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