 Open Access
 Total Downloads : 496
 Authors : H.Raghupathi, B.Karunaiah, K.V.Mulari Mohan
 Paper ID : IJERTV2IS90482
 Volume & Issue : Volume 02, Issue 09 (September 2013)
 Published (First Online): 19092013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
A BiLevel FHCDMA Scheme for Wireless Communication Over Fading Channels
H.Raghupathi, B.Karunaiah,K.V.Mulari Mohan Student M.Tech., Associate Professor, Professor Holymary Institute of Technology and Science
Abstract
In this project, we propose a Bilevel frequency hopping codedivision multipleaccess (FHCDMA) scheme for wireless communication systems. The new scheme provides flexibility in the selection of modulation codes and FH patterns. By partitioning the modulation codes, our Bi level scheme can be modified to support more possible users without increasing the number of FH patterns. The performance and spectral efficiency (SE) of the scheme are analyzed. Our results show that the partitioned Bilevel FHCDMA scheme supports higher data rate and greater SE than Goodmans frequencyshiftkeying FHCDMA scheme under some conditions.

Introduction
FREQUENCYHOPPING codedivision multiple access (FHCDMA) provides frequency diversity and helps mitigate multi path fading and diversify interference. Major advantages of FH CDMA over directsequence CDMA include better resistance to multiple access interference (MAI), less stringent power control, and reduced nearfar problem and multi path interference. By assigning a unique FH pattern to each user, a FHCDMA system allows multiple users to share the same transmission channel simultaneously. MAI occurs when more than one simultaneous users utilize the same carrier frequency in the same time slot. Onehit FH patterns have been designed in order to minimize MAI.
In addition, good man, proposed to add Marry frequency shift keying(MFSK) atop FHCDMA in order to increase data rate by transmitting symbols, instead of data bits.
we propose a partitioning method on the modulation codes, such that modulation codes with lower cross correlation values are grouped together. Using different groups of modulation codes as an additional level of address signature, the partitioned Bi level
FHCDMA scheme allows the assignment of the same FH pattern to multiple users, thus increasing the number of possible users. The performance of our Bi level FHCDMA scheme over additive white Gaussian noise (AWGN), and Rayleigh and Rician fading channels are analyzed algebraically.

ORTHOGONAL MCCDMA

MCCDMA is a form of CDMA or spread spectrum, but we apply the spreading in the frequency domain (rather than in the time domain as in Direct Sequence CDMA).

MCCDMA is a form of Direct Sequence CDMA, but after spreading, a Fourier Transform (FFT) is performed.

MCCDMA is a form of Orthogonal Frequency Division Multiplexing (OFDM), but we first apply an orthogonal matrix operation to the user bits. Therefore, MC CDMA is sometimes also called "CDMA OFDM".

MCCDMA is a form of Direct Sequence CDMA, but our code sequence is the Fourier Transform of a Walsh Hadamard sequence.

MCCDMA is a form of frequency diversity. Each bit is transmitted simultaneously (in parallel) on many different sub carriers. Each subcarrier has a (constant) phase offset. The set of frequency offsets form a code to distinguish different users.
The MCCDMA method described here is NOT the same as DSCDMA using multiple carriers. In the latter system the spread factor per sub carrier can be smaller than with conventional DSCDMA. Such a scheme is sometimes called MCDSCDMA. This does not use the special OFDMlike waveforms to ensure dense spacing of overlapping, yet orthogonal sub carriers. MCDSCDMA has advantages over DSCDMA as it is easier to synchronize to this type of signals.
Fig 2.1: possible implementation of an MultiCarrier spreadspectrum transmitter
MCCode Division Multiple Access systems allow simultaneous transmission of several such user signals on the same set of subcarriers. In the downlink multiplexer, this can be implemented using an Inverse FFT and a Code Matrix.
Fig2.4 : Alternative implementation of a Multi Carrier spreadspectrum transmitter, using the Direct sequence principle.

RECEIVER DESIGN
Because of delay spread and frequency dispersion due to multipath fading, subcarriers are received with different amplitudes. An importance aspect of the receiver design is how to treat the individual subcarriers, depending on their amplitude ri. Options are
Fig 2.2 : FFT implementation of an MCCDMA base station multiplexer and transmitter.
MCCDMA as a special case of DSCDMA
Fig 2.3 : possible implementation of a Multi Carrier spreadspectrum transmitter.
The above transmitter can also be implemented as a DirectSequence CDMA transmitter, i.e., one in which the user signal is multiplied by a fast code sequence. However, the new code sequence is the Discrete Fourier Transform of a binary, say, Walsh Hadamard code sequence, so it has complex values.

Linear combining, by weighting the ith subcarrier by a factor di according to

Maximum Ratio Combining: di = ri. This optimally combats noise, but does not exploit interference nulling. (See also MRC diversity)

Equal Gain: di = 1. The simplest solution. (See also EGC diversity)

Equalization: di = 1/ri. This perfectly restores orthogonality and nulls interference, but excessively boosts noise.
i
i

Wiener filtering: di = ri/(r 2 + c). This gives the best postcombiner signaltonoiseplusinterference ratio.


Maximum likelihood detection


ADVANTAGES OF MCCDMA
Compared to Direct Sequence (DS) CDMA. DSCDMA is a method to share spectrum among multiple simultaneous users. Moreover, it can exploit frequency diversity, using a RAKE receiver. However, in a dispersive multipath channel, DS CDMA with a spread factor N can accommodate N simultaneous users only if highly complex interference cancellation techniques are used. In practice this is difficult to implement. MCCDMA can handle N simultaneous users with good BER, using standard receiver techniques.
Compared to OFDM. To avoid excessive bit errors on subcarriers that are in a deep fade, OFDM typically applies coding. Hence, the number of subcarriers needed is larger than the number of bits or symbols transmitted simultaneously. MCCDMA replaces this encoder by an N x N matrix operation. Our initial results reveal an improved BER



Multiple Access Techniques
Multiple access schemes are used to allow many simultaneous users to use the same fixed bandwidth radio spectrum. In any radio system, the bandwidth, which is allocated to it, is always limited. For mobile phone systems the total bandwidth is typically 50 MHz, which is split in half to provide the forward and reverse links of the system.

Frequency Division Multiple Access (FDMA):
In Frequency Division Multiple Access (FDMA), the available bandwidth is subdivided into a number of narrower band channels. Each user is allocated a unique frequency band in which to transmit and receive on. During a call, no other user can use the same frequency band.
Each user is allocated a forward link channel (from the base station to the mobile phone) and a reverse channel (back to the base station), each being a single way link. The transmitted signal on each of the channels is continuous allowing analog transmissions. The bandwidths of FDMA channels are generally low (30 kHz) as each channel only supports one user. FDMA is used as the primary breakup of large allocated frequecy bands and is used as part of most multichannel systems.
Fig. 5 TDMA/FDMA hybrid, showing that the bandwidth is split into frequency channels and time slots.

Code Division Multiple Access:
Code Division Multiple Access (CDMA) is a spread spectrum technique that uses neither frequency channels nor time slots. In CDMA, the narrow band message (typically digitized voice data) is multiplied by a large bandwidth signal, which is a pseudo random noise code (PN code). All users in a CDMA system use the same frequency band and transmit simultaneously. The transmitted signal is recovered by correlating the received signal with the PN code used by the transmitter. Fig. 1.6 shows the general use of the spectrum using CDMA.

CDMA Generation:
CDMA is achieved by modulating the data signal by a pseudo random noise sequence (PN code), which has a chip rate higher then the bit rate of the data. The PN code sequence is a sequence of ones and zeros (called chips), which alternate in a random fashion. The data is modulated by modular2 adding the data with the PN code sequence. This can also be done by multiplying the signals, provided the data and PN code is represented by 1 and 1 instead of 1 and 0. Fig.8 shows a basic CDMA transmitter.

CDMA SYSTEM :
The rapid worldwide growth in cellular telephone subscribers over the past decade has evidently showed that the wireless communication is an effective means for transferring information in todays society. Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) are two approaches that have contributed to this advancement in the telecommunications industry.
However, the widespread success of these communications systems has led to the development for newer and higher technology techniques and standards in order to facilitate highspeed communication for multimedia, data and video in addition to voice transmissions . Code Division
Multiple Access (CDMA) is todays dominant technology for the evolution of third generation (3G) mobile communications systems with the development of two major schemes: Wideband CDMA (WCDMA) and CDMA2000.
The WCDMA technology otherwise known as the Universal Mobile Telecommunications System (UMTS) is designed with the intention of providing an upgrade path for the existing Global System for Mobile Communications (GSM) while CDMA2000 is based on the fundamental technologies of IS95, IS95A (cdmaOne) as well as the 2.5G IS95B systems . These two schemes are similar for their ability to provide high data rates and the efficient use of bandwidth but are incompatible as they use different chip rates. The following sections of this chapter will describe and explain the basic concepts behind the CDMA technology.


Spread Spectrum Multiple Access
The spread spectrum modulation techniques are originally developed for use in the military and intelligence communications systems due to their resistance against jamming signals and low probability of interception (LPI). They are immune to various kinds of noise and multipath distortion.
A number of modulation techniques have been developed to generate spread spectrum signals. These can be generally classified as directsequence spread spectrum (DSSS), frequencyhopping spread spectrum (FHSS), timehopping spread spectrum (THSS), chirp modulation and the hybrid combination modulation . We will look into the functionality of the DSSS and how this modulation technique is incorporated to the CDMA system.

DirectSequence Spread Spectrum (DSSS):
The DSSS technique is one of the most popular forms of spread spectrum. This is probably due to the simplicity with which direct sequencing can be implemented. Figure 1 shows the basic model and the key characteristics that make up the DSSS communications system. In this form of modulation, a pseudorandom noise generator creates a spreading code or better known as the pseudonoise (PN) code sequence. Each bit of the original input data is directly modulated with this PN sequence and is represented by multiple bits in the transmitted signal. On the receiving end, only the same PN sequence is capable of Demodulating the spread spectrum signal to successfully recover the input data .
Fig:3.1 Basic model of the directsequence spread spectrum communications system
The bandwidth of the transmitted signal is directly proportional to the number of bits used for the PN sequence. A 7bit code sequence spreads the signal across a wider frequency band that is seven times greater than a 1bit code sequence, otherwise termed as having a processing gain of seven. Figure 2.2 illustrates the generation of a DSSS signal using an exclusiveOR (XOR) operation. The XOR obeys the following rules:
Fig:3.2 Generation of a DSSS signal with processing gain = 7
Note that an input data bit of zero causes the PN sequence coding bits to be transmitted without inversion, while an input data bit of one inverts the coding bits. Rather than to represent the binary data with bits 0s and 1s, the input data and PN sequence are converted into a bipolar waveform with amplitude values of Â±1. This is further illustrated in figure 3.
Fig: 3.3 Transmitter of the DSSS system
we are also able to identify two criteria that need to be met in order to be considered as a DSSS system. The bandwidth of the transmitted signal s(t) is much wider as compared to the input data m(t); and This wide bandwidth is caused by the modulation of the spreading signal c(t) and the intended receiver requires this identical signal for retrieving the message signal m(t). In the next few sections, we will look into the functionality of the various components for a DirectSequence Code Division Multiple Access (DSCDMA) system.


. New BiLevel FHCDMA Scheme Description
In our Bilevel FHCDMA scheme, the available transmission bandwidth is divided into Mh frequency bands with Mm. carrier frequencies in each band, giving a total of MmMh carrier frequencies. In the first first level, a number of serial data bits is grouped together and represented by a symbol.
Table slots (i.e., pattern length). The elements in the modulation codes and FH patterns determine the carrier While an element of a modulation code defines the carrier frequency used in a frequency band in a given time slot, an element of the FH pattern determines which frequency band to use.

Partitioned Bilevel FHCDMA Scheme

In general, the number of possible users in a FH CDMA system is limited by the number of available FH patterns. However, our Bilevel FHCDMA scheme can flexibly increase the number of possible users by trading for lower data rate through a reduction of symbol size. It is done by partitioning
the modulation codes into several groups and each group contains reduced number of modulation codes.

Performance Analyses and Comparisons
In FHCDMA systems, MAI depends on the cross correlation values of FH patterns. For our two level FHCDMA scheme, the crosscorrelation values of the modulation codes impose additional (symbol) interference and need to be considered. Assume that onehit FH patterns of dimensions MhXLh and the transmission band is divided into MmMh frequencies.
The probability that a frequency of an interferer its with one of the wmfreuencies of the desired user is given by
the probability that the dehopped signal contains n entries an undesired row is given by
Over AWGN, and Rayleigh and Rician fading channels, false alarms and deletions may introduce detection errors to the received FHCDMA signals. A falsealarm probability, ,is the probability that a tone is detected in a receiver when none has actually been transmitted. The False Alarm probability is generally by
In this section, we compare the performances of the new Bilevel FHCDMA and Goodmans MFSK/FH CDMA schemes uder the condition of same transmission parameters.
Also shown in the figure is the computer simulation result for validating our theoretical analysis. The computer simulation of our twolevel FHCDMA scheme is performed as follows. The FH pattern assigned to each user is arbitrarily selected from all 472 possible(11 Ã— 47, 11, 0, 1) prime
sequences constructed from GF(47)and then all 112 possible (4 Ã— 11, 4, 0, 1) prime sequences constructed from GF(11) are used as the modulation codes for each user. For each simulation point in the figure, the total number of data bits involved in the simulation ranges from104 to 106, depending on the targeted error probability.
Fig: 5.1 BEPs of the partitioned Bilevel FHCDMA and Goodmans FHCDMAschemes versus the number of simultaneous users K over a Rayleighfading chanel , where MgxLg=44×47,wg=wm=4, MmxLm=4×11,MhxLh=11×47.
Fig. 5. BEPs of the Bilevel FHCDMA scheme versus the number of simultaneous users K over AWGN, and Rayleigh and racial fading channels, where wm=4,Mm=4×11,MhxLh=11×47

Conclusion
In this project, we proposed a new Bilevel FHCDMA scheme. The prime/FHCDMA and RS/FHCDMA schemeswere special cases of our scheme. The performance analysesshowed that the Bilevel FHCDMA scheme provided atradeoff between performance and data rate. The partitioned Bilevel FHCDMA scheme increased the number of possible users and exhibited higher data rate and
greater SE than Goodmans MFSK/FHCDMA scheme. In summary, the new scheme offered more flexibility in the design of FHCDMA systems to meet different operating requirements.

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