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Emerging wireless technology
ABSTRACT
New and increasingly advanced data
services are driving up wireless traffic, which is being further boosted by
growth in voice applications in advanced market segments as the migration from
fixed to mobile voice continues. This is already putting pressure on some
networks and may be leading to difficulties in maintaining acceptable levels of
service to subscribers.
For the past few decades the lower
band width applications are growing but the growth of broad band data
applications is slow. Hence we require technology which helps in the growth of
the broad band data applications. WiMAX is such a technology which helps in
point-to-multipoint broadband wireless access with out the need of direct line
of sight connectivity with base station.
This paper explains about the WiMAX
technology, its additional features in physical layer and MAC layer and the
benefits of each feature.
This paper focuses on the major
technical comparisons (like QOS and coverage) between WiMAX and other
technologies. It also explains about the ability of the WiMAX to provide
efficient service in multipath environment.
II. Introduction:
For the past couple decades, low-bandwidth applications such as
downloading ring tones and SMS are experiencing sharp growth, but the growth of
broadband data applications such as email and downloading/ uploading files with
a laptop computer or PDA has been slow. The demand for
broadband access continues to escalate worldwide and lower-bandwidth wire line
methods have failed to satisfy the need for higher bandwidth integrated data
and voice services. WiMAX is radio technology that promises two-way Internet
access at several megabits per second with ranges of several miles. It is
believed that the technology can challenge DSL (Digital Subscriber Line) and
cable broadband services because it offers similar speeds but is less expensive
to set up. The intention for WiMAX is to provide fixed, nomadic, portable and,
eventually,
III.What is wimax?
WiMAX is an acronym that stands for “Worldwide Interoperability for Microwave Access”. IEEE 802.16
is working group number 16 of IEEE 802, specializing in point-to-multipoint
broadband wireless access. It also is known as WiMAX.
There are at least four 802.16 standards: 802.16, 802.16a, 802.16-2004
(802.16), and 802.16e.
WiMAX does not conflict with WiFi but actually complements it. WiMAX is a wireless metropolitan area network
(MAN) technology that will connect IEEE 802.11 (WiFi) hotspots to the Internet
and provide a wireless extension to cable and DSL for last km broadband access.
IEEE 802.16 provides up to 50 km of linear
service area range and allows
user’s connectivity without a direct line of sight to a base station. The
technology also provides shared data rates up to 70 Mbit/s.
The portable version of WiMAX, IEEE 802.16 utilizes Orthogonal Frequency
Division Multiplexing Access (OFDM/OFDMA) where the spectrum is divided into
many sub-carriers. Each sub-carrier then uses QPSK or QAM for modulation. WiMAX
standard relies mainly on spectrum in the 2 to 11 GHz range. The WiMAX specification
improves upon many of the limitations of the WiFi standard by providing
increased bandwidth and stronger encryption
For years, the wildly successful 802.11 x or WiFi wireless LAN
technology has been used in BWA applications. When the WLAN technology was
examined closely, it was evident that the overall design and feature set
available was not well suited for outdoor Broadband wireless access (BWA)
applications. WiMAX is suited for both indoor and outdoor BWA; hence it solves
the major problem.
In reviewing the standard, the technical details and features that
differentiate WiMAX certified equipment from WiFi or other technologies can
best be illustrated by focusing on the two layers addressed in the standard,
the physical (PHY) and the media access control (MAC) layer design.
III. a)
WIMAX PHY Layer
The first version of the 802.16 standard released addressed
Line-of-Sight (LOS) environments at high frequency bands operating in the 10-66
GHz range, whereas the recently adopted amendment, the 802.16a standard, is
designed for systems operating in bands between 2 GHz and 11 GHz. The
significant difference between these two frequency bands lies in the ability to
support Non-Line -of-Sight (NLOS) operation in the lower frequencies, something
that is not possible in higher bands. Consequently, the 802.16a amendment to
the standard opened up the opportunity for major changes to the PHY layer
specifications specifically to address the needs of the 2-11 GHz bands. This is
achieved through the introduction of three new PHY-layer specifications (a new
Single Carrier PHY, a 256 point FFT OFDM PHY, and a 2048 point FFT OFDMA PHY);
Some of the other PHY layer features of 802.16a that are instrumental in
giving this technology the power to deliver robust performance in a broad range
of channel environments are; flexible channel widths, adaptive burst profiles,
forward error correction with concatenated Reed-Solomon and convolutional
encoding, optional AAS (advanced antenna systems) to improve range/capacity,
DFS (dynamic frequency selection)-which helps in minimizing interference, and
STC (space-time coding) to enhance performance in fading environments through
spatial diversity. Table 1 gives a high level overview of some of the PHY layer
features of the IEEE 802.16a standard.
b) IEEE 802.16a MAC Layer
The 802.16a standard uses a slotted TDMA protocol scheduled by the base station to allocate capacity to subscribers in a point-to-multipoint network topology. By tarting with a TDMA approach with intelligent scheduling, WiMAX systems will be able to deliver not only high speed data with SLAs, but latency sensitive services such as voice and video or database access are also supported. The standard delivers QoS beyond mere prioritization, a technique that is very limited in effectiveness as traffic load and the number of subscriber’s increases. The MAC layer in WiMAX certified systems has also been designed to address the harsh physical layer environment where interference, fast fading and other phenomena are prevalent in outdoor operation.
IV.WiMAX
Scalability:
At
the PHY layer the standard supports flexible RF channel bandwidths and reuse of
these channels (frequency reuse) as a way to increase cell capacity as the
network grows. The standard also specifies support for automatic transmit power
control and channel quality measurements as additional PHY layer tools to
support cell planning/deployment and efficient spectrum use. Operators can
re-allocate spectrum through sectorization and cell splitting as the number of
subscribers grows.
In
the MAC layer, the CSMA/CA foundation of 802.11, basically a wireless Ethernet
protocol, scales about as well as does Ethernet. That is to say - poorly. Just
as in an Ethernet LAN, more users results in a geometric reduction of throughput,
so does the CSMA/CA MAC for WLANs. In contrast the MAC layer in the 802.16
standard has been designed to scale from one up to 100's of users within one RF
channel, a feat the 802.11 MAC was never designed for and is incapable of
supporting.
a) Coverage:
The BWA standard is designed for optimal performance in all types
of propagation environments, including LOS, near LOS and NLOS environments, and
delivers reliable robust performance even in cases where extreme link
pathologies have been introduced. The robust OFDM waveform supports high
spectral efficiency over ranges from 2 to 40 kilometers with up to 70 Mbps in a
single RF channel. Advanced topologies (mesh networks) and antenna techniques
(beam-forming, STC, antenna diversity) can be employed to improve coverage even
further. These advanced techniques can also be used to increase spectral
efficiency, capacity, reuse, and average and peak throughput per RF channel. In
addition, not all OFDM is the same. The OFDM designed for BWA has in it the ability
to support longer range transmissions and the multi-path or reflections
encountered. In contrast, WLANs and 802.11
systems have at their core either a basic CDMA approach or use OFDM with a much
different design, and have as a requirement low power consumption limiting the
range. OFDM in the WLAN was created
with the vision of the systems covering tens and maybe a few hundreds of meters
versus 802.16 which is designed for higher power and an OFDM approach that
supports deployments in the tens of kilometers.
b) Quality of
service:
The 802.16a MAC relies on a Grant/Request protocol for access to
the medium and it supports differentiated service The protocol employs TDM data
streams on the DL (downlink) and TDMA on the UL (uplink), with the hooks for a
centralized scheduler to support delay-sensitive services like voice and video.
By assuring collision-free data access to the channel, the 16a MAC improves
total system throughput and bandwidth efficiency, in comparison with
contention-based access techniques like the CSMA-CA protocol used in WLANs. The
16a MAC also assures bounded delay on the data. The TDM/TDMA access technique
also ensures easier support for multicast and broadcast services. With a
CSMA/CA approach at its core, WLANs in their current implementation will never
be able to deliver the QoS of a BWA,
802.16 systems.
V. ROLE OF ‘OFDMA’ IN MULTIPATH ENIRONMENT:
Technologies using DSSS
(802.11b, CDMA) and other wide band technologies are very susceptible to
multipath fading, since the delay time can easily exceed the symbol duration,
which causes the symbols to completely overlap (ISI). The use of several
parallel sub-carriers for OFDMA enables much longer symbol duration, which
makes the signal more robust to multipath time dispersion
a). Multipath: Frequency Selective Fading
This type
of fading affects certain frequencies of a transmission and can result in deep
fading at certain frequencies. One reason this occurs is because of the wide
band nature of the signals. When a signal is reflected off a surface, different
frequencies will reflect in different ways. In Figure below, both CDMA (left)
and OFDMA (right) experience selective fading near the center of the band. With
optimal channel coding and interleaving, these errors can be corrected. CDMA
tries to overcome this by spreading the signal out and then equalizing the
whole signal. OFDMA is therefore much more resilient to frequency selective
fading when compared to CDMA.
VI. OFDMA with Adaptive Modulation and Coding (AMC):
Both W-CDMA (HSDPA) and OFDM
utilize Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude
Modulation (QAM). It should be noted here that for WCDMA, AMC is only used on
the downlink, since the uplink still relies on WCDMA which uses QPSK but not QAM.
Modulation and coding rates can be changed to achieve higher throughput, but
higher order modulation will require better Signal to Noise Ratio. Figure illustrates how
higher order modulations like QAM 64 are used closer to the base station, while
lower order modulations like QPSK are used to extend the range of the base
station .
Performance
results conducted for one of the 3GPP Working Groups [2], show that while OFDM
is able to achieve the maximum throughput of 9.6 Mbps (16QAM), WCDMA does not
exceed 3 Mbps. From these results, it appears that even higher discrepancy may
be found when utilizing higher modulation and code rates to yield even higher
throughput for OFDM.
Adaptive Modulation and
Coding (AMC) in a multipath environment may give OFDMA further advantages since
the flexibility to change the modulation for specific sub-channels allows you
to optimize at the frequency level. Another alternative would be to assign
those sub channels to a different user who may have
better channel conditions for that particular
sub-channel. This could allow users to
concentrate transmit power on specific
sub-channels, resulting in improvements to the uplink
budget and providing greater range. This technique
is known as Space Division Multiple Access (SDMA).
In Figure below, you can see how sub-channels could be chosen
depending on the received signal strength. The sub-channels on which the user
is experiencing significant fading are avoided and power is concentrated on
channels with better channel conditions. The signals on the top indicate the
received signal strength, while the bottom part of the figure indicates which
sub-carriers are then chosen for each signal.
With OFDMA, the client device could choose sub channels based on
geographical locations with the potential of eliminating the impact of deep
fades. CDMA-based technologies utilize the same frequency band regardless of
where the user is.
VII.ADVANCED RADIO TECHNIQUES:
a) Transmit and receive diversity
schemes:
Transmit and Receive Diversity schemes are used to
take advantage of multipath and
reflected signals that occur in NLOS environments.
By utilizing multiple antennas (transmit
and/or receive), fading, interference and path loss can be reduced. The OFDMA transmit
diversity option uses space time
coding. For receive diversity, techniques such as maximum ratio combining (MRC) take advantage of two separate receive paths.
b) Smart Antenna Technology:
Adaptive antenna systems (AAS)
are an optional part of the 802.16 standard. AAS equipped base stations can
create beams that can be steered, focusing the transmit energy to achieve
greater range as shown in the figure. When receiving, they can focus in the
particular direction of the receiver. This helps eliminate unwanted interference
from other locations.
VIII. Conclusion:
Thus
WiMAX systems for portable/nomadic use will have better performance,
interference rejection, multipath tolerance, high data quality of service
support (data oriented MAC, symmetric link) and lower future equipment costs
i.e., low chipset complexity, high spectral efficiencies. And hence WiMAX can
complement existing and emerging 3G mobile and wireline networks, and play a
significant role in helping service provides deliver converged service
offerings
IX.
BIBLIOGRAPHY:
Ø Understanding
“WiMAX”- Joe Laslo & Michael gartenberg
Ø www.intel.com/ebusiness/pdf/wireless/intel
Ø www.intel.com/netcomms/technologies/wimax
Ø P.
S. Henry, “Wi-Fi: What’s next?” IEEE Communications Magazine
Ø WiMAX Handbook –Frank ohrtman
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