Thursday, 12 July 2012

Multi User CDMA System Model

Let's have CDMA System with $K$ Users indexed by $j=1 \cdots K$. Let Each User Transmits BPSK Modulated Signal Simultaneously. Then the Transmitted Signal for one such User $j$ can be Represented as:

$$ S_j(t)= \sqrt{2 P_j}\,c_j(t)\,b_j(t)\,Cos(\omega_c t+\theta_j) $$ Where,

$$c_j(t)=\sum_{n=-\infty}^{\infty}\,c_j^{(n)}\,p_{T_c}(t-n\,T_c) $$ is the $j$th User Signature Waveform, with the Signature bits

$$c_j^{(n)} \in {-1,+1} $$

$$ b_j(t)=\sum_{n=-\infty}^{\infty}\,b_j^{(n)}\,p_T(t-n \, T) $$ is the BPSK Modulated Signal for User $j$

$$b_j^{(n)} \in {-1,+1} $$ and

$$ T=N\,T_c $$ Where $N$ is the Spreading Factor or Processing Gain. The Received Signal at the Receiver can be Represented as

$$ r(t)= \sum_{j=1}^{K} \sqrt{2 P_j}\,c_j(t-\tau_j)\,b_j(t-\tau_j)\,Cos(\omega_c t+\phi_j) + \eta(t) $$ Where $\tau_j$ is Relative Time offset and $$\tau_j \in \left [ 0 \: T \right ] $$ $\phi_j$ is Phase Offset such that $$\phi_j  \in \left [ 0 \: 2 \pi \right ]$$ and $$\phi_j=\theta_j-\omega_c \tau_j $$ $\eta(t)$ is Zero Mean AWGN Process with PSD $N_0$

1 comment:

  1. Performance Analysis can be Obtained Using matched Filter Detection of Each User.Lets do the Detection for User 1.

    $$ z_1= \int_{0}^T r(t)c_1(t)Cos(\omega_c t) dt $$