Florida Institute of technologies ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 13: Frequency allocation and channelization Spring 2011

Florida Institute of technologies Page 2  Frequency reuse and co-channel interference  DS-SS and CDMA Outline Important note: Slides present summary of the results. Detailed derivations are given in notes.

Florida Institute of technologies Frequency reuse  High capacity achieved thorough frequency reuse  Channel reuse increases interference in the system  Minimum reuse distance depends on system’s capability to handle interference  Prediction of interference oComplicated in general case oSpecial tools used for prediction and mitigation  Simplified analysis gives some indications of under laying principles  Assumptions oRegular cell layout oInterference beyond first tier of reuse - neglected oSites have the same configuration Page 3 C/I – carrier to interference ratio R – distance to serving site D i – distance to ith interferer n – path loss exponent i 0 – number of first tier interferers C/I estimate:

Florida Institute of technologies Interference estimation Page 4 Define frequency reuse ratio Q Neglecting differences between different D i ‘s Note 1: C/I depends on N and n Note 2: C/I increases with N – separation of cells increased Note 3: C/I increase with n – signal decays faster Reuse scheme N C/I [dB] C/I estimate for some common reuse schemes n = 3.84 is assumed Cellular geometry allows some values for N. Allowed values satisfy condition i and j are integers

Florida Institute of technologies Examples of C/I requirements  C/I depends on cellular technology air interface properties  Different technologies have different oModulation oCoding oDiversity oMobility patterns, etc.  For nominal design purposes a number is provided for a given technology Page 5 TechnologyC/I [dB]Reuse N GSM124 GSM with FH93 iDEN2112 cdma WCDMA-41 C/I requirements for some deployed technologies Note 1: Theory so far, assumes fixed frequency allocation and FDMA Note 2. Results derive assuming non-adaptive air interface and worst case scenario Estimate of N given C/I requirement

Florida Institute of technologies DS-SS and CDMA  DS-SS: Direct Sequence Spread Spectrum  CDMA – Code Division Multiple Access  DS-SS/CDMA – air interface scheme adopted by 3G technologies (cdma2000/EvDO and WCDMA/HSPA)  Wideband channel oCdma2000/EvDO – 1.3MHz oWCDMA/HSPA – 5MHz  Frequency reuse of N=1  Duplexing – FDD  All users co-channel and co-time oUsers separated by using orthogonal codes Page 6 CDMA TDMA FDMA Commonly used analogy for three access schemes

Florida Institute of technologies DS SS Systems - basic principles  Three basic stages oSpreading o“RF Modem” oDe-spreading Page 7 Processing of the signal for a single CDMA user RF modem part is independent of CDMA

Florida Institute of technologies Page 8 DS CDMA - multiple users  After spreading signals from multiple users are summed  Signals from multiple users co-exist in time and frequency  The spreading codes have to be orthogonal

Florida Institute of technologies Example of DS CDMA - two users same PG  Processing gain (PG) is the ratio of chip and bit rates Page 9

Florida Institute of technologies Example – WCDMA voice  Vocoder rate 12.2kbps  Chip rate 3.84Mbps 10 Processing gain Required S/N ratio for voice after de- spreading is around 5dB Signal can have S/(N+I) of -20dB and still be received successfully DS CDMA allows demodulation of signals that are below interference and/or noise floor At RF (before de-spreading) At the base-band (after de-spreading) Note: processing gain is derived through reshaping of the power spectrum density in the frequency domain

Florida Institute of technologies Homework  Homework 4 assigned Page 11