INTERFERENCE MITIGATION BASED ON CARRIER INTERFEROMETRY CODES IN THE DOWNLINK LOW EARTH ORBIT SATELLITE CHANNEL

The multi-carrier code division multiple access (MC-CDMA) system employs the Hadamard-Walsh (HW) code in the low earth orbit (LEO) satellite systems. It has been used as spreading code to maintain the multi-access interference (MAI) in the downlink LEO satellite channel. However, due to the non-orthogonality of the HW codes, these interferences are observed at the MC-CDMA satellite receiver. As a result, the LEO satellite system performance is degraded. In this paper, we propose to employ the orthogonal Carrier Interferometry (CI) code with MC-CDMA in the downlink LEO satellite channel. A new system is called CI/MC-CDMA which is used to mitigate the MAI. Moreover, the performance of the CI/MC-CDMA satellite system in terms of bit error rate (BER) is analyzed and simulated. To confirm the analysis, the system is compared to the HW/MC-CDMA system. It has been shown that CI/MC-CDMA satellite system outperforms HW/MC-CDMA.


INTRODUCTION
In recent years, multicarrier code division multiple access (MC-CDMA) is accepted technology for LEO satellite systems [1].Specifically, when the bandwidth of the transmitted signal is more than the coherence bandwidth of the channel leading to frequency selective fading channel [2], [3].Many studies have been done for MC-CDMA techniques which provide improved robustness concerning DS-CDMA techniques in the frequency-selective multipath channel [4].
In the MC-CDMA system, each user is assigned a unique orthogonal code, where each user's data symbol is spread by that code.The number of spreading In the mobile wireless communication system, CI codes are employed as spreading codes for the MC-CDMA system [7].It is introduced in frequencyselective Rayleigh fading channels.However, the CI codes have many advantages compared to other codes, for example, the CI codes can be designed for any length N (N∈I).Also, it is capable of supporting greater than N users.In [8], CI codes are proposed for the MC-CDMA system, and applied in downlink frequency-selective mobile wireless fading channels.This confirmed that CI/MC-CDMA system provides improved performances in terms of probability-of-error compared with HW/MC-CDMA system.In [9], carrier interferometry spreading codes are proposed for DS-CDMA systems to reduce the multi-beam interference in LEO satellite systems.Moreover, The CI codes are proposed for the MC-CDMA system over a hybrid Satellite/Underwater Acoustic Channel.
In this paper, we propose to apply CI/MC-CDMA in downlink frequencyselective LEO satellite systems.This will facilitate the MC-CDMA receiver to separate subcarrier components from the received signal by performing an FFT operation and accumulating them in the frequency domain.
This work demonstrates the performance improvement achievable through applying CI codes in the MC-CDMA system, and transmission over frequencyselective LEO satellite channel.The rest of this paper is organized as follows.In section II the CI/ MC-CDMA system model is given.Section III presents the channel model.Section IV presents the Performance Analysis.Simulation results and conclusions are provided in section V.

CI/ MC-CDMA SYSTEM MODEL
The transmitter for the user in the MC-CDMA system is shown in Fig. each user are converted from serial to parallel.The parallel data symbols are spread by a unique orthogonal code.A Synchronous MC-CDMA transmission system employing two different orthogonal spread codes (WH & CI) is considered.In the MC-CDMA system, each transmitter applies a unique orthogonal code to maintain orthogonality between different users.Each spreading symbol is modulated by orthogonal subcarriers using N point IFFT.The output of N point IFFT is converted to the serial data stream, and cyclic prefix (CP) is added to the final form.The output of the point of IFFT ( ) is represented as: Where √ is a constant that ensures symbol energy of unity ( ).The spread code with length corresponding to the user is: For CI codes And for Hadamard-Walsh codes ( The pass-band representation of the transmitted signal for user is: where ( ) is pulse shaping filter with duration , where is symbol duration, and cyclic prefix duration.The frequency of subcarrier + , where is the carrier frequency, and subcarrier separation .

Journal of Alasmarya University: Basic and Applied Sciences
The total transmitted signal considering all users' for MC-CDMA system is The wideband MC-CDMA signal ( ) of ( 6) is transmitted over frequency-selective downlink LEO satellite channel.The signal arriving at the receiver is: where and are fading gain coefficients, phase offset, and time delay parameters for of subcarrier, respectively.The line of sight (LOS) component is given by and multipath components are given by .The additive white Gaussian noise (AWGN) is presented by ( ) with power spectral density .The MC-CDMA receiver model is illustrated in Fig. 2.

CHANNEL MODELING
In this study, Loo's statistical channel model is used for the land mobile satellite (LMS) channel [10].The channel model parameters were taken from linear regression fits of experimental measurement results of real-world LMS channels in the environment of rural areas [11].The channel model for land mobile satellite systems is usually modeled as composite fading distributions to describe more accurately the amplitude fluctuation of the signal envelope.Although some mathematical models, such as the Loo model have been developed to describe the satellite channel.It provides a significantly less computational burden than other channel models.In this model, the amplitude of LOS is characterized by the Nakagami distribution, and the multipath part is the Rayleigh distribution.The fading channel of the satellite links can be modeled as.
Where ( ) is the stationary random phase process with uniform distribution over [0,2π), while is the deterministic phase of the LOS components.The independent stationary random process ( ) and ( ), which are also independent of ( ) are the amplitudes of the scatter and the LOS components, following Rayleigh and Nakagami distributions, respectively.

Journal of Alasmarya University: Basic and Applied Sciences
Where ,the average power of the scattering components, ( ) is the gamma function, is the Nakagami parameter with Var [.], and ,is the average power of the LOS component.
The case of is associated with suburban and rural areas with partial obstruction of the LOS component.The received signal envelope | ( )| for the mobile satellite channel has probability density function (PDF) defined by: where ( ) is the confluent hypergeometric function.To take care of propagation effects regardless of transmitted signal characteristics, the most general approach is wideband modeling which is proposed.Therefore, we propose to extend narrowband Abdi's channel to wideband and employ it in our simulation.A block diagram of the channel model is presented in Fig. 3.
In this particular case, the wideband LMS channel is the tapped delay-line model, where each tap is described by a narrowband model.This channel model is defined as a time-varying impulse response consisting of four paths.The first path represents the amplitude of the LOS component, following Nakagami distribution ( ) .The remaining three paths represent multipath part following Rayleigh distribution ( ) (t) for ( ).The multipath part parameters are determined by the channel delay profile (CDP) given in Table I, where represents the average power of taps, and ( ) is channel delay.One way of incorporating the effect of the elevation angle in a statistical LMS channel model is to use empirical expressions for the parameters of the envelope PDF in terms of the elevation angle [11].We use least-square polynomial fits of preliminary wideband measurement data to calculate these parameters: The the parameter shown in the figure is used to control multipath power.Requiring that the overall average power of the MP part is equal to unity, we have that

THE MODULES ANALYSIS
In this section, we provide an analytical comparison of the performance of traditional HW/MC-CDMA and CI/MC-CDMA in downlink frequency-selective LEO satellite channels.The received signal ( ) ( ) is down-converted after CP removal.The receiver employs fast Fourier transform (FFT) to separate orthogonal subcarriers, then the output of FFT q point (q=0,1,…N-1) is projected over the set of orthogonal subcarriers N, to form Where ( ) the frequency response of the LMS channel for user, which is represented by: The frequency response of the MMSE equalizer for user is:

Fig. 4 .
Fig.4.Simulation results for CI / MC-CDMA and HW / MC-CDMA systems over downlink LEO satellite channel (Half load)