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ECE 5221 Personal Communication Systems

ECE 5221 Personal Communication Systems. Prepared by: Dr . Ivica Kostanic Lecture 15: Capacity of CDMA systems. Spring 2011. Outline . Single cell capacity in CDMA systems Noise rise Pole point Capacity of a multiple cell system Examples.

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ECE 5221 Personal Communication Systems

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

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

  3. 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 • Power control is perfect • Isolated cell (i.e., no ‘out of cell’ interference) • All mobiles have same Eb/Nt requirement at the baseband (after de-spreading) • All mobiles have same activity (voice activity) Single CDMA cell Processing gain in dB

  4. CDMA – single cell capacity (2) After de-spreading the Eb/Nt ratio for each mobile Pi - average received power from the ith user Rb - bit rate of the ith user a - activity of the ith user It - total power in the channel W - channel bandwidth 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 Solving for received power of the ith mobile Portion of total power dedicated to the ith user

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

  6. 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 Noise rise as a function of loading Assuming all mobiles are the same

  7. Pole point • Capacity of a CDMA system depends on • Processing gain • Voice activity • Signal to noise requirements of individual users Example: Consider IS-95 based CDMA system. Following parameters are known: chip rate – 1.2288Mc/sec, bit rate - 14.4kb/sec, voice activity – 0.5, Eb/Nt = 7dB. Calculate maximum theoretical single cell capacity. Answer: 35

  8. CDMA – multi-cell capacity • Mobiles in all cells are co-spectrum and co-time • ‘Out-of-cell’ increase the overall noise rise Define The loading in multi-cell case Typical value for Iadj is 0.66

  9. 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 • At low loading coverage is larger • As loading increases – coverage decreases • Change of coverage as a function of loading – cell breathing • Coverage design in CDMA is at a given assumed loading – a.k.a. “Engineering capacity” Change of CDMA coverage due to system loading Review attached link budget

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