Lamens CDMA

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)

How CDMA Works

• 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

How Walsh Code works

A 0-bit is converted into 2 numbers of 2-bit walsh codes. then it is converted to 4 numbers of 4-bit walsh codes. CDMA uses 64 bit walsh code. 0 00 00 00 01 01 01 00 11 01 10
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
Now Let us decode and get A
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

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