Iaetsd evaluation ofdma & sc-ofdma performance on

EVALUATION: OFDMA & SC-OFDMA PERFORMANCE ON
WIRELESS COMMUNICATION
G. MANIKANDAN1
, T. JENISH2
, S. PRASHANTHY3
1
Research Scholar
St. Peter’s University
St. Peter’s Institute of Higher Education and Research
Avadi, Chennai-600 054
2
Assistant Professor, Department of Electronics and Communication Engineering,
Kodaikanal Institute of Technology,Tamilnadu, India
3
UG Student , Department of Computer Science & Engineering,
Kodaikanal Institute of Technology,Tamilnadu, India
Abstract— This paper investigates OFDMA and SC-
FDMA techniques combined with the Orthogonal
Frequency Division Multiplexing (OFDM). Single
carrier frequency Division Multiple Access(SC-FDMA)
is a notable application of Multiple Input Multiple
Output(MIMO) system. SC-FDMA is one of the most
promising techniques aiming at solving the capacity
problem of wireless communication systems and
achieving higher spectral efficiency, depending on
multiplexing signals based on their spatial signature.
On the other hand most third generation mobile phone
systems are using Code Division Multiple Access
(CDMA) as their modulation technique. CDMA is not
so complicated to implement as OFDM based systems.
As CDMA has a wide bandwidth, it is difficult to
equalize the overall spectrum - significant levels of
processing would be needed for this as it consists of a
continuous signal and not discrete carriers. Not as easy
to aggregate spectrum as for OFDM. For this reason,
CDMA is also investigating so that the performance of
OFDM-SC-FDMA can be compared. Various OFDMA
techniques are investigated including LTE or Long
Term Evolution is the brand name given to the efforts
of 3GPP 4th Generation technology development efforts
mostly in Europe and UMB (Ultra-Mobile Broadband)
is the brand name for similar efforts by 3GPP2, linear
detection schemes, minimum mean square error,
ordered successive cancellation, and maximum
likelihood methods. Promising results are obtained to
enhance spectral efficiency on the expense of
computational complexity which needs to be addressed.
Keywords- Single carrier frequency division Multiple
Access (SC-FDMA); Orthogonal Frequency Division
Multiple Access (OFDMA); Multiple input multiple
output(MIMO); Signal-to-noise ratio (SNR); (UMB)
Ultra-Mobile Broadband; Long Term Evolution(LTE)
I. Wireless Technology Overview
Wireless technologies enable one or more
devices to communicate without an actual wired
connection. Radio frequency is used to transmit
the data. Such technologies are rapidly evolving
to meet a variety of communications needs, from
simple to complex wireless communications
needs can all be classified in one of three ways,
based on the distance they are meant to cover.
These include: wireless personal area networks
(WPAN), wireless local area networks (WLAN),
and wireless wide area networks (WWAN). The
evolution is stated in figure: 1.1 [9]
Figure: 1.1 Evolution of wireless technology
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II. Verizon Wireless and LTE Mobile
Broadband Technology
Wireless carriers are keenly interested in
choosing the best technology for their
customers—for both today and tomorrow. For
Verizon Wireless, selecting the right technology
is imperative. As a leader in the wireless
industry, Verizon Wireless is committed to the
potential technology advances offered by LTE.
Verizon Wireless is currently conducting
laboratory and field tests using LTE technology
and plans to launch its 4G mobile network in
2010. This deployment will help the company
realize its goal of delivering improved wireless
Internet connectivity and mobility to its
customers. For the mobile user, connectivity
means an un tethered experience and true
mobility.[7] Users can work and communicate
almost whenever and wherever they want.
LTE’s improved speeds will allow wireless
carriers to offer a number of business-specific
applications and services, such as video
conferencing, direct connectivity, and mobile
applications that bring the desktop experience to
mobile devices.[8]
III. LTE frame structure
As mentioned previously in this paper, LTE is a
4G wireless technology that Verizon Wireless
and numerous leading wireless carriers have
chosen as their upgrade path beyond 3G
technologies. Verizon Wireless will operate LTE
in the 700 MHz spectrum, which translates to
unprecedented performance and data access is
shown in figure 3.1 and 3.2[8]
Figure: 3.1 LTE frame structure[1]
Figure: 3.2 LTE frame structure [2]
IV. Evolution of LTE
As mentioned previously in this paper, LTE is a
4G wireless technology that Verizon Wireless
and numerous leading wireless carriers have
chosen as their upgrade path beyond 3G
technologies. Verizon Wireless will operate LTE
in the 700 MHz spectrum, which translates to
unprecedented performance and data access.[8]
The 3GPP body began its initial investigation of
the LTE standard as a viable technology in 2004.
In March 2005, 3GPP began a feasibility study
whose key goals were to agree on network
architecture and a multiple access method, in
terms of the functional split between the radio
access and the core network[5]. The 3GPP
decided to use OFDMA in the downlink
direction and use SC-FDMA in the uplink
direction. The specifications for the LTE
standard were approved by 3GPP in January
2007. The specifications are now under change
control, leading to their inclusion in 3GPP
Release 8. While the LTE requirements are
finalized, the standard is not fully completed.
LTE Release 8 was completed by late 2008 as
shown in figure 4.1[8]
Figure: 4.1 Evolution of LTE
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V. MIMO-OFDMA
Multiple Input Multiple Output (MIMO) is one
of the most popular Advanced Antenna
Technologies which is supported both by LTE
and UMB. The salient features of MIMO is that
it offers higher throughput for a given bandwidth
and higher link range for a given power value. A
detailed discussion of the MIMO technology is
beyond the scope of this survey and we provide
a cursory glance at the key features of the
technology. In MIMO the transceiver and
receiver have multiple antennas giving MIMO
multiple flavors based on the number of
antennas present on each side. However, the key
idea is that a transmitter sends multiple streams
on multiple transmit antennas 9 of 15 and each
transmitted stream goes through different paths
to reach each receiver antenna .The different
paths taken by the same stream to reach multiple
receivers allow canceling errors using superior
signal processing techniques. MIMO also
achieves spatial multiplexing to distinguish
among different symbols on the samefrequency.
MIMO thus helps in achieving higher spectral
efficiency and Link reliability.[9]
VI. SNR-OFDMA
Signal-to-noise ratio (SNR) is broadly defined as
the ratio of the desired signal power to the noise
power. SNR estimation indicates the reliability
of the link between the transmitter and receiver.
In adaptive system design, SNR estimation is
commonly used for measuring the quality of the
channel. Then, the system parameters are change
adaptively based on this measurement. For
example, if the measured channel quality is low,
the transmitter adds some redundancy or
complexity to the information bits (more
powerful coding), or reduces the modulation
level (better Euclidean distance), or increases the
spreading rate (longer spreading code) for lower
data rate transmission. Therefore, instead of
fixed information rate for all levels of channel
quality, variable rates of information transfer can
be used to maximize system resource utilization
with high quality of user experience.
One particular interest for SNR estimation is to
use it for Adaptive Orthogonal Frequency
Division Multiplexing (AOFDM) based wireless
communication systems. Adaptive modulation
that employs different level of modulation for
each (or a group) of sub-carriers depends
strongly on accurate estimation of SNR value
[1]-[3]. There are many other applications that
can exploit SNR information, like channel
estimation through interpolation and optimal soft
information generation for high performance
decoding algorithms[4],[ 5].
In previous SNR estimation techniques, the SNR
measurement is considered as an indication of
long-term fading statistics due to shadowing and
log-normal fading. This long-term SNR estimate
is often calculated using regularly. Transmitted
training (or pilot) sequences. Instead of using
training sequences, the data symbols can also be
used for this purpose. For example,
Balachandran, who uses SNR information as a
channel quality indicator for rate adaptation,
exploits the cumulative Euclidean metric
corresponding to the decoded trellis path for
channel quality information [6]. Jacobsmeyer
describes another method for channel quality
measurement. He proposes the use of the
difference between the maximum likelihood
decoder metrics for the best path and the second
best path [7]. In a sense, he uses some sort of
soft information for channel quality indication.
But, this approach does not provide any
information about the strength of the interferer
or the desired signal. There are several other
SNR measurement techniques which can be
found in [8] and reference listed therein.[12]
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VII. Comparison of parameters of UMTS,
HSPA, HSPA+ and LTE
Figure:4.1 Comparison of UMTS, HSPA,
HSPA+ and LTE
VIII. What is OFDM?
Orthogonal frequency division multiplexing
(OFDM) is a widely used modulation and
multiplexing technology, which has become the
basis of many telecommunications standards
including wireless local area networks (LANs),
digital terrestrial television (DTT) and digital
radio broadcasting in much of the world.[3]
In the past, as well as in the present, the OFDM
is referred in the literature as Multi-carrier,
Multi-tone and Fourier Transform. The OFDM
concept is based on spreading the data to be
transmitted over a large number of carriers, each
being modulated at a low rate. The carriers are
made orthogonal to each other by appropriately
choosing the frequency spacing between them.
A multicarrier system, such as FDM (aka:
Frequency Division Multiplexing), divides the
total available bandwidth in the spectrum into
sub-bands for multiple carriers to transmit in
parallel.[15] It combines a large number of low
data rate carriers to construct a composite high
data rate communication system. Orthogonality
gives the carriers a valid reason to be closely
spaced with overlapping without ICI. [16]
IX. Why OFDM?
In contrast to conventional Frequency Division
Multiplexing, the spectral overlapping among
subcarriers are allowed in OFDM since
orthogonality will ensure the subcarrier
separation at the receiver, providing better
spectral efficiency and the use of steep band
pass filter was eliminated. OFDM transmission
system offers possibilities for alleviating many
of the problems encountered with single carrier
systems. It has the advantage of spreading out a
frequency selective fade over many symbols.
This effectively randomizes burst errors caused
by fading or impulse interference so that instead
of several adjacent symbols being Completely
destroyed, many symbols are only slightly
distorted. This allows successful reconstruction
of majority of them even without forward error
correction. Because of dividing an entire signal
bandwidth into many narrow sub-bands, the
frequency response over individual sub-bands is
relatively flat due to sub-band are smaller than
coherence bandwidth of the channel. Thus,
equalization is potentially simpler than in a
single carrier system and even equalization may
be avoided altogether if Differential encoding is
implemented.[3]
X. Principle of OFDM
In digital communications, information is
expressed in the form of bits. The term symbol
refers to a collection, in various sizes, of bits
[17]. OFDM data are generated by taking
symbols in the spectral space using M-PSK,
QAM, etc, and convert the spectra to time
domain by taking the Inverse Discrete Fourier
Transform (IDFT). Since Inverse Fast Fourier
Transform (IFFT) is more cost effective to
implement, it is usually used instead [16]. The
main features of a practical OFDM system are as
follows:
 Some processing is done on the source
data, such as coding for correcting
errors, interleaving and mapping of bits
onto symbols. An example of mapping
used is QAM.
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 The symbols are modulated onto
orthogonal sub-carriers. This is done by
using IFFT.
 Orthogonality is maintained during
channel transmission. This can be
achieved by adding a cyclic prefix to the
OFDM frame to be sent. The cyclic
prefix consists of the L last samples of
the frame, which are copied and placed
in the beginning of the frame. It must be
longer than the channel impulse
response.
 Synchronization: cyclic prefix can be
used to detect the start of each frame.
This is done by using the fact that the L
first and last samples are the same and
therefore correlated.
 Demodulation of the received signal by
using FFT.
 Channel equalization: the channel can
be estimated either by using a training
sequence or sending known so-called
pilot symbols at predefined sub-carriers.
Decoding and de-interleaving.
XI. Basic OFDM system
The OFDM signal generated by the system in
Fig 1 & 2 is at baseband ; in order to generate a
radio frequency (RF) signal at the desired
transmit frequency filtering and mixing is
required. OFDM allows for a high spectral
efficiency as the carrier power and modulation
scheme can be individually controlled for each
carrier. However in broadcast systems these are
fixed due to the one-way communication.
The basic principle of OFDM is to split a high-
rate data stream into a number of lower rate
streams that are transmitted simultaneously over
a number of subcarriers. The block diagram
showing a simplified configuration for an
OFDM transmitter and receiver is given in Fig.1
& Fig. 2. [3]
XII. Orthogonality
The main aspect in OFDM is maintaining
orthogonality of the carriers. If the integral of
the product of two signals is zero over a time
period, then these two signals are said to be
orthogonal to each other. Two sinusoid with
frequencies that are integer multiples of a
common frequency can satisfy this criterion.
Therefore, orthogonality is defined by: Where n
and m are two unequal integers; fo is the
fundamental frequency; T is the period over
which the integration is taken. For OFDM, T is
one symbol period and fo set to 1/T for optimal
effectiveness [15][16].
XIII. OFDM Applications:
OFDM technique is the most prominent
technique of this era .Some of its applications is
given below.
 DAB: DAB - OFDM forms the basis for
the Digital Audio Broadcasting (DAB)
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standard in the European market [18].
Digital Audio Broadcasting (DAB)
using OFDM has been standardized in
Europe [19] and is the next step in
evolution beyond FM radio broadcasting
providing interference free transmission.
 HDTV
 Wireless LAN Networks
 5.3.1 HIPERLAN/2
 IEEE 802.11g
 IEEE 802.16 Broadband Wireless
Access System.
 Wireless ATM transmission system
 IEEE 802.11a
XIV. SC-FDMA
Single-carrier FDMA (SC-FDMA) was chosen
to reduce Peak to Average Ratio (PAR), which
has been identified as a critical issue for use of
OFDMA in the uplink where power-efficient
amplifiers are required in mobile devices.
Another important requirement was to maximize
the coverage. For each time interval, the base
station scheduler assigns a unique time-
frequency interval to a terminal for the
transmission of user data, thereby ensuring
intracell orthogonality. Slow power control, for
compensating path loss and shadow fading, is
sufficient as no near-far problem is present due
to the orthogonal uplink transmissions.
Transmission parameters, coding, and
modulation are similar to the downlink
transmission.
The chosen SC-FDMA solution is based on
using a cyclic prefix to allow high-performance
and low-complexity receiver implementation in
the e-Node B. As such, the receiver
requirements are more complex than in the case
of OFDMA for similar link performance, but
this is not considered to be a problem in the base
station. The terminal is only assigned with
contiguous spectrum blocks in the frequency
domain to maintain the single-carrier properties
and thereby ensure power-efficient transmission.
This approach is often referred to as blocked or
localized SC-FDMA.[9]
XV. SC-FCDMA & OF-DMA in LTE
Physical Layer
The multiple access scheme in LTE downlink
uses Orthogonal Frequency Division Multiple
Access and uplink uses Single Carrier
Frequency. These multiple access solutions
provide orthogonality between the users,
reducing the interference and improving the
network capacity. The multiple access schemes
are illustrated in Figure 15.1.
Figure:15.1 Physical layer in LTE
Date Rate: For 20 MHz spectrum, the target for
peak data rate is 50 Mbps (for uplink) and 100
Mbps (for downlink). Bandwidth: In 3GPP
technology family, there were considered both
the wide band(WCDMA with 5MHz) and the
narrowband (GSM with 200 kHz). Therefore,
the new system is now required to facilitate
frequency allocation flexibility with 1.25/2.5, 5,
10, 15 and 20 MHz allocations [7].
Peak Spectral Efficiency: The peak spectral
efficiency requirement for downlink Is 5 bps/Hz
or higher, and for uplink is 2.5 bps/Hz or higher.
Spectral Efficiency of Cell Edge: The
requirement for spectral efficiency of cell edge
is 0.04-0.06 bps/Hz/user for downlink and 0.02-
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0.03 bps/Hz/user for uplink, with assumption of
10 users/cell.
Average Cell Spectral Efficiency: The average
cell spectral efficiency required for downlink is
1.6-2.1 bps/Hz/cell and for uplink it is 0.66-1.0
bps/Hz/cell.
Latency: The LTE control-plane latency
(transition time to active state) is less than 100
ms (for idle to active), and is less than 50 ms
(for dormant to active). The user plane latency is
less than 10 ms from UE (user end) to server.
Security & Mobility: Security and mobility in
3GPP technology is used at good level with the
earlier systems starting from GSM and it is
sustained at that level and higher.[19]
XVI. Transceiver result
This simulates model of OFDMA and SC-
FDMA in Matlab. The block diagrams of
OFDMA and SC-FDMA are shown in Figure
16.1 and Figure 16.2 respectively, below. The
block diagrams of OFDMA and SC-FDMA are
similar to OFDM system, except the additional
subcarrier mapping and the position of some
blocks.[19]
Figure:16.1 Transmitter
Figure: 16.2 Receiver
XVII. BER vs SNR of OFDMA and SC-
FDMA
The BER vs SNR of OFDMA and SC-FDMA
are shown in Figures 17.1 & 17.2 and the
corresponding values in Tables 4.2 and 4.3
respectively. In Tables 18.1 and 18.2, the
observations are taken for a specific value of
BER (1e-3). In both OFDMA and SC-
International
FDMA, the BPSK and QPSK have same SNR
values of 6.8 and 6.5 respectively, but a sudden
change occur in 16 (16.4) which shows that 64
Figure 17.1: BER vs SNR of OFDMA with
Adaptive Modulation.
Figure: 17.2 BER vs SNR of SC-FDMA with
Adaptive Modulation
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XVIII. BER VS SNR for OFDMA
Error Probability of OFDMA and SC-FDMA for
Adaptive Modulation The error probability
graphs of OFDMA and SC-FDMA are shown in
Figures 18.1 and 18.2, and the corresponding
values in Tables 18.3 and 18.4 respectively.
Figure 18.1: Power Spectral Density of OFDMA
Table 18.3: Error probability of OFDMA Table
18.4: Error probability of SC-FDMA From
Tables 18.3and 18.4, it can be seen that for a
specific value of Pe (1e-0.5) the BPS modulation
has less value of SNR as compared to other
modulations.
Figure 18.2: Power Spectral Density of SC-
FDMA
Table 18.3: Error probability of OFDMA
Table 18.4: Error probability of SC-FDMA
The 64-QAM has higher SNR values in both
OFDMA and SC-FDMA
XIX. Power Spectral Density of OFDMA
and SC-FDMA:
The power spectral density of OFDMA and SC-
FDMA are shown in figure 19.1and figure 19.2
respectively.
Figure 19.1: Power Spectral Density of OFDMA
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Figure 19.2: Power Spectral Density of SC-
FDMA
Figure 19.1 and 19.2 shows the power spectral
density of the OFDMA and SCFDMA
respectively. We can observe that the average
power of all SC-FDMA symbols (512) is nearly
-375dB, whereas, in case of OFDMA the
average power of all symbols is nearly -400dB.
This shows that the SC-FDMA symbols have
inherently more average power as compared to
OFDMA at all frequencies. This result also
shows the transmit power requirements of
OFDMA and SC-FDMA symbols which is
covered in next section of PAPR.
XX. PAPR of OFDMA and SC-FDMA for
Adaptive Modulation
a) BPSK and QPSK: The PAPR of
OFDMA and SC-FDMA for BPSK and
QPSK modulations are shown in Figure
20.1 and Figure 20.2 respectively.
Figure20.1:PAPR of OFDMA and SC-FDMA
for BPSK.
Figure20.2: PAPR of OFDMA and SC-FDMA
for QPSK
b) 16-QAM and 64-QAM: The PAPR of
OFDMA and SC-FDMA for 16-QAM
and 64-QAM are shown in Figures 20.4
and 20.4 respectively.
Figure 20.3: PAPR of OFDMA and SC-FDMA
for 16-QAM
Figure 20.4: PAPR of OFDMA and SC-FDMA
for 64-QAM.
From Figures 4.11 and 4.12, it can be observed
that by increasing the order of modulation, the
PAPR of SC-FDMA increases from 7.5 dB to 8
dB (in case of 16-QAM) and becomes 9.8 db (in
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case of 64-QAM). Thus, for SC-FDMA the
PAPR increases for higher order modulation
XXI. Conclusion
LTE and BER is a key parameter for indicating
the system performance of any data link. From
the simulated results, it can be observed that for
a fix value of SNR, the BER increases for high
order modulation (16-QAM and 64-QAM) in
both the multiple access techniques (OFDMA
and SC-FDMA) used in LTE system. On the
other hand, the lower order modulation schemes
(BPSK and QPSK) experience less BER at
receiver thus lower order modulations improve
the system performance in terms of BER and
SNR. If the bandwidth efficiency of these
modulation schemes is considered, the higher
order modulation accommodates more data
within a given bandwidth and is more bandwidth
efficient as compared to lower order modulation.
Thus, there exists a tradeoff between BER and
bandwidth efficiency among these modulation
schemes used in LTE. It is also concluded from
the results that, the error probability increases as
order of modulation scheme increases.
Therefore, the selection of modulation schemes
in adaptive modulation is quite crucial based on
these results. The power consumption at the user
end such as portable devices is again a vital
issue for uplink transmission in LTE system.
From the simulation results, it can be concluded
that the higher order modulation schemes have
an impact on the PAPR of both OFDMA and
SC-FDMA. The PAPR increases in SC-FDMA
and slightly decreases in OFDMA for higher
order modulation schemes. The overall value of
PAPR in SC-FDMA is still less than that of
OFDMA in all modulation schemes, and that is
why it has been adopted for uplink transmission
in LTE system. Based on the results obtained, it
can be concluded to adopt low order modulation
scheme i.e. BPSK, QPSK and 16-QAM for
uplink in order to have less PAPR at user end. In
nutshell, SC-FDMA is more power efficient.
Here our future work is focused toward the
study of achievable "Enhancement Survey on
Security in 4G".
XXII. REFERENCES
[1] Broadband MIMO-OFDM Wireless
Communications : PROCEEDINGS OF THE
IEEE, VOL. 92, NO. 2, FEBRUARY 2004
[2] Design and BER Performance of MIMO-
OFDM for Wireless Broadband
Communications, International Journal of
Modern Engineering Research (IJMER) Vol.3,
Issue.3, May-June. 2013 pp-1382-1385.
[3] A Review on OFDM: Concept, Scope & its
Applications, IOSR Journal of Mechanical and
Civil Engineering (IOSRJMCE) ISSN : 2278-
1684 Volume 1, Issue 1 (May-June 2012), PP
07-11
[4] IMPLEMENTING SC-FDMA &OFDMA IN
MATLAB: International Journal of
Computing and Corporate Research ISSN
(Online) : 2249-054X Volume 3 Issue 6
November 2013 International Manuscript ID :
2249054XV3I6112013-05 1
[5] A Modified Fast FFT Algorithm for OFDM
Based Future Wireless Communication
System: International Journal of Soft
Computing and Engineering (IJSCE) ISSN:
2231-2307, Volume-1, Issue-6, January 2012
[6] Fairness Aware Group Proportional
Frequency Domain Resource Allocation in L-
SC-FDMA Based Uplink*: Int. J.
Communications, Network and System
Sciences, 2011, 4, 487-494
doi:10.4236/ijcns.2011.48060 Published
Online August 2011
(http://www.SciRP.org/journal/ijcns)
[7] Orthogonal Frequency Division Multiplexing
Based Wireless Communication System for
Digital Broadcast Applications: IP Multimedia
Communications A Special Issue from IJCA -
www.ijcaonline.org
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[8] white paper: LTE: The Future of Mobile
Broadband Technology
[9] Long Term Evolution (LTE) & Ultra-Mobile
Broadband (UMB) Technologies for Broadband
Wireless Access
[10] LTE: The Future of Mobile Broadband
Technology
[11] Theory of Frequency Division Multiplexing:
http://zone.ni.com/devzone/cda/ph/p/id/269
[12] Noise Power and SNR Estimation for OFDM
Based Wireless Communication Systems Acosta,
Guillermo. “OFDM Simulation Using
MATLAB” 2000
[13] Litwin, Louis and Pugel, Michael. “The
Principles of OFDM” 2001
[14] Understanding an OFDM transmission:
http://www.dsplog.com/2008/02/03/understandin
g-an-ofdm-transmission/
[15] Minimum frequency spacing for having
orthogonal sinusoidal
http://www.dsplog.com/2007/12/31/minimum-
frequencyspacing-for-having-orthogonal-
sinusoidal
[16] G. Fay, "Wireless Data Networking,"
International Journal of Network Management, 8
March 1992, pp. 8-17.
[17] ETSI, "Radio Broadcast Systems: Digital
Audio Broadcasting (DAB) to mobile, portable
and fixed receivers," ETSI final draft ETS 300
401, Nov 1994.
[18] A Modified Fast FFT Algorithm for OFDM
Based Future Wireless Communication System:
International Journal of Soft Computing and
Engineering (IJSCE) ISSN: 2231-2307, Volume-
1, Issue-6, January 2012.
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