Wireless Communication
Lecture CDMA Hardware/ System view
Wireless Communications 2nd Ed pp 5, 36, 329-331, 448, 458, 481, 74; cdma2000 32,36; cdmaOne 27; IS-95 567-580
What is a CDMA System?
CDMA (Code Division Multiple Access) is a system that does an XOR operation on data with a PN code to allow multiple users to transmit on the same frequency.
Transmitter See figure 6.49a
Receiver See figure 6.49b
PN Code Generator
figure 6.48 and Aly’s Circuit
Example Using data rate of 1 bit per second and 10 chips per second
PN code 1
1101100100
1101100100 XOR of data bit ‘0’ and PN code
PN code 2
1011011001
0100100110 XOR of data bit ‘1’ and PN code
Random Noise
1100011010 Noise 1
0001101101 Noise 2
0100110011 Noise 3
1101100100 Signal 1
0100100110 Signal 2
Received Signal
0100100110 Received signal is sum of all transmitted signals and noise.
Received Signal XORd with PN code 1
0100100110 received signal
1101100100 PN code 1
1001000010 = 0
Received Signal XORd with PN code 2
0100100110 rec
1011011001 PN code 2
1111111111 = 1
In class example in groups of 5 or on board if time doesn’t permit
Processing Gain / Jamming Immunity
PG = chips per bit
PG in dB = 10 log PG
figure 6.50
Advantages
1. The transmission bandwidth is larger than the coherence bandwidth, eliminating
frequency selective fades.
2. Bandwidth efficient
3. Soft Capacity Limit; More users don’t block, they just degrade performance.
4. Can adjust data rate for changing channel conditions.
5. Inherent Privacy Due to Unique PN code
6. Soft Handoff to another base station.
Disadvantages
1. Larger Noise Bandwidth
2. Self-Jamming from users with PN codes that aren’t exactly orthogonal.
3. Chip length is usually much less than the channel delay spread requiring a RAKE
receiver or equalizer.
4 Near-far problem requires fast power control.
Current CDMA Systems
Interim Standard 95 (IS-95)
1.25 MHz Bandwidth
14.4 KHz Data Rate
Uses 900 MHz and 1900 MHz Frequencies
Table 11.4 p 569 Forward Traffic Channel
Fig.l 11.14 p 570Transmitter
cdmaOne IS-95A
UMTS W-CDMA Wideband CDMA
5 MHz bandwidth requirements
2 Mbps Data Rate – Future data rates up to 8 Mbps