ECE 5221 Personal Communication Systems

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

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

3. 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 • Complicated in general case • Special tools used for prediction and mitigation • Simplified analysis gives some indications of under laying principles • Assumptions • Regular cell layout • Interference beyond first tier of reuse - neglected • Sites have the same configuration C/I – carrier to interference ratio R – distance to serving site Di– distance to ith interferer n – path loss exponent i0– number of first tier interferers C/I estimate:

4. Interference estimation C/I estimate for some common reuse schemes Define frequency reuse ratio Q Neglecting differences between different Di‘s n = 3.84 is assumed Cellular geometry allows some values for N. Allowed values satisfy condition 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 i and j are integers

5. Examples of C/I requirements C/I requirements for some deployed technologies • C/I depends on cellular technology air interface properties • Different technologies have different • Modulation • Coding • Diversity • Mobility patterns, etc. • For nominal design purposes a number is provided for a given technology Estimate of N given C/I requirement Note 1: Theory so far, assumes fixed frequency allocation and FDMA Note 2. Results derive assuming non-adaptive air interface and worst case scenario

6. 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 • Cdma2000/EvDO – 1.3MHz • WCDMA/HSPA – 5MHz • Frequency reuse of N=1 • Duplexing – FDD • All users co-channel and co-time • Users separated by using orthogonal codes FDMA TDMA CDMA Commonly used analogy for three access schemes

7. DS SS Systems - basic principles Processing of the signal for a single CDMA user • Three basic stages • Spreading • “RF Modem” • De-spreading RF modem part is independent of CDMA

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

9. Example of DS CDMA - two users same PG • Processing gain (PG) is the ratio of chip and bit rates

10. Example – WCDMA voice At RF (before de-spreading) • Vocoder rate 12.2kbps • Chip rate 3.84Mbps Processing gain At the base-band (after de-spreading) • 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 Note: processing gain is derived through reshaping of the power spectrum density in the frequency domain

11. Homework • Homework 4 assigned