The document discusses an improved multiuser OFDM system that enhances performance by multiplexing user data into a single OFDM symbol, achieving double the capacity and better signal quality compared to traditional systems. Utilizing a UWB channel and polarization diversity reduces receiver complexity while effectively allocating resources to multiple users, addressing challenges of previous systems. Simulations demonstrate significant reductions in bit error rate (BER) and improved signal-to-noise ratio (SNR), with plans for future work to further enhance receiver efficiency.

1. INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
VOLUME 5 ISSUE 1 – MAY 2015 - ISSN: 2349 - 9303
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Improving The Performance Of Multiuser OFDM
Wireless System With UWB Channel
Bhuvaneshwari P1
1
SNS College of Technology,
ME Communication Systems, Department of ECE,
bhuvaneshwari2531@gmail.com
Jayanthi k2
Associate Professor
SNS College of Technology,Department of ECE,
jayanthiecesnsct06@gmail.com
Abstract— The multi user OFDM system can be used to produce a highly flexible and effective communication
system. In the existing multi user OFDM system, resource allocation to each user is the major problem. Also the existing
multi user OFDM system uses frequency diversity technique that requires sparse bandwidth and multiple receivers
improving the complexity. In order to increase the system performance, the proposed system allows multiple user data to
be multiplexed together to form a single OFDM symbol where the resources are allocated equally to all users. The
proposed multiuser OFDM system achieves double the capacity and better performance in terms of Signal-to-Noise Ratio
and Bit Error Rate of a single user OFDM system and the UWB channel is used to securely transmit the data through the
channel. Also, the proposed system reduces the receiver complexity by using polarization diversity to receive the
multipath components for multiple users.
Index Terms— BER, Cyclic Prefix, Diversity, Inter Symbol Interference, Rayleigh channel, SNR.
——————————  ——————————
1. INTRODUCTION
1.1 OFDM
In OFDM, a large number of closely spaced orthogonal
subcarriers are used to carry data on many simultaneous
parallel data streams or channels. The sub-carriers are modulated
with Quadrature Amplitude Modulation (QAM) or Phase-Shift
Keying (PSK) which maintains total data rates similar to existing
single-carrier modulation schemes in the same bandwidth at a
low symbol rate. The key advantage of OFDM over single-carrier
schemes is its ability to deal with with severe channel conditions
without complex equalization filters by using orthogonality
property. Channel equalization is simplified because OFDM may
be viewed as using many slowly modulated narrow band signals
rather than one quickly modulated wideband signal. Inter Symbol
Interference (ISI) can be eliminated by the use of guard intervals
between symbols and exploit echoes and time-spreading to
achieve a diversity gain, i.e. a signal-to-noise ratio up gradation.
This also facilitates the design of Single Frequency Networks
(SFNs), in which several adjoining transmitters send the same
signal concurrently at the same frequency.
1.2 DIVERSITY
Diversity is a technique that is used to give back for
fading channel impairments. Diversity technique can be
implemented by using two or more receiving antennas that
receives multipath components. To counteract the effect of ISI,
equalization technique is used whereas Diversity is usually
employed to reduce the intensity and extent of the fades
experienced by a receiver in the fading channel[2].
The information signal is modulated through M different carriers
in frequency diversity system. Here each carrier should be
isolated from the other subcarrier by at least the coherence
bandwidth, so that different copies of the signal go through
independent fading. The optimal combining of independently
faded copies give a guide for decision at the receiver. The optimal
combiner used is the maximum ratio combiner.
1.3 FREE SPACE CHANNELS
A channel has a certain capacity for transmitting the
information signal from the transmitter to the receiver. There are
three commonly used wireless channels such as AWGN channel,
Rayleigh channel, Rician fading channel.
1.3.1 AWGN Channel
Additive white Gaussian noise (AWGN) channel is a basic or
frequently used channel model for analyzing modulation
schemes. In this model, a white Gaussian noise is added to the
signal that passes through the AWGN channel. This implies that
the channel’s amplitude, frequency response is flat with
unlimited or boundless bandwidth and phase frequency response
is linear for all frequencies without any amplitude loss and phase
distortion and the modulated signals pass through the channel.
Fading does not exist for this type of channel[7]. The transmitted
signal gets distorted only by AWGN process.
1.3.2 Rayleigh Channel
Constructive and destructive interference, and phase shifting of
the signal occurs due to the effect of multipath transmission. This
causes Rayleigh fading. There is no line of sight (NLOS) path in
which no direct path between transmitter and receiver exists in
Rayleigh fading Channel[6]. If the signal bandwidth of the
channel is smaller than coherence bandwidth, the channel is
called flat; or else it is frequency-selective fading channel.
1.3.3 Rician Channel
Rician distribution can be obtained when Rayleigh fading have a
strong line of sight (LOS)[7]. In environments where there is a
dominant Line-of-Sight (LOS) path between the transmitter and
the receiver, the complex Gaussian distributed fading coefficient

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should be modelled with a non-zero mean, which can be called
Rician fading.
1.3.4. UWB Channel
Due to the large bandwidth of operation, the UWB channel
influences new effects in the receiver as compared with narrow
band wireless channels. Due to the combination of reflected,
delayed, scattered and diffracted signal components introduces
severe multipath fading in the radio channel environment [9].
This fading degrades the Carrier to Signal Noise Ratio (CNR)
leading to higher Bit Error Rate (BER). The most prominent
indoor channel model in UWB systems is Saleh-Valenzuela (S-
V) approach which is based on arrival of multipath components
[5]. The multipath components arrived are divided into two
categories as cluster arrival rate and ray arrival rate.
1.4 FIXED AND DYNAMIC SUBCARRIER
ALLOCATION
The existing multiuser OFDM system uses static and
dynamic subcarrier allocation schemes. In the fixed resource
allocation scheme, many users suffer from poor channel gains
due to path loss and fading since the subcarriers allocated to each
user are fixed. On the other hand the fading parameters for
different users are mutually independent. The subcarriers that
appearing in deep fade for one user may not be in deep fade to
other users. Hence instantaneous channel state information is
required to each user for allocation of subcarriers. Instead a
decentralized approach is used by the dynamic subcarrier
allocation scheme and the instantaneous channel response of each
user are needed to be considered in parallel [3]. This dynamic
subcarrier allocation algorithm divides the available subcarriers
into a number of partitions and the partition with the highest
average channel gain is allocated to each user. In some cases two
or more users are assigned with same partition. Hence the
instantaneous channel information of all users are taken into
account. In this technique there is a risk that subcarriers in deep
fades would be modulated using higher order schemes due to a
high average channel gains. Also the complexity of the system
increases with increase in the number of partitions. On the other
hand, increasing the number of partitions will have an impact on
the probability of the successful subcarrier allocation plan. In the
adaptive resource allocation technique, within a single partition
all the subcarriers are modulated using same modulation scheme.
A disadvantage of such scheme is that subcarrier in deep fades
would be modulated using higher order modulation due to a high
average channel gain which would result in less than optimum
BER performance. But, for implementing such system with
higher complexity at both transmitter and receiver structure is
impractical.
2. PROPOSED SYSTEM
In multiuser OFDM, multiple users are assigned with a
subset of subcarriers. This allows simultaneous low data rate
transmission from several users. Hence the proposed system
multiplexes the data of multiple users to form a single data. The
OFDM transmitter converts the multiplexed digital data from
different users to be transmitted, are modulated using
conventional modulation technique. It then transforms this
frequency representation of the data to the time domain using an
Inverse Discrete Fourier Transform (IDFT). The Inverse Fast
Fourier Transform (IFFT) performs the same operations as an
IDFT, and it is much more computationally efficient, and is used
in all practical systems.
In order to transmit the OFDM signal the time domain signal is
then mixed up to the required frequency to be transmitted through
the channel. The reverse operation of the transmitter is performed
at the receiver by mixing the RF signal to base band for
processing, then by using a Fast Fourier Transform the signal is
analyzed in the frequency domain. The subcarriers amplitude and
phase are then picked out and converted back to digital data. The
FFT and the IFFT are complement to eachother and the most
appropriate term depends on whether the signal is being received
or generated. The term FFT and IFFT are used interchangeably
where the signal is independent.
.
Fig.2 Block Diagram of Proposed Multiuser OFDM System
2.1 MODULATION
BPSK is the simplest form of the Phase Shift Keying
(PSK). In BPSK, individual data bits are used to control the
phase of the carrier. The modulator shifts the carrier to one of two
possible phases during each bit interval, which are 180 degrees or
π radians apart. This modulation is the stoutest of all the PSKs
since it takes the highest level of noise or distortion to make the
demodulator reach an inaccurate decision.
2.2 SERIAL TO PARALLEL CONVERSION
Data to be transmitted is typically in the form of a serial
data stream. In OFDM, each symbol transmits nearly 4000 bits,
and so a serial to parallel conversion stage is needed to convert
the input serial bit stream to the parallel data to be transmitted in
each OFDM symbol[4]. The data allocated to each symbol
depends on the modulation scheme used and the number of
subcarriers. When an OFDM transmission occurs in a multipath
radio environment, frequency selective fading can result in
groups of subcarriers being heavily attenuated, which in turn can
result in bit errors. These nulls in the channel can cause the
information sent in neighboring carriers to be destroyed, resulting
in a clustering of the bit errors in each symbol. Most Forward
Error Correction (FEC) schemes tend to work more to improve
the performance most systems by utilizing data scrambling as
part of the serial to parallel conversion stage. This is implemented
by randomizing the subcarrier allocation of each sequential data
bit. At the receiver the original sequencing of the data bits is
obtained with a cluster of bit errors so that they are
approximately uniformly distributed in time.
2.3 FFT AND IFFT
After the subcarrier modulation stage each of the data
subcarriers is set to amplitude and phase based on the data being
sent and the modulation scheme. All unused subcarriers are set to
zero and this OFDM signal is in the frequency domain. The

3. INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
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obtained frequency domain is converted into time domain by
IFFT, allowing it to be transmitted. In the frequency domain,
before applying the IFFT, each of the discrete samples of the
IFFT belongs to an individual subcarrier. The outer subcarriers
are unmodulated and set to zero amplitude. These zero
subcarriers provide a frequency guard band before the Nyquist
frequency and effectively act as an interpolation of the signal and
allows for a realistic roll off in the analog anti-aliasing
reconstruction filters.
2.4 CYCLIC PREFIX
One of the most important properties of OFDM
transmissions is the robustness against the multipath delay
spread. A long symbol period is used to achieve this property,
which minimizes the inter-symbol interference. The level of
robustness, can actually be increased even more by the addition
of a guard interval between transmitted symbols. The guard
period allows the multipath signals of previous symbol to die
away before the information from the current symbol is gathered.
The cyclic extension of the symbol is the most effective guard
interval that can be used. The Fast Fourier Transform used at the
receiver transforms a cyclic time domain signal to its equivalent
frequency spectrum. The signal  kxn
is not necessary form of a
cyclic signal of N+L-1 samples by repeating the last L-1 samples
 kxn
at the beginning of the signal. This technique is called guard
interval by cyclic prefix.
2.5 PROTECTION AGAINST ISI
In an OFDM signal to maintain orthogonality among the
subcarriers, the amplitude and phase of the subcarrier must
remain constant over the period of the symbol. Inter-Carrier
Interference may occur if they are not constant and the spectral
shape of the subcarriers will not have the correct sinusoidal shape
resulting in the incorrect placement of nulls at correct
frequencies. At the symbol boundary the amplitude and phase
changes abruptly to the new value required for the next data
symbol.
ISI causes spreading of the energy between the symbols,
resulting in changes in the amplitude and phase of the subcarrier
at the start of the symbol in multipath environments. The length
of these transient effects depends on the radio channel delay
spread. This signal is a result of each multipath component
arriving at slightly different times.
2.6 CHANNEL MODEL
In wireless communication various channels can act as a
medium for the effective communication between the transmitter
and the receiver. Hence a Rayleigh fading channel is considered.
This is a frequency selective fading. The Rayleigh fading is
suitable for environments when no LOS path exists in between
transmitter and receiver, but only has the indirect path. The
resultant signal received at the receiver will be the sum of all the
reflected and scattered waves from the channel [8].
3. RESULTS AND DISCUSSION
3.1 PERFORMANCE OF MULTIUSER OFDM
The parameter specifications of the multi user OFDM
system is shown in the table 3.1,
Table.3.1 Specification of the Multi User OFDM System
The proposed multiuser OFDM system is simulated with
multiplexed data of 3 users. The total number of subcarriers is
128. Out of 128 subcarriers 8 subcarriers are used as pilot
symbols to estimate the channel condition and 24 subcarriers are
left unused to act as guard band. This reservation of subcarriers
for guard band helps to reduce out of band radiation. In this
project BPSK modulation with Rayleigh channel are used.
The Fig. 3.1 shows the performance of the multi user OFDM
system in terms of BER and SNR. Since the data of multiple
users are multiplexed together that resembles as a single user
OFDM symbol the multi user OFDM system gives a better
performance than single user OFDM system.
Fig. 3.1 BER Vs SNR of the Multi User OFDM system
The simulated result shows that the BER decreases with increase
in SNR.
3.2 PERFORMANCE COMPARISONS OF SINGLE
USER AND MULTI USER OFDM
The parameter specifications used in he designed single
user and the multi user OFDM system are compared in the
following table 3.2.
Table.3.2 Specification of the Single User and the Multi User OFDM System
Parameters
Single
User
Multi User
Number of Users 1 3
Number of Sub
Carriers
64 128
FFT Length 64 128
Number of Pilot
Symbols
4 8
BER at 30dB
Parameters Specification
Number of Users 3
Number of Subcarriers 128
FFT Length 128
Number of Data Subcarriers 96
Cyclic Prefix Length 16
Number of Pilot Symbols 8
Unused Subcarriers for
Guard Interval
24

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From the table 3.2 it is inferred that the proposed multiuser
OFDM system gives improved SNR and reduced BER when
compared to a single user OFDM system.
Fig. 3.2 BER and SNR comparison of Single user and multi user OFDM
The graph in the Fig 3.2 shows that the BER of multiuser OFDM
system follows the single user OFDM system till 35dB SNR
value. The BER of the multi user OFDM system reduces further
at SNR above 35 dB.
4. CONCLUSION
The performance of the proposed multiuser OFDM
system is obtained in terms of SNR and BER. The MATLAB
simulation results shows considerable improvement in BER
performance for a multiuser OFDM system compared to single
user OFDM. The proposed system achieves a BER of
for an SNR of 35dB which is a slight improvement than a single
user OFDM system. The proposed multiuser OFDM system is
modulated using BPSK modulation technique in the presence of a
Rayleigh fading channel. Also, it reduces the complexity of
receiver by using only one receive antenna instead of multiple
receive antennas for each user by multiplexing and
demultiplexing of their data into a single user data. This
eliminates the problem of resource allocation such as transmit
power and subcarrier to each user.
5. FUTURE WORK
My future work is to obtain the performance of multi user
OFDM with polarization diversity at the receiver in the presence
of the UWB channel to get improved SIR and reduced BIR. An
UWB signal can be transmitted at a much higher data rate in the
range of Gbps. Hence the UWB channel can be used to transmit
multi user OFDM signal. Also to reduce the receiver complexity,
Polarization diversity is used at the receiver that can be used to
overcome the drawbacks of existing diversity techniques.
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