09-10-2012, 05:18 PM
SPATIAL PROCESSING, POWER CONTROL, AND CHANNEL ALLOCATION FOR OFDM WIRELESS COMMUNICATIONS
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Introduction
Broadband Wireless Communications
In recent years, wireless communication and networking has experienced a rapid growth,
and it promises to become a globally important infrastructure. The advances in integrated
circuit technology and digital signal processing algorithms have made wireless communication
technology accessible to millions of people. The light weight, long operational time, and
a®ordable prices of portable devices have resulted in ever increasing demand for wireless
services.
The spectral growth of video, voice, and data communication over the Internet, and
equally rapid pervasion of mobile telephony, justify great expectation for mobile multimedia.
However, the current wireless communication systems and standards, such as the 3G cellular
standard or the WLAN IEEE802.11 standard family, do not fully support the new emerging
multimedia applications. The quality of service (QoS) they can provide is not competitive
with the QoS wire-line service providers can o®er. The large volume and high sensitivity of
multimedia data require the development and deployment of wireless communication systems
that can guarantee reliable data transmission at high data rates. Research and development
are taking place all over the world to de¯ne the next generation of Wireless Broadband Mul-
timedia Communication Systems (WBMCS) consisting of various components at di®erent
scales rating from global networks to residential small networks. The demand for wireless
mobile, Internet, and multimedia communications is growing exponentially. Therefore it is
imperative that wireless, Internet, and multimedia should be brought together. Thus, in
the near future, wireless Internet Protocol (IP) and Wireless Asynchronous Transfer Mode
(WATM) will play an important role in the development of WBMCS.
While present communication systems are primarily designed for one speci¯c application,
such as speech on a mobile telephone or high-rate data in a Wireless Local Area Network
(WLAN), the next generation of WBMCS will integrate various functions and applications.
Designing wireless communication systems that supporting large data rates with su±cient
robustness to radio channel impairments, faces the challenge of devising modulation, coding,
and signal processing techniques that can combat the adverse e®ects of the radio signal
propagation environment, such as multipath fading and interference, more e®ectively.
To implement the wireless broadband communication systems, the following challenges
must be considered: Frequency allocation and selection, Channel characterization, Multiple
access techniques, Protocol and networks, and ¯nally System development with e±cient
modulation, coding and smart antenna techniques.
Standardization and Frequency Bands
Inspired by the successful application of the cellular concept, the wireless evolution has so far
gone through two generations. First generation (1G) wireless systems (like AMPS, TACS)
use analog transmission and support voice services. Second generation (2G) systems (like
GSM, IS-95. PDC) employ digital technology and provide circuit-switched data communi-
cation services at low speeds in addition to voice. On the other hand, the so-called 2.5G
system (like EDGE/GPRS, HDR), which currently operate in most countries, support more
advanced services, such as moderate rate (up to 100 kbps) packet-switched data.
In 1G and 2G systems, the main focus was on increasing system capacity in terms of
the number of established connections, which have constant, low rate streams. However,
recent evolutions in the telecommunications arena indicate a clear trend towards enhanced,
rate-demanding services that are expected to °ourish in the next years. The idea of the
third generation (3G) systems became evident by the need to support high and diverse
data rates for heterogeneous application, such as home-networking, video conferencing, fast
wireless/mobile Internet access and multimedia communications. 3G systems, such as UMTS
and CDMA2000 are envisioned to support rates in the order of 1 or 2 Mbps [1].
There are several forums for the standardization of wireless broadband systems; namely
IEEE802.11 [2], European Telecommunication Standards Institute Broadband Radio Ac-
cess Networks (ETSI BRAN) [3], Multimedia Mobile Access Communications (MMAC) [4],
IEEE802.16/WiMAX [5], and IEEE802.20 [6]. IEEE 802.11 made the ¯rst WLAN standard
for 2.4 GHZ Industrial, Scienti¯c, and Medical band (ISM), and 5GHZ Unlicensed National
Information Infrastructure (UNII) band. The legacy version of WLAN speci¯es the medium
access control and three di®erent physical layers; direct sequence spread spectrum, frequency
hopping, and infrared which give a data rate of upto 2Mbps. Later, the committee pro-
posed new versions, namely IEEE802.11b using high speed direct sequence spread sequence
physical layer for the speed of 11Mbps, IEEE802.11a with Orthogonal Frequency Division
multiplexing (OFDM) for 54Mbps in 5GHZ band, and IEEE802.11g for high the speed of
up to 54Mbps in ISM band.
ETSI BRAN and MMAC jointly used OFDM for high speed wireless transmission in 5
GHZ band. ETSI High Performance Local Area Network type 2 (HIPERLAN/2) consists
of a family of standards one of which is an OFDM-based standard that is very similar to
IEEE802.11a. MMAC is used in Japan and supports both IEEE802.11a and HIPERLAN/2
standards. Note that Japan has only 100 MHZ available in the 5-GHZ band, while the
United States and Europe provides 300 and 455 MHZ, respectively.
Fixed wireless technologies (also called ¯xed wireless access, wireless broadband access, or
broadband wireless access) are not new but because of recent advances, this technology has
been successful in rural communities that are out of reach of installed ¯xed lines. When used
at high frequencies, ¯xed wireless can carry more data but has limited range and requires
more complex equipment and line of sight. At lower frequencies, the range is further and
the equipment is cheaper, but the transmission rates are low. Multi-point Microwave Dis-
tribution Systems (MMDS) and local multi-point distribution systems (LMDS) were viewed
as promising technologies, but a lack of uniform standards has hampered their deployment.
IEEE 802.16 and 802.16a are new ¯xed-wireless standards that should be able to transmit
32-56 km with maximum data rates close to 70 Mbit/s. Again, the higher frequencies require
line of sight but it provides high-capacity links. At lower frequencies, line of sight is not
required but speeds are lower. This technology is a high-speed wireless backbone designed
to link distant ISPs to the Internet. Wireless LAN technologies would then be used for the
connection to the user.
The IEEE 802.16 standard is about to revolutionize the broadband wireless access indus-
try. The 802.16 standard, the Air Interface for Fixed Broadband Wireless Access Systems, is
also known as the IEEE Wireless-MAN (WMAN) air interface. This technology is designed
from the ground up to provide wireless last-mile broadband access in the Metropolitan Area
Network (MAN), delivering performance comparable to traditional cable, DSL, or T1 of-
ferings. The principal advantages of systems based on 802.16 are multi-fold: the ability to
quickly provision service, even in areas that are hard for wired infrastructure to reach; the
avoidance of steep installation costs; and the ability to overcome the physical limitations
4
of traditional wired infrastructure. Providing a wired broadband connection to a currently
underserved area through cable or DSL can be a time-consuming, expensive process, with
the result that a surprisingly large number of areas in the US and throughout the world
do not have access to broadband connectivity. 802.16 wireless technology provides a °ex-
ible, cost-e®ective, standards-based means of ¯lling existing gaps in broadband coverage,
and creating new forms of broadband services not envisioned in a wired world. Drawing
on the expertise of hundreds of engineers from the communications industry, the IEEE has
established a hierarchy of complementary wireless standards. These include IEEE 802.15
for the Personal Area Network (PAN), 802.11 for the Local Area Network (LAN), 802.16
for the Metropolitan Area Network, and the proposed IEEE 802.20 for the Wide Area Net-
work (WAN). Each standard represents the optimized technology for a distinct market and
usage model and is designed to complement the others. A good example is the proliferation
of home and business wireless LANs and commercial hot spots based on the IEEE 802.11
standard.
WiMAX (the Worldwide Interoperability for Microwave Access Forum) is a non-pro¯t
corporation formed by equipment and component suppliers, including Intel Corporation, to
promote the adoption of IEEE 802.16 compliant equipment by operators of broadband wire-
less access systems. The organization is working to facilitate the deployment of broadband
wireless networks based on the IEEE 802.16 standard by helping to ensure the compatibility
and interoperability of broadband wireless access equipments.
In an e®ort to bring interoperability to broadband wireless access, WiMAX is focusing
its eorts on establishing a unique subset of baseline features grouped in what is referred to
as System Pro¯les that all compliant equipments must satisfy. These pro¯les will establish a
baseline protocol that allows equipment from multiple vendors to interoperate, and that also
provides system integrators and service providers with the ability to purchase equipment from
more than one supplier. System Pro¯les can address the regulatory spectrum constraints
faced by operators in di®erent geographies. WiMAX will establish a structured compliance
procedure based upon the proven test methodology results in a complete set of test tools
available to equipment developers so they can design in conformance and interoperability
during the earliest possible phase of product development. Ultimately, the WiMAX suite
will enable service providers to choose from multiple vendors of broadband wireless access
equipment that conforms to the IEEE 802.16a standard and that is optimized for their unique
operating environment.
By choosing interoperable, standards-based equipment, the operator reduces the risk of
deploying broadband wireless access systems. Economies of scale enabled by the standard
help reduce monetary risk. Operators are not locked in to a single vendor because base
stations will interoperate with subscriber stations from di®erent manufacturers. Ultimately,
operators will bene¯t from lower-cost and higher-performance equipment, as equipment man-
ufacturers rapidly create product innovations based on a common, standards-based platform.
On December 2002, the IEEE Standards Board approved the establishment of IEEE
802.20, the Mobile Broadband Wireless Access (MBWA) Working Group. The mission of
IEEE 802.20 is to develop the speci¯cation for an e±cient packet based air interface that is
optimized for the transport of IP based services. The goal is to enable worldwide deploy-
ment of a ordable, ubiquitous, always-on and interoperable multi-vendor mobile broadband
wireless access networks that meet the needs of business and residential end user markets.
MBWA Scope is the speci¯cation of physical and medium access control layers of an air
interface for interoperable mobile broadband wireless access systems, operating in licensed
bands below 3.5 GHZ, optimized for IP-data transport, with peak data rates per user in
excess of 1 Mbps. It supports various vehicular mobility classes up to 250 Km/h in a MAN
environment and targets spectral e±ciencies, sustained user data rates and numbers of active
users that are all signi¯cantly higher than achieved by existing mobile systems.
The 802.20 interface seeks to boost real time data transmission rates in wireless metropoli-
tan area networks to speeds that rival DSL and cable connections (1Mbps or more) based
on cell ranges of up to 15 kilometers or more, and it plans to deliver those rates to mobile
users even when they are travelling at speeds up to 250 kilometers per hour (155 miles per
hour). This would make 802.20 an option for deployment in high-speed trains. The 802.16e
project authorization request speci¯es only that it will support subscriber stations moving
at vehicular speeds. Essentially, 802.16e is looking at the mobile user walking around with
a PDA or laptop, while 802.20 will address high-speed mobility issues.
Wireless Networks: The Layered Architecture
The inherent volatility of the wireless medium constitutes the major di±culty in the design
of wireless networks. The quality of a narrow-band wireless link between a transmitter and a
receiver depends both on radio propagation parameters (path loss, shadow fading, multipath
fading) and cochannel interference.
The OSI (Open Systems Interconnection) model de¯nes a layered architecture and the
protocols de¯ned in each layer are responsible for communicating with the same peer protocol
layer running in the opposite computer, and providing services to the layer above it (except
for the top-level application layer). The techniques of layered protocols were developed to
logically decompose a complex network into smaller, more understandable parts (layers), to
provide standard interfaces between network functions, and to allow each layer to perform
the same functions as its counterpart in other nodes of the network,
In the following we will brie°y describe the characteristics of the main layers for a wireless
networks.
Physical Layer
Physical layer-based techniques are employed on a link basis, in order to achieve high data
rate, while maintaining an acceptable Bit Error Rate (BER) at the receiver, irrespective of
link quality. The parameters that are considered as adaptable are modulation and coding,
Interleaving, transmission power level, use of multiple antenna, ...
Modulation Level
Modulation is a fundamental component of a digital communications system. It is the
process of mapping the digital information to analog form so it can be transmitted over the
channel. Consequently every digital communication system has a modulation that performs
the task. Closely related to modulation is the inverse process, called demodulation, done by
the receiver to recover the transmitted digital information. Modulation is done by changing
the amplitude, phase or frequency of the transmitted Radio Frequency (RF) signal. The main
design issue of the modulator is the choice of the constellation, which is the set of M points
(constellation size) that can be transmitted on a single symbol. This choice a®ects several
important properties of a communication system; for example BER, Peak to Average Power
Ratio (PAPR), and RF spectrum shape. Each block of b = log2M bits from the coded
bit stream constitutes a symbol and each symbol is mapped to one of M waveforms for
transmission over the channel. The single most important parameter for a constellation is
the "minimum distance", dmin, which is the smallest distance between any two points in
the constellation. It depends on several factors; constellation size, average power, and the
shape of constellation. The modulation and demodulation can be done either coherently, or
non-coherently.