Florida Institute of technologies ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 15: Capacity of CDMA systems Spring 2011

Florida Institute of technologies Page 2 Single cell capacity in CDMA systems Noise rise Pole point Capacity of a multiple cell system Examples Outline Important note: Slides present summary of the results. Detailed derivations are given in notes.

Florida Institute of technologies CDMA – single cell capacity All mobiles are transmitting co-frequency and co-time All mobiles are tightly power controlled so that they transmit at minimum transmit power Simplifying assumptions oPower control is perfect oIsolated cell (i.e., no out of cell interference) o All mobiles have same Eb/Nt requirement at the baseband (after de-spreading) oAll mobiles have same activity (voice activity) Page 3 Single CDMA cell Processing gain in dB

Florida Institute of technologies CDMA – single cell capacity (2) Page 4 After de-spreading the Eb/Nt ratio for each mobile P i - average received power from the ith user R b - bit rate of the ith user - activity of the ith user I t - total power in the channel W - channel bandwidth Solving for received power of the ith mobile Portion of total power dedicated to the ith user Note 1: received power of all users are assumed the same Note 2: mobiles implement activity detection and does not transmit when there is no data to be transmitted

Florida Institute of technologies Noise rise Page 5 Noise rise in dB One may write Therefore kT - PSD of thermal noise (~4e-18mW/Hz) F - noise figure of the receiver W - bandwidth of the channel (~ chip rate) Increase of noise in CDMA channel

Florida Institute of technologies Pole point Noise rise – depends on system loading As loading approaches 1 (100%), the noise rise becomes infinite When the noise approaches infinity system cannot operate: Pole point Pole point – maximum theoretical limit of CDMA capacity Page 6 Noise rise as a function of loading Assuming all mobiles are the same

Florida Institute of technologies Pole point Capacity of a CDMA system depends on oProcessing gain oVoice activity oSignal to noise requirements of individual users Page 7 Example: Consider IS-95 based CDMA system. Following parameters are known: chip rate – Mc/sec, bit rate kb/sec, voice activity – 0.5, Eb/Nt = 7dB. Calculate maximum theoretical single cell capacity. Answer: 35

Florida Institute of technologies CDMA – multi-cell capacity Mobiles in all cells are co-spectrum and co-time Out-of-cell increase the overall noise rise Page 8 Define The loading in multi-cell case Typical value for I adj is 0.66

Florida Institute of technologies CDMA- Multi-cell pole point Pole point is a fundamental limit for CDMA capacity – systems cannot operate at pole point Systems are designed to operate at a fraction of pole point: beta between 30 and 75% Noise rise may be seen as a link budget loss In CDMA capacity and coverage are interdependent oAt low loading coverage is larger oAs loading increases – coverage decreases oChange of coverage as a function of loading – cell breathing Coverage design in CDMA is at a given assumed loading – a.k.a. Engineering capacity Page 9 Change of CDMA coverage due to system loading Review attached link budget