wireless
Local Loop).
Although several operators paid large sums for the
right to use 25 MHz in one or more of the five regions
of the country, and two of the winning deployed in
several cities, the overall plan failed, and in 2006 the
licenses were revoked.
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Motives
1) Lack of standards for providing this type of
service.
2) Requirements for line of sight. The existing
commercial solutions required line of sight
between the base station and the subscriber,
implying that the installations for each
subscriber had to be carried out by qualified
personnel
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IEE802.16
EEE802.16 was born as a solution for metropolitan networks,
with a reach of tens of kilometers and transmission rates of
several Mbit/s. Originally aimed at operating frequencies above
10 GHz, did not have much impact. In 2003, an amendment of
the standard extended the range of operation to the frequencies
between 2 and 11 GHz. This allows for greater range and, and,
with the use of OFDM (Orthogonal Frequency Diversity
Modulation), it can overcome the line of sight requirement. The
purpose of the new standard was to address the basic
constraints of 802.11 as an outdoor access technology:
Range
Quality of Service (QoS).
For this purpose, 802.16 uses a different medium access
method and offers a great variety of modulation and channel-
sharing techniques, hence great versatility with both licensed
and unlicensed spectrum solution
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802.16 goals
IEEE 802.16 is an ambitious protocol geared
to satisfy the needs of broadband wireless
operators and industry. In many cases the
decision towards a specific technology
depends on tens of aspects such as efficient
spectrum utilization, legislation, access to
technology and investment capacity.
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802.16 Standard: MAN Pt-Mp
Base Station connected to the public Network
Feeds Subscriber stations (SS)
Both Types of BS
SS Serves a building
Multiple services with QoS
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Designed from the Ground Up for Metropolitan Area
Networks
In January 2003, the IEEE approved the 802.16a standard
which covers frequency bands between 2 GHz and 11 GHz.
This standard is an extension of the IEEE 802.16 standard
for 10 – 66 GHz published in April 2002. These sub 11 GHz
frequency ranges enable non line-of-sight performance,
making the IEEE 802.16a standard the appropriate technology
for last-mile applications where obstacles like trees and
buildings are often present and where base stations may
need to be unobtrusively mounted on the roofs of homes
or buildings rather than towers on mountains.
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EEE 802.16 d-2004
This amendment considers only one medium access mechanism,
but three physical layers: one based on single carrier, another
that uses OFDM (Orthogonal Frequency Division Multiplexing)
with 256 subcarriers, and a third one with 2048 OFDM
subcarriers. This agrees with the ETSI (European
Telecommunication Standards Institute) HiperMAN standard,
except for the fact that the latter uses OFDM only. The
modulation and codification schemes can be adaptive depending
on the distance from the subscriber to the base station, giving
priority to service to the farthest subscribers over transmission
rate. Spectral efficiency is greater than the one for 802.11a/g, but
smaller than the one for 802.11n
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EEE 802.16 d-2004 MAC
The medium access mechanism is TDM/TDMA (Time
Division Multiplexing/Time Division Multiple Access),
which means that the base station transmits to
different subscribers in different time slots. When a
SS needs to send a frame, it sends a request to the
BS, which will allocate a certain number of time slots
in the uplink channel depending on the solicited
service and availability. The capabilities of the SS will
be manifested at initialization.
There are three ways to handle the two directions of
traffic: FDD (Frequency Diversity Duplexing) , TDD
(Time Diversity Duplexing) and Half Duplex
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Throughput, Scalability, QoS and Security
By using a robust modulation scheme, IEEE
802.16 delivers high throughput at long ranges with a high
level of spectral efficiency that is also tolerant of signal
reflections. Dynamic adaptive modulation allows the base
station to tradeoff throughput for range. For example, if
the base station cannot establish a robust link to a distant
subscriber using the highest order modulation scheme, 64
QAM, the modulation order is reduced to 16 QAM or QPSK,
which reduces throughput and increases effective range.
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Transmission Rates
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802.11 and 802.16
Other than the coverage and mobility, a key
difference between the 802.11 and the 802.16 is
the MAC. Unlike the 802.11, which supports
10’s of users, the 802.16 MAC is designed to
support thousands of users using a grant-
request mechanism. The QoS support for voice
and video is designed from ground up, and
differentiated service levels are also introduced
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General Features
Wideband Channels (5~28 MHz)
Múltiple Access, TDM/TDMA
Adaptive both Upstream and Downstream
TDD, FDD o Half Duplex
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Burst Adaptive Profile
(Burst Profile)
Modulation and FEC
Dynamically assigned according to the
conditions of the link : Capacity and
robustness interchange
SS features are known at the time the link is
established
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Duplexing Techniques
Downlink:
a burst is provided at every SS
Uplink:
every SS is provided with a variable length
time slot
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Quality of Service
The grant/request characteristics of the 802.16
Media Access Controller (MAC) enables an operator
to simultaneously provide premium guaranteed levels
of service to businesses such as T1-level service,
and high-volume “best-effort” service to homes,
similar to cable-level service, all within the same BS
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Features of 802.16 e
Scalable OFDMA
Variable channel configurations for multiuser
diversity exploitation
Multiple-input multiple-output (MIMO) and
advanced antenna systems
Advanced channel coding and hybrid-ARQ
QoS and service classes.
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Frame structure
The scalable OFDMA supports both TDD and FDD frame
structures, with the frame size ranging from 2 ms to 20 ms.
Each frame is divided into four regions:
DL (downlink transmission),
TTG (transmit transition gap),
UL (uplink transmission),
RTG (receive transition gap).
The TTG and RTG provide guard periods against round trip
delay in TDD operation as well as a ramping down period of
the power amplifiers.
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FDD Frames
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TDD
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OFDM
The IEEE 802.16-2004 standard-specified OFDM as the
transmission method for NLOS connections. The OFDM signal is
made up of many orthogonal characters, and each individual carrier
is digitally modulated with a relatively slow symbol rate. This
method has distinct advantages in multipath propagation because,
in comparison with the single carrier method at the same
transmission rate, more time is needed to transmit a symbol. The
BPSK, QPSK, 16QAM, and 64QAM modulation modes are used,
and the modulation is adapted to the specific transmission
requirements. Transmission rates of up to 75 Mbit/s are possible.
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OFDM Flavors
In IEEE 802.16-2004, a distinction is made between
two methods: OFDM and OFDMA. In the normal
OFDM mode, 200 carriers are available for data
transmission and both TDD and FDD methods are
used. In the OFDMA mode, various subscribers can
be served simultaneously by assigning each
subscriber a specific carrier group
(subchannelization) that carries the data intended for
that subscriber. The number of carriers can vary over
a wide range depending on permutation zones and
FFT base (128, 512, 1024, 2048).
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OFDM Flavors
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OFDMA Advantages
OFDMA supports multiple access, which allows
user devices to transmit only through the sub-
channel(s) allocated to them. In OFDMA
with 2048 carriers and 32 sub-channels, if only
one sub-channel is allocated to a device,
all the transmit power will be concentrated in
1/32 of the spectrum available and may
bring a 15 dB gain over OFDM
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OFDMA Advantages
SOFDMA brings an additional advantage over
OFDMA. It scales the size of the Fast
Fourier Transform (FFT) to the channel
bandwidth in order to keep the carrier spacing
constant across different channel bandwidths
Constant carrier spacing results in a higher
spectrum efficiency in wide channels, and a cost
reduction in narrow channels.
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802.16: Specifically Designed for the Outdoor MAN
802.11 802.16 Technical
Sub ~ 50 m indoor . 802.16 PHY tolerates greater
Up to 40 km
Range (add access points for
greater coverage)
Average cell size 7 – 12 km
multi-path delay spread
(reflections)
NLOS performance
Optimized for indoor,
Coverage short range
Standard support for advanced antenna
techniques
802.16: 256 OFDM (vs. 64 OFDM);
adaptive modulation
Channel b/w is flexible to accommodate Only 3 non-overlapping 802.11b
Channel bandwidth is wide
both licensed and license exempt channels; 5 for 802.11a
Scalability (20 MHz) and fixed -> Cell
planning is constrained
bands -> easier cell planning 802.16: limited by available
spectrum
2.7 bps/Hz peak
3.6 bps/Hz peak 802.16: MAC efficiency constant
Bit rate Up to 54 Mbps in 20 MHz
channel
Up to 50 Mbps in a 14 MHz channel with PHY rate increase
802.11: contention-based MAC
QoS No QoS support -> 802.11e
working to standardize
QoS built into MAC -> voice/ video,
differentiated services possible (CSMA)
802.16: scheduled MAC
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Coverage
In addition to supporting a robust and dynamic
modulation scheme, the IEEE 802.16 standard also
supports technologies that increase coverage, including
mesh topology and “smart antenna” techniques. As
radio technology improves and costs drop, the ability to
increase coverage and throughput by using multiple
antennas to create “transmit” and/or “receive diversity”
will greatly enhance coverage in extreme environments.
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QoS
Each connection in 802.16 is associated with
a data service and each data service is
associated with a set of QoS parameters that
reflect the service requirement. The 802.16
standard defines four types of services:
Unsolicited Grant Service (UGS)
Real-time Polling Service (rtPS)
Non-realtime
Polling Service (nrtPS)
Best Effort (BE).
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IEEE 802.16 Summary
802.16 aims to overcome the 802.11
limitations in outdoor applications by using a
deterministic medium access layer, which
ensures quality of service, important feature
for voice applications. In addition, it is well
suited for long distance transmission and a
high number of nodes. 802.16 also introduces
new modulation schemes and different
channel bandwidths, which makes it a more
versatile but more complex system.
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WAN
Wide Area Network
IMT-2000
802.22 MAN
Metropolitan Area Network
802.20 HiperMAN
802.16e HiperACCESS
802.16d
LAN
Local Area Network
802.11
HiperLAN
802.15
HiperPAN
PAN
Personal Area Network
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Worldwide Interoperability for
Microwave Access
The Worldwide Interoperability for Microwave
Access Forum (WiMAX) was formed with the
following objective:
Promote the wide-scale deployments of
fixed broadband wireless access networks
operating above 2 GHz by using a global
standard and certifying the interoperability
of products and technologies.
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WiMAX scope
According to the application two different
markets are identified:
a) In developing countries, the limited amount of
land lines makes room for a fixed wireless
access technology, based on the IEEE 802.16-
2004 amendment known as 802.16d. b) In
developed countries, mobile broadband services
are better served with the 2005 amendment of
the standard, dubbed 802.16 e .
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WiMAX Roadmap
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Example: Redline
Redline’s 802.16 Systems are Designed
for High Performance in Real World Environments
RANGE CAPACITY RELIABILITY VERSATILITY
Range: beyond High data rate – CIR/PIR support Supports TDM transport
30 mi/50 km Ethernet (T1 & E1)
(QPSK) 37.5 - 45.5 Mbps Standards based
Over air rate – interface Operates on Licensed
256OFDM for ~70 Mbps (802.1p/Q) and Unlicensed Bands
Non-Line-of -Sight
operation: reflective, Can be configured Dynamic Adaptive PTP and PMP modes
refractive & diffractive for one-way latency of Modulation (DAM)
<4 msec Easy deployment –
Forward error minutes vs. hours or
High Spectral Efficiency
– correction, Time days
Division Duplex (TDD)
5 bps/Hz Air Rate,
2.7 – 3.3 bps/Hz
Ethernet Data Rate Encryption for
enhanced security
(DES/AES)
256 OFDM for
robust NLOS
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Redline’s AN-100
Worlds First Standards Compliant Complete
WirelessMAN Solution
First Shipping IEEE 802.16a Product (Feb 2004)
Mature Product - Over 1yr in Operation
Winner of 2004 SUPERQuest Award
“Backbone/Edge Networking Equipment” category
Delivers…
Ability to Guarantee Multiple QoS Levels
Low Latency, High Capacity (14MHz Channel)
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Modulation schemes
(Reach & Rate Data – Field Proven)
7 MHz Channel Estimate PMP Link Budget PTP Link Budget
64 QAM/ -75 dBm .6 mile NLOS 1 mile NLOS
3 miles OLOS 8 miles OLOS
18.7 Mbps
5 miles LOS 12 miles LOS
22.5 Mbps
16QAM/ -82 dBm .8 mile NLOS 1.5 mile NLOS
5 miles OLOS 12 miles OLOS
12.4 Mbps
8.5 miles LOS 17 miles LOS
15 Mbps
QPSK/ -89 dBm 1.1 mile NLOS 2.4 mile NLOS
6.2 Mbps 9 miles OLOS 18 miles OLOS
7.5 Mbps 14 miles LOS 24.5 miles LOS
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AN-100 SC And SS System
Components
Outdoor Radio
Sector Subscriber
Equipment
Controller Station
Mount Common
to SS & SC
Indoor Terminal
Equipment
Sector Controller Indoor Unit Subscriber Station Indoor Unit
AN-100C AN-100S
Split IF Design (Radio ODU,
Terminal IDU)
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NLOS
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Example of NLOS capability
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WiBRO
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WiBro is corean standard based
on 802.16e
Demoed in
October 2005.
High speed
handoff
Three corean
operators
paid 100 M$ each
fo 2,3 GHz
Spectrum during
7 years
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WiBro frequency allocation
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New spectrum allocation for
WiMAX by ITU
http://www.dailywireless.org/2007/09/10/luxor-wimaxed/
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WiMax is now part of ITU
standards
IMT-2000 extension band between 2,5 GHz
and 2,69 GHz
Part of IMT-2000 standards
W-CDMA
CDMA- 2000
TD-SCDMA
OFDMA TDD WMAN
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Conclusions
From a technical point of view it is clear that WiMAX
meets most current challenges relating outdoor
wireless links. When it comes to selecting a
technology, one must consider not only the technical
aspects, but the whole scenario we are planning to
build. We should consider what are the functionalities
we want to implement, and whether the price and
benefits of these functionalities is justified. As of
today, WiFi solutions, or the ones that make use of
proprietary extensions are the most cost effective.
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