Dr
Rupesh Singh
Associate Professor & HOD ECE, HMRITM,
Dr Nidhi Singh
Associate
Professor,
Abstract
A number of new
technologies are being integrated by the telecommunications industry as it
prepares for the next generation mobile services. One of the key changes incorporated
in the multiple channel access techniques is the choice of Orthogonal Frequency
Division Multiple Access (OFDMA) for the air interface. This paper presents a
survey of various multiple channel access schemes for 4G networks and explains
the importance of these schemes for the improvement of spectral efficiencies of
digital radio links. The paper also discusses about the use of Multiple
Input/Multiple Output (MIMO) techniques to improve signal reception and to
combat the effects of multipath fading. A comparative performance analysis of
different multiple access schemes such as Time Division Multiple Access (TDMA),
FDMA, Code Division Multiple Access (CDMA) & Orthogonal Frequency Division
Multiple Access (OFDMA) is made vis-à-vis design parameters to highlight the
advantages and limitations of these schemes. Finally simulation results of
implementing some access schemes in MATLAB are provided.
I. INTRODUCTION
4G (also known
as Beyond 3G), an abbreviation of Fourth-Generation, is used for describing the
next complete evolution in wireless communications. A 4G system will be a
complete replacement for current networks and will be able to provide a comprehensive
and secure IP solution. Here, voice, data, and streamed multimedia can be given
to users on an "Anytime, Anywhere" basis, and at much higher data
rates than the previous generations [1], [2], [3]. 4G wireless communication standard
has the following specifications:
·
A
spectrally efficient system (in bits/s/Hz and bits/s/Hz/site)
·
High
network capacity i.e., more simultaneous calls per cell
·
A
nominal data rate of 100 Mbit/s when the client physically moves at high speeds
relative to the station, and 1 Gbit/s when client and base station are in
relatively fixed positions Smooth handoff across heterogeneous networks.
·
Seamless
connectivity and global roaming across multiple networks.
·
High quality of
service for next generation multimedia support e.g. real time audio, high speed
data, HDTV video content, mobile TV etc.
·
Interoperability
with existing wireless standards.
·
An
all-IP packet switched network.
Actually the 4G
system should dynamically share and utilize the network resources to meet the
requirements of all the 4G enabled users. The principal design techniques to be
exploited in 4G mobile wireless networks are [4], [5], [6], [7]:
·
Adaptive radio
interface
·
Modulation,
spatial processing including multi-antenna and multi-user Multiple
Input/Multiple Output (MIMO) to attain ultra high spectral efficiency
·
Multiple
access to exploit the frequency selective channel property
·
Turbo principle
to minimize the required SNR at the reception side
·
Relaying,
including fixed relay networks (FRNs), and the cooperative relaying concept,
known as multi-mode protocol
According to the
4G working groups, the infrastructure and
the terminals of 4G will have almost all the standards from 2G to 4G
implemented. Although legacy systems are in place to adopt existing users, the
infrastructure for 4G will be only packet-based (all-IP). Some proposals suggest
having an open internet platform. With the wireless standards evolution, the
access techniques used also increased in efficiency, capacity and scalability.
The first generation wireless standards used plain Time division Multiple
Access (TDMA) and Frequency Division Multiple Access (FDMA). In the wireless
channels, TDMA is less efficient in handling the high data rate channels
because it requires large guard periods to alleviate the multipath impact. Again,
FDMA consumes more bandwidth for guards for avoiding inter carrier
interference. So two branches came in second generation systems, one branch of
standard used the combination of FDMA and TDMA and the other introduced a new
access scheme called Code Division Multiple Access (CDMA) [8]. Usage of CDMA increased
the system capacity. Data rate is also increased as CDMA is efficient to handle
the multipath channel. This enabled the third generation systems to use CDMA as
the access scheme of IS-2000, UMTS, HSXPA, 1xEV-DO, TD-CDMA and TD-SCDMA. The
only issue with CDMA is that it suffers from poor spectrum flexibility and
scalability. Recently, new access schemes like Orthogonal Frequency Division
Multiple Access (OFDMA), Single Carrier FDMA (SC-FDMA), Interleaved FDMA
(IFDMA) and Multi-carrier CDMA (MC-CDMA) are gaining more importance for the next
generation systems. WiMax uses OFDMA in the downlink and in the uplink [9]. For
the next generation UMTS, OFDMA is being considered for the downlink. By contrast,
IFDMA is being considered for the uplink since OFDMA contributes more to the
Peak-to-Average Power Ratio (PAPR) related issues and results in nonlinear
operation of amplifiers. IFDMA provides less power fluctuation and thus avoids
amplifier issues. Similarly, MC-CDMA is in the proposal for the IEEE 802.20
standard. The advantages of these access schemes are that they offer the same
efficiencies as older technologies like CDMA, scalability and higher data rates
but they require less complexity for equalization at the receiver. This is an
added advantage especially in the MIMO environments as the spatial multiplexing
transmission of MIMO systems requires high complexity equalization at the receiver.
With these advantages in these multiplexing systems, improved modulation
techniques are also used. At the present data rates of 15-30 Mbit/s, 4G is
capable of providing users with streaming high-definition television. At rates
of 100 Mbit/s, the content of a DVD-5 (for example a movie), can be downloaded
within about 5 minutes for offline access [10]. The rest of the paper is
organized as follows. Section II provides descriptions of several multiple
access schemes for 4G mobile wireless networks. Section III provides a comparative
performance analysis of the above schemes vis-àvis design parameters of 4G
mobile wireless networks viz., throughput, Access scheme, Bit Error Rate (BER),
Intersymbol Interference (ISI), equalization, Bandwidth and security issues.
Simulation results of implementing CDMA and
OFDMA channel access schemes in MATLAB, is provided in section IV. Section V
concludes the paper with some highlights on future work.
II. MULTIPLE
ACCESS SCHEMES FOR 4G MOBILE WIRELESS NETWORKS
A. FDMA
FDMA gives users
an individual allocation of one or several frequency bands, or channels. It is
a basic technology in the analog Advanced Mobile Phone Service (AMPS), the most
widely-installed cellular phone system installed in North America. With FDMA,
each channel can be assigned to only one user at a time. FDMA is also used in
the Total Access Communication System (TACS). The Digital-Advanced Mobile Phone
Service (D-AMPS) also uses FDMA but adds TDMA to get three channels for each
FDMA channel, tripling the number of calls that can be handled on a channel.
The use of frequency division multiplexing is to provide multiple and
simultaneous transmissions to a single transponder. In FDMA, each transmitter
is assigned a distinct frequency channel so that receivers can discriminate
among them by tuning to the desired channel. TDMA and CDMA are always used in
combination with FDMA, i.e., a given frequency channel may be used for either
TDMA or CDMA independently of signals on other frequency channels. In 1989, the
Cellular Telecommunications Industry Association (CTIA) chose TDMA over
Motorola’s FDMA (today known as narrowband analog mobile-phone service [NAMPS])
narrowband standard as the technology of choice for existing 800 MHz cellular
markets and for emerging 1.9 GHz markets. Crosstalk is a major limitation of
FDMA, which causes interference between the other frequency bands and disturbs the
transmission. The features of FDMA are as follows.
• FDMA requires
high-performing filters in the radio hardware, in contrast to TDMA and CDMA.
• FDMA is not
vulnerable to timing problems like TDMA. Since a predetermined frequency band
is available for the entire period of communication, stream data can easily be used
with FDMA.
• As frequency
filtering is there, FDMA is not sensitive to near-far problem which we get in
CDMA.
• There is
different frequency slot for every user transmission and reception happens at
different frequencies.
There is a
difference between FDMA and frequency-division duplexing (FDD). While FDMA
allows multiple users to simultaneously access a certain system, FDD refers to
how the radio channel is shared between the uplink and downlink (for instance,
the traffic going back and forth between a mobilephone and a base-station).
Again, Frequency Division Multiplexing (FDM) is different from FDMA. FDM is a physical
layer technique that combines and transmits low bandwidth channels through a
high-bandwidth channel. FDMA, on the other hand, is an access method in the
data link layer [11]. Low PAPR and low sensitivity to carrier frequency offset
are some of the useful properties of FDMA. A hybrid system may be formed by
combining TDMA and FDMA.
B. TDMA
TDMA is a
channel access method for shared medium networks. It allows several users to
share the same frequency channel by dividing the signal into different time
slots. The users transmit in rapid succession, one after the other, each at his
time slot. This allows multiple stations to share the same transmission medium
(e.g. radio frequency channel) while using only a part of its channel capacity.
TDMA is used in the digital 2G cellular systems such as Global System for
Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN,
and in the Digital Enhanced Cordless Telecommunications (DECT) standard for
portable phones. It is also used extensively in satellite systems, and
combat-net radio systems. TDMA is a method used to enable multiple earth
stations or VSAT terminals to transmit intermittently on the same frequency,
but with the timing of their transmissions so arranged that the bursts do not
overlay when they arrive at the satellite but arrive in sequence and thus are
all successfully received by the teleport hub modem burst demodulator. The
operation of TDMA requires an out link control to all the remote sites which
contains some control information. This out link carrier also had a frame structure
that provides accurate timing information for all the remote sites. The teleport
hub equipment computer tells each VSAT site what particular time slot to use in
the TDMA frame and this time plan information is broadcast to all sites
periodically. The burst time plan may be fixed, so as to allocate each site a particular
proportion of the total TDMA frame time or is may be dynamic, whereby the time
slot allocated is adjusted in response to the traffic needs of each site [12]. Fig.1
shows the TDMA frame structure where a data stream is divided into frames and
those frames divided into time slots. TDMA is a type of time-division
multiplexing, with the special point that instead of having one transmitter
connected to one receiver, there are multiple transmitters. In the case of the
uplink from a mobile phone to a base station this becomes particularly
difficult because the mobile phone can move around and vary the timing advance
required to make its transmission match the gap in transmission from its peers.
The features of TDMA are as follows.
• Shares single
carrier frequency with multiple users
• Non-continuous
transmission makes handoff simpler
• Slots can be
assigned on demand in dynamic TDMA
• Less stringent
power control than CDMA due to reduced intra cell interference
• Higher
synchronization overhead than CDMA
• Advanced
equalization may be necessary for high data rates if the channel is
"frequency selective" and creates Inter symbol interference
• Cell breathing
(borrowing resources from adjacent cells) is more complicated than in CDMA
• Frequency/slot
allocation complexity
• Pulsating
power envelop: Interference with other devices
Fig.
1 TDMA Frrame Structure
TDMA can also be
dynamic in nature. In dynamic TDMA, a scheduling algorithm dynamically reserves
a variable number of time slots in each frame to variable bit-rate data
streams, based on the traffic demand of each data stream. applications.HIPERLAN/2
broadband radio access network, IEEE 802.16a WiMax, Bluetooth, Packet Radio
Multiple Access (PRMA) method for combined circuit switched voice communication
and packet data, TD-SCDMA, ITU-T are some of the application areas of Dynamic
TDMA.
C. CDMA
CDMA employs
spread-spectrum technology and a special coding scheme (where each transmitter
is assigned a code) to allow multiple users to be multiplexed over the same
physical channel. By contrast, TDMA divides access by time, while FDMA divides
it by frequency. CDMA is a form of "spreadspectrum" signaling, since
the modulated coded signal has a much higher data bandwidth than the data being
communicated. CDMA has been widely used in mobile phones and in satellite
system for transportation logistics. The scheme of CDMA is briefly described
below. CDMA uses the spread spectrum technique, which spreads the bandwidth of
the data uniformly for the same transmitted power. Spreading code is a
pseudo-random code which has a narrow ambiguity function unlike other narrow
pulse codes. In CDMA a locally generated code runs at a much higher rate than
the data to be transmitted. Data for transmission is simply logically XOR
(exclusive OR) added with the faster code. The figure shows how spread spectrum
signal is generated. The data signal with pulse duration of Tb is XOR added
with the code signal with pulse duration of Tc. (Note: bandwidth is proportional
to 1 / T where T = bit time) Therefore, the bandwidth of the data signal is 1 /
Tb and the bandwidth of the spread spectrum signal is 1 / Tc. Since Tc is much
smaller than Tb, the bandwidth of the spread spectrum signal is much larger than
the bandwidth of the original signal. The ratio Tb / Tc is called spreading
factor or processing gain and determines to certain extent the upper limit of
total number of users supported simultaneously by a base station. Fig. 2 shows the
frame structure of CDMA. Each user in a CDMA system uses a different code to modulate
their signal. Choosing the codes used to modulate the signal is very important
in the performance of CDMA systems. The best performance will occur when there
is good separation between the signal of a desired user and the signals of
other users.
Fig.
2 CDMA Frame Structure
The separation
of the signals is made by correlating the received signal with the locally
generated code of the desired user. If the signal matches the desired user's
code then the correlation function will be high and the system can extract that
signal. If the desired user's code has nothing in common with the signal the
correlation should be as close to zero as possible (thus eliminating the
signal); this is referred to as cross correlation. If the code is correlated
with the signal at any time offset other than zero, the correlation should be
as close to zero as possible. This is referred to as auto-correlation and is
used to reject multi-path interference. Flexible allocation of resources and
privacy protection in due to antijamming capabilities of PN sequences are some
of the advantages of CDMA. Fig.3 shows 3-D representation of FDMA, TDMA/FDMA
hybrid and CDMA. FDMA shows that each narrow band channel is allocated to a
single user; FDMA/TDMA hybrid shows that the bandwidth is split into frequency
channels and time slots; CDMA shows that each user is allocated a different
code in same frequency and time slot.
FDMA TDMA/FDMA
Hybrid CDMA
Fig
3 3D Representation of Access Schemes
There are
basically two types of CDMA: single carrier CDMA and multicarrier CDMA.
a) Single
Carrier CDMA
Fig.4 shows the
general structure of a single carrier transmission system. The transmitted
symbols are pulse formed by a transmitter filter. After passing the multipath channel
in the receiver a filter matched to the channel is used to maximize signal to
noise ratio and the device used to extract the data. The scenario we are
dealing with is Digital Video Broadcasting-Terrestrial (DVB-T), which is
characterized by the following conditions [13]:
·
Transmission
Rate: R =1/T = 7.4 M sym/s
·
Maximum
channel delay: τmax = 224 μs
·
ISI
[14] : tmax
/T » 1600
b) Multi Carrier
CDMA
The complexity
involved in removing the interference in the receiver of single carrier CDMA is
tremendous. Using the approach in Fig.4 will only lead to sub-optimal results.
This is the main reason why the multi carrier approach shown in Fig.5 has
become so popular.
The
original data stream of rate R is multiplexed into N parallel
data streams of rate Rmc = 1/ Tmc = R/N. Each of the data streams is
modulated with a different frequency and the resulting signals are transmitted
together in the same band. Correspondingly the receiver consists of N parallel
receiver paths. Due to the prolonged distance in between transmitted symbols,
the ISI for each sub system reduces to tmax / Tmc = tmax /
N*T. In the case of DVB-T we have N=8192 leading to an ISI of tmax
/ Tmc = 0.2. Such little ISI can often be tolerated and no extra counter
measure such as an equalizer is needed. But as far as the complexity of a
receiver is concerned a system with 8192 parallel paths still isn't feasible.
This asks for a slight modification of the approach which leads us to the concept
of OFDM.
Fig.
5 Multi Carrier CDMA
D. OFDMA
OFDMA is a
multi-user version of the popular OFDM digital modulation scheme. For achieving
multiple accesses subsets of subcarriers are provided to the individual users
in OFDMA. This allows simultaneous low data rate transmission from several
users. OFDMA is also a candidate access method for the IEEE 802.22 Wireless
Regional Area Networks (WRAN). The project aims at designing the first
cognitive radio based standard operating in the VHF-low UHF spectrum (TV spectrum)
[15]. FDMA also supports demand assignment with fixed assignment. Demand
assignment allows all users apparently continuous access of the radio spectrum
by assigning carrier frequencies on a temporary basis using a statistical
assignment process. Single-carrier FDMA (SC-FDMA) is a kind of FDMA scheme,
which is basically a multi-user version of Single-carrier
Frequency-Domain-Equalization (SC-FDE) modulation scheme [16]. SC-FDE can be
viewed as a linearly precoded OFDM scheme, and SC-FDMA can as a linearly precoded
OFDMA scheme, henceforth LP-OFDMA. It can also be viewed as a single carrier
multiple access scheme. One prominent advantage over conventional OFDM and
OFDMA is that the SC-FDE and LP-OFDMA/SC-FDMA signals have lower
peak-to-average power ratio (PAPR) because of its inherent single carrier
structure [17].
A.
Characteristics
Based on
feedback information about the channel conditions, adaptive user-to-subcarrier assignment
can be achieved. If the assignment is done sufficiently fast, this further
improves the
OFDM robustness
to fast fading and narrow-band co channel interference, and makes it possible
to achieve even better system spectral efficiency. Different number of
sub-carriers can be assigned to different users, for supporting different Quality
of Service (QoS), i.e. to control the data rate and error probability
individually for each user. OFDMA resembles CDMA spread spectrum, where users
can achieve different data rates by assigning a different code spreading factor
or a different number of spreading codes to each user. OFDMA can be seen as an
alternative to combining OFDM with TDMA or time-domain statistical
multiplexing, i.e. packet
mode
communication. Low-data-rate users can send continuously with low transmission
power instead of using a "pulsed" high-power carrier. Constant delay,
and shorter delay, can be achieved. In OFDMA, the resources are partitioned in
the timefrequency space, and slots are assigned along the OFDM symbol index as
well as OFDM sub-carrier index. OFDMA is considered as highly suitable for
broadband wireless networks, due to advantages including scalability and MIMO friendliness,
and ability to take advantage of channel frequency selectivity. In spectrum
sensing cognitive radio, OFDMA is a possible approach to filling free radio
frequency bands adaptively.
B. Principle of
Operation
OFDM is a subset
of FDM in which a single channel utilizes multiple sub-carriers on adjacent
frequencies. In addition the sub-carriers in an OFDM system are overlapping to maximize
spectral efficiency. Ordinarily, overlapping adjacent channels can interfere
with one another. However, sub-carriers in an OFDM system are precisely
orthogonal to one another. Thus, they are able to overlap without interfering.
Two conditions must be considered for the orthogonality between the subcarriers
[15].
1. Each
subcarrier has exactly an integer number of cycles in the FFT interval.
2. The number of
cycles between adjacent subcarriers differs by exactly one.
C. Orthogonality
of Sub-Channel Carriers
OFDM
communications systems are able to more effectively utilize the frequency
spectrum through overlapping subcarriers. These sub-carriers are able to
partially overlap without interfering with adjacent sub-carriers because the maximum
power of each sub-carrier corresponds directly with the minimum power of each
adjacent channel. OFDM channels are different from band limited FDM channels
how they apply a pulse-shaping filter. With FDM systems, a sinc-shaped pulse is
applied in the time domain to shape each individual symbol and prevent ISI.
With OFDM systems, a sinc-shaped pulse is applied in the frequency domain of
each channel. As a result, each sub-carrier remains orthogonal to one another.
D.
Transmitter/Receiver
In order to use
multiple sub-carriers to transmit an individual channel, an OFDM communications
system must perform several steps, described in Fig.6.
Fig.
6 OFDM Communication System
1. Serial to
Parallel Conversion - In an OFDM system, each channel can be broken into
various sub-carriers. The use of sub-carriers makes optimal use out of the
frequency spectrum but also requires additional processing by the transmitter
and receiver. This additional processing is necessary to convert a serial bit
stream into several parallel bit streams to be divided among the individual
carriers. Once the bit stream has been divided among the individual
sub-carriers, each sub-carrier is modulated as if it was an individual channel
before all channels are combined back together and transmitted as a whole. The
receiver performs the reverse process to divide the incoming signal into
appropriate sub-carriers and then demodulating these individually before reconstructing
the original bit stream.
2. Modulation
with the Inverse FFT - The modulation of data into a complex waveform
occurs at the Inverse Fast Fourier Transform (IFFT) stage of the transmitter.
Here, the modulation scheme can be chosen completely independently of the
specific channel being used and can be chosen based on the channel
requirements. In fact, it is possible for each individual sub-carrier to use a
different modulation scheme. The role of the IFFT is to modulate each
sub-channel onto the appropriate carrier.
3. Cyclic Prefix
Insertion - Wireless
communications systems are susceptible to multi-path channel reflections; a
cyclic prefix is added to reduce ISI. A cyclic prefix is a repetition of the
first section of a symbol that is appended to the end of the symbol. In
addition, it is important because it enables multipath representations of the
original signal to fade so that they do not interfere with the subsequent
symbol. Fig.7 shows the block diagram of Cyclic Prefix Insertion
Fig.
7 Cycle Prefix Insertion
4. Parallel to
Serial Conversion - Once
the cyclic prefix has been added to the sub-carrier channels, they must be transmitted
as one signal. Thus, the parallel to serial conversion stage is the process of
summing all sub-carriers and combining them into one signal. As a result, all
subcarriers are generated perfectly simultaneously. Several common commercial
protocols, such as DVB, asymmetric digital subscriber line (ADSL), and wireless
Ethernet (WiFI) implement OFDM. With WiFI, the IEEE 802.11a and IEEE 802.11g
implementations specifically use
OFDM techniques.
With IEEE 802.11g, each channel occupies 16.25 MHz of bandwidth at the 2.4GHz
frequency range. In addition, each channel is divided into 52 sub-carriers of
312.5 kHz. Together, these sub-carriers overlap to fully utilize the 16.25 MHz
channel bandwidth dedicated per channel. In addition, each sub-carrier can use
a unique modulation scheme. More specifically,
WiFI can use BPS, QPSK, 16-QAM, or 64-QAM depending on the characteristics
of the physical
channel being used. One of the newest wireless internet protocols, WiMAX, also
used OFDM technology.
III. PERFORMANCE
ANALYSIS
Several
parameters are considered here for the comparative performance analysis of the
above schemes vis-à-vis design parameters of 4G mobile wireless networks.
• Throughput:
In communication networks, throughput or network throughput is the average rate
of successful message delivery over a communication channel. This data may be delivered
over a physical or logical link, or pass through a certain network node.
• Access
Scheme: In telecommunications and computer networks, a channel access
method or multiple access method allows several terminals connected to the same
multi-point transmission medium to transmit over it and to share its capacity.
Examples of shared physical media are wireless networks, bus networks, ring
networks, hub networks and half-duplex point-to-point links. A channel-access
scheme is based on a multiplexing method, which allows several data streams or
signals to share the same communication channel or physical medium.
• Delay: In a network based on packet
switching, transmission delay is the amount of time required to push all of the
packet's bits into the wire. In other words, this is the delay caused by the
data-rate of the link. Transmission delay is a function of the packet's length
and has nothing to do with the distance between the two nodes. This delay is
proportional to the packet's length in bits, It is given by the following
formula: DT = N / R where DT is the transmission
delay N is the number of bits, and R is the rate of transmission
(say in bits per second)
• Bandwidth:
Bandwidth is the difference between the upper and lower frequencies in a
contiguous set of frequencies. It is typically measured in hertz, and may
sometimes refer to passband bandwidth, sometimes to baseband
bandwidth, depending on context.
• Dynamic
Power Management: Dynamic power management is a design methodology aiming
at controlling performance and power levels of digital circuits and systems,
with the goal of extending the autonomous operation time of battery powered systems,
providing graceful performance degradation when supply energy is limited, and
adapting power dissipation to satisfy environmental constraints.
• BER: In
digital transmission, the number of bit errors is the number of received bits
of a data stream over a communication channel that has been altered due to
noise, interference, distortion or bit synchronization errors. It is the number
of bit errors divided by the total number of transferred bits during a studied
time interval.
• ISI: In
telecommunication, inter symbol interference (ISI) is a form of distortion of a
signal in which one symbol interferes with subsequent symbols. This is an
unwanted phenomenon as the previous symbols have similar effect as noise, thus
making the communication less reliable
• Equalization:
It is the process of adjusting the volume of certain frequencies within a
signal.
• Security:
Communications security is the discipline of preventing unauthorized
interceptors from accessing telecommunications in an intelligible form, while
still delivering content to the intended recipients
• PAPR:
The peak-to-average power ratio (PAPR) is a measurement of a waveform,
calculated from the peak amplitude of the waveform divided by the RMS value of
the waveform. Table I gives a comparative performance analysis of FDMA, TDMA,
CDMA and OFDMA with respect to these design parameters.
IV. SIMULATION
RESULTS
This section
provides the simulation results of implementing two different channel access
schemes in MATLAB. Our simulation is restricted to OFDMA as this technology
surpass the other two technologies viz., FDMA and TDMA in terms of different
network parameters for 4G mobile wireless networks as seen from Table I. Fig. 8
show the frequency response characteristics of OFDMA signals as simulated in
MATLAB.
Table I Comparative Analysis
Fig
8 Frequency Response Characteristics of OFDMA Signal
V.
CONCLUSION
Table I shows
that OFDMA surpasses all the other schemes in terms of various parameters. So
it is the right choice of multiple channel access schemes for 4G mobile
wireless networks. However in spite of all the utilities of OFDMA, there are
certain obstacles in using OFDM in transmission system in contrast to its
advantages. Firstly OFDM signal exhibits a very high PAPR. Therefore, RF
power amplifiers should be operated in a very large linear region. Otherwise,
the signal peaks get into non-linear region of the power amplifier causing
signal distortion. This signal distortion introduces inter modulation among the
subcarriers and out of band radiation. Thus, the power amplifiers should be
operated with large power back-offs. On the other hand, this leads to very
inefficient amplification and expensive transmitters. Thus, it is highly
desirable to reduce the PAPR. Secondly it is very sensitive to frequency errors
caused by frequency differences between the local oscillators in the transmitter
and the receiver. Hence carrier frequency offset causes a number of impairments
including attenuation and rotation of each of the subcarriers and ICI between subcarriers.
In the mobile radio environment, the relative
movement between
transmitter and receiver causes Doppler frequency shifts; in addition, the
carriers can never be perfectly synchronized. These random frequency errors in OFDM
system distort orthogonality between subcarriers and thus ICI occurs. A Number
of methods have been developed to reduce this sensitivity to frequency offset. Thirdly
asynchronous data communication services
such as web access are characterized by short communication bursts at high data
rate. Few users in a base station cell are transferring data simultaneously at
low constant data rate. Fourthly dealing with co-channel interference from
nearby cells is more complex in OFDM than in CDMA. It would require dynamic
channel allocation with advanced coordination among adjacent base stations.
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http://en.wikipedia.org/wiki/4G
[17]
http://en.wikipedia.org/wiki/Single-carrier_FDMA
