Lecture 8: CDMA

1. Lecture 8: CDMA

2. Code Division Multiple Access • What is CDMA? • Cocktail party story! • What are the benefits? • Increased capacity ! • Increased quality ! • Simplified system planning !

3. Benefits - continued • Enhanced privacy • Improved coverage (fewer cells) • Increased talk time (battery life!) • Bandwidth on demand Are they true? Or just an hype!

4. CDMA Technology • Spread spectrum technology • Direct sequence (DS) • Frequency hop (FH) • Time hop (TH) • Multicarrier (MC) • Hybrid techniques

5. Spread Spectrum • This techniques “spreads” data-transmission to a wider range of frequencies. • That is, wider than the required bandwidth is used to transmit data. • As a result, multiple users can transmit provided the interference can be “tolerated”.

6. Initial Motivation • Originates from military needs where transmission takes place in an hostile environment. Typical way to jam a narrowband is to send strong noise signal. Also, it is easy to detect whether someone is communicating or not in a narrowband Spread spectrum avoids this and actual communication will look like “Gaussian Noise”.

7. Magic of CDMA • In traditional TDMA/AMPS system, adjacent cells can’t reuse frequency since they will interfere with each other. That is the fear of one worst-case situation prevents reuse of the frequency • In CDMA, there is no one user who will interfere “badly”. Each user will contribute little bit to interference and thus the average interference is low compared to signal strength, then you are ok! That is, this system is not sensitive to worst-case but rather to average-case interference.

8. CDMA • High capacity, noise-like carrier waves etc. was first suggested decades ago by Claude Shannon. • Why was this not popular then?

9. Capacity Determination • Capacity is determined by the balance between the required SNR for each user and the spread spectrum processing gain (ratio of total bandwidth to transmission rate). • SNR E/N = (Energy per bit) / N Where N = Power of spectral density of noise + interference

10. Energy per bit and Noise • E = S/R where S is received signal power and R is rate of transmission • N = [Sum of Xi . Si + I + n0 ] / W where Xi is activity factor of user i, Si received power of user i, I is total external interference power n0 is power density of background noise

11. SNR E/N = (SW/R)/( (n-1)S + I + n0) where we make a crucial assumption that the received signal strength of each user is S Here, n is the number of users supported by the system. n increases as E/N (tolerated by the receiver) decreases.

12. Near-Far Problem • Major assumption we made is that Si’s are roughly equal. • Consider two mobile unit, one near the tower and the other far away. Suppose both transmit at the same power. • Received power of closer unit is much much higher than that of the farther one.

13. Near-Far Problem • Suppose there is 60dB difference in SNR of two users (due to received power difference) • Calculation from previous equations show that the spreading bandwidth must be 10000 times the data rate (which is 10K). • Therefore you need 100MHz bandwidth! • For worse efficiency !!

14. Air-interface Details • CDMA uses a bandwidth of 1.25MHz • Why 1.25? Design compromise based on receiver design and complexity and delay characteristics • Higher rate will result in multipath collision where as lower rate will result in Rayleigh fading and adverse effect of “law of large numbers”. • Several logical channels (64 in total) • Pilot channel • Sync. channel • 7 Paging channels • 55 Traffic channels

15. Pilot Channel • Assigned channel code is 0 • Universal search target of mobiles • It is a demodulation reference for the mobile receivers • It also helps in measuring handoff levels • It carries no information except a short code that the mobile unit uses to demodulate. • Short codes (2^15 chips) are identical for all towers. (except for a “pilot offset” which is used as reference in demodulation)

16. Sync Channel • Carries a repeating message that identifies the station and the phase of pilot sequence. • As a consequence, the repeating message that conveys the timing and system configuration info to the mobile unit

17. Paging Channel • Conveys page info to mobile units • Also conveys, access acknowledge (similar to FOCC of AMPS). • After successful access, a traffic channel is allocated to the mobile unit

18. Traffic Channel • Carries data in 20ms frames. • Carry variable rate traffic frames: 1, ½, ¼, 1/8, of the maximum rate (9600 or 14,400) • Rate variation is accomplished by code repetition. • Are we wasting bits? • No, since we will power-down when we repeat codes

19. Power Control Subchannel • Power control info is carried on the traffic channel by “puncturing” 2 from every 24 transmitted symbols

20. Handoff • Soft Handoff • Each participating tower transmit same traffic over its assigned channel. • The code channel assignments are independent. • The signal is “added”.

21. RAKE receivers • Multipath signals arrive at different time. • So “combine” signals arrive within a delay (say roundtrip “delay”) to boost the signal strength. Each such delay is called a “finger” • This is done at the receiver. • IS-95 uses 3-4 finger RAKE reception

22. Vocoders • Statistics show that a typical conversation consists of only 45% information. • So why not power-down during “blank” or “silent” time. • This power-down reduces “noise” to other users and thus can potentially increase number of “sustainable” users in the system.