Improving The Performance Of Multiuser OFDM Wireless System With UWB Channel

INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 1 – MAY 2015 - ISSN: 2349 - 9303

Improving The Performance Of Multiuser OFDM Wireless System With UWB Channel Jayanthi k2

Bhuvaneshwari P1

Associate Professor SNS College of Technology,Department of ECE, jayanthiecesnsct06@gmail.com

1

SNS College of Technology, ME Communication Systems, Department of ECE, bhuvaneshwari2531@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 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.

1.1 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.

1.3 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.

1.2 DIVERSITY

1.3.2 Rayleigh Channel

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

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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INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 1 – MAY 2015 - ISSN: 2349 - 9303 should be modelled with a non-zero mean, which can be called Rician fading.

1.3.4. UWB Channel

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.

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 (SV) 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 ALLOCATION

DYNAMIC

SUBCARRIER

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.

.

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

2. PROPOSED SYSTEM

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.

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

2.3 FFT AND IFFT

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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

INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 1 – MAY 2015 - ISSN: 2349 - 9303 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 xn k  is not necessary form of a

Parameters Number of Users Number of Subcarriers FFT Length Number of Data Subcarriers

Specification 3 128 128 96

Cyclic Prefix Length Number of Pilot Symbols Unused Subcarriers for Guard Interval

16 8 24

cyclic signal of N+L-1 samples by repeating the last L-1 samples xn k  at the beginning of the signal. This technique is called guard

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.

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.

Fig. 3.1 BER Vs SNR of the Multi User OFDM system

The simulated result shows that the BER decreases with increase in SNR.

2.6 CHANNEL MODEL

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.

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

Table.3.2 Specification of the Single User and the Multi User OFDM System

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

Parameters Number of Users Number of Sub Carriers FFT Length Number of Pilot Symbols BER at 30dB

35

Single User

Multi User

1

3

64

128

64

128

4

8

INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 1 – MAY 2015 - ISSN: 2349 - 9303 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.

[5]

[6]

[7]

[8] 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.

[9]

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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[4] Jiang M. and Hanzo L., “Multiuser MIMO-OFDM for next

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