Diversity Combining for DS CDMA System in a Rayleigh Fading Channel with Three Spreading Codes

Fading problem is a major impairment of the wireless communication channel. In this paper we considers different diversity techniques to mitigate the fading problem in wireless channel for DS CDMA (Direct sequence Code Division Multiple Access) system. Diversity is an effective method for increasing the received signal-to-noise ratio of a wireless communication system in a flat Rayleigh fading channel. Diversity branches can be established through frequency, time, antenna, or any combination of these diversity mechanisms. The branch outputs can be processed using schemes such as selection combining, equal gain combining, or maximal ratio combining. The diversity combining technique used in this paper improved the performance of the DS CDMA system. For this system, three spreading code sequences are used to spread the spectrum of the signal namely, an M-sequence, a Gold sequence, and an Orthogonal Gold sequence. The BER performance in a two path flat Raleigh fading environment for DS CDMA system with diversity combining is evaluated using MATLAB 7 computer simulation software. خلا ـ صلا ـــــــ ة توفخلا ةلكشم (Fading) يدؤت ًتلا ةٌسٌئرلا لكاشملا نم ىلإ فاعضإ ةراشلإا لاصتلاا تاونق ًف لا ًكلسلا (Wireless Communication) . عٌونتلا تاٌنقت فلتخم ةسارد تمت ثحبلا اذه ًف (Diversity) ًتلا للقت ةلكشم نم تاونق ًف توفخلا لا لاسرلإا ماظنل ًكلسلا DS CDMA (Direct Sequence Code Division Multiple Access) . عٌونتلا (Diversity) ةبسن ةداٌزل ةرثؤم ةقٌرط ًه ةراشلإا ىلإ ةملتسملا ًف ءاضوضلا تاونق ًف ا لاصتلا ا ل ًكلسلا ةانقل توفخ (Rayleigh) . عٌونتلا (Diversity) اهنٌوكت نكمم ًف (Frequency, Time, Antenna) وأ يأ . تاٌنقتلا هذه نٌب داحتا جراخم عٌونتلا (Diversity) اهتجلاعم نكمملا نم ب مادختسا ةمظنأ لثم (diversity combining) ًهو ( (Selection, Equal Gain, or Maximal Ratio Combining . نٌسحت مت دقل ءادلأا ماظنل (DS CDMA) ةٌنقت مادختساب (Diversity Combining) . ماظنلا اذه ًف ثلاث مادختسا مت عاونأ نم عاونأ ارفشلا ضرغل ت لمع راشتنا فٌطلل (Spread Spectrum) ًهو (M-Sequence, a Gold Sequence, and an Orthogonal Gold Sequence) . مت دقل تبلا اطخ ةٌلامتحا ةبسن باسح (Bit Error Rate) BER ماظنل (DS CDMA) ةٌنقت مادختساب (Diversity Combining) ةراشلإا رمت امدنع قٌرط نع ةبساحلا مادختساب ةاكاحملا ةٌلمع تمت دقل .توفخلل نٌراسمب (MATLAB7 Computer Software) . 1. Fading in Communication Channels Asst. Lect. Ghanim Abd Al-Kareem Al-Rubyai Electrical Engineering Dept., College of Eng. Al-Mustansiriya University, Baghdad, Iraq Journal of Engineering and Development, Vol. 11, No. 1, March (2007) ISSN 1813-7822 14 Fading is the term used to describe the rapid fluctuations in the amplitude of the received radio signal over a short period of time. Fading is a common phenomenon in Mobile Communication Channels, where it is caused due to the interference between two or more versions of the transmitted signals which arrive at the receiver at slightly different times. The resultant received signal can vary widely in amplitude and phase, depending on various factors such as the intensity, relative propagation time of the waves, bandwidth of the transmitted signal etc... [1,2] . 1.1 Mathematical Model Consider a transmitted signal s(t) = Acos (2πfct) through a fading channel. The received signal can be expressed as (ignoring the effects of noise):