10-cdma.ppt

1 CDMA munication & IS-95 CDMA munication & IS-95 Most of the slides are stolen from Prof. Abhay Karandikar ’s lecture 2 Spread Spectrum Spread Spectrum Priniciples Priniciples ? Does not attempt to allocate disjoint frequency or time slot resources – Instead, this approach allocates all resources to simultaneous users, controlling the power transmitted by each user to the minimum required to maintain a given SNR ? Each user employs a noise-like wideband signal occupying the entire frequency allocation ? Each user contributes to the background noise affecting all users, but to the least extent possible. 3 Spread Spectrum Spread Spectrum Priniciples Priniciples ? This restriction on interference limits capacity, but because time and bandwidth resource allocations are unrestricted, the resulting capacity is significantly higher than the conventional system 4 Spread Spectrum Spread Spectrum Priniciples Priniciples ? Suppose each user use a wideband Gaussian noise carrier ? Suppose each user ’ s transmission is controlled so that all signals received at the BS are of equal power ? Let P s be the power of each user, and the background noise be negligible. ? Then the total interference power, I, presented to each user ’ s demodulator is I = (K-1) P s (1) where K is the number of users 5 Spread Spectrum Spread Spectrum Priniciples Priniciples ? Let ’ s say demodulator of each user operates at bit-energy-to-noise-density level of E b /N 0 . ? So the noise density received by each user ’ s demodulator is N 0 = I/W (2), where W Hz is the bandwidth of the wideband noise carriers ? The received energy per bit is the received signal power divided by the data rate R (bits/s), ., E b = P s /R (3) 6 Spread Spectrum Spread Spectrum Priniciples Priniciples ? Combining (1), (2) and (3) we get K – 1 = I/P s = (W/R) / (E b /N 0 ) (4) ? If W >> R then the capacity of the system can be large – ., transmission bandwidth should be much