Advantages of CDMA.
- Requires less number of towers
- Call capacity is 3 to 5 times more than GSM
- Call capacity is 10 times more than AMPS
- CDMA uses spread spectrum (GSM uses FDMA/TDMA)
- When you speak, your voice is compressed with a vocoder.(Now your speech is 9600bits per sec)
- The no. of bits in the output is doubled by convolutional encoder for error correction.(now the speech is 19200 bps)
- Each bit is replicated 64 times and XOR-ed with walsh code which is unique for each call.
- This is again XOR-ed with a PseduoRandom Number which is unique for each cell sector.
- The output is modulated with the carrier frequency (in the range of 800mhz or 1900 mhz)
- Each channel uses 1.25 MHz. But it is spread spectrum.
- Compare :A TV channel needs 6MHz,FM radio needs 18KHz
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Data Transmitted by A : 1011 | ||||||||||||||||||||||||
Walsh code allotted for Mr. A : 10 | ||||||||||||||||||||||||
11 | 00 | 11 | 11 | Data of Mr.A :each bit repeated two times | ||||||||||||||||||||
10 | 10 | 10 | 10 | walsh code allotted for Mr.A | ||||||||||||||||||||
+1+1 | -1-1 | +1+1 | +1+1 | Data of Mr.A : 1 is plus1,0 is minus1 | ||||||||||||||||||||
+1-1 | +1-1 | +1-1 | +1-1 | walsh code of Mr.A 1 is plus1,0 is minus1 | ||||||||||||||||||||
+1-1 | -1+1 | +1-1 | +1-1 | Final data transmitted by Mr.A: Data and Walsh code Multiplied |
Data Transmitted by B : 0011 | ||||
Walsh code for B : 11 | ||||
00 | 00 | 11 | 11 | Data of B :each bit repeated two times |
11 | 11 | 11 | 11 | walsh code of B |
-1-1 | -1-1 | +1+1 | +1+1 | Data of B : 1 is plus1,0 is minus1 |
+1+1 | +1+1 | +1+1 | +1+1 | walsh code of B |
-1-1 | -1-1 | +1+1 | +1+1 | B: Data and Walsh code Multiplied |
+1
|
-1
|
-1
|
+1
|
+1
|
-1
|
+1
|
-1
|
<< output released by A |
-1
|
-1
|
-1
|
-1
|
+1
|
+1
|
+1
|
+1
|
<< output released by B |
0
|
-2
|
-2
|
0
|
2
|
0
|
2
|
0
|
<< Resultant Interference of A + B |
0
|
-2
|
-2
|
0
|
2
|
0
|
2
|
0
|
received input at the receiver |
+1
|
-1
|
+1
|
-1
|
+1
|
-1
|
+1
|
-1
|
<<Walsh code of A is 10 ie +1-1 |
0
|
+2
|
-2
|
0
|
+2
|
0
|
+2
|
0
|
Multiply walshcode and received input |
2
|
-2
|
2
|
2
|
Convert 8 items into 4 items by ADDing | ||||
1 | 0 | 1 | 1 | All the values greater than 0 are 1,else 0. This is the data sent by A |