TLM2005 Tutorial 6
CDMA and Cellular Networks
1. The figure below depicts a simplified scheme for CDMA encoding and decoding. There
are seven logical channels, all using DSSS with a spreading code of 7 bits. Assume that all
sources are synchronized. All seven sources transmit a data bit, in the form of a 7-bit
sequence. The signals from all sources are combined at the receiver. Then the data bit for
a desired channel is decoded.
(a)
What are the spreading codes for channel 0 and channel 1?
(b)
Determine the composite signal received at the receiver.
(c)
Assume that the receiver wants to decode channel 0.
(i)
Determine the receiver output after the multiplier.
(ii)
Determine the receiver output after the adder.
(iii) Determine the decoded bit value.
Channel
Number
Data
Value
(0)
1
(1)
0
(2)
1
(3)
0
(4)
0
(5)
0
(6)
1
Desired Channel’s Code
+
X
Composite Signal
Individual Channel’s
Transmitted
Waveforms
+
2. Assume a CDMA system and answer the following:
a. If the symbol of user 1 is -1 and the code is <-1 1 1 -1 1>, what is the transmitted signal?
b. If the symbol of user 1 is 1 and the transmitted signal is <-1 1 -1 1 -1>, what is the code?
c. If the transmitted signal is <1 1 -1 -1 1> and code is <-1 -1 1 1 -1>, what is the symbol?
TLM2005 Wireless Communications, Trimester 2
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3. Consider a CDMA system with three users and a receiver R. The codes and the symbols
transmitted by the users are as follows:
User
Code
Tx Symbol
U1
<1,1,-1,-1>
<-1,-1,+1,+1>
U2
<-1,1,1,-1>
<-1,+1,+1,-1>
U3
<-1,1,-1,1>
<-1,+1,-1,+1>
Using this information, answer the following questions:
i. What is the signal received by the receiver R? (Ignore Noise)
ii. Decode the symbol transmitted by U1.
iii. Decode the symbol transmitted by U2.
iv. Decode the symbol transmitted by U3.
4. An FDMA cellular system has a total of 40 MHz of bandwidth and uses two 15-kHz
simplex channels to provide full duplex voice channels. What is the number of channels
available per cell for a frequency reuse factor of (1) 4 cells, (2) 7 cells, and (3) 12 cells?
What is the number of simultaneous communications that can be supported by a single cell
in each case?
5. A cellular system uses FDMA with a spectrum allocation of 12.5 MHz, a guard band of 50
kHz at each edge of the allocated spectrum, and a channel bandwidth of 90 kHz. What is
the number of available channels?
TLM2005 Wireless Communications, Trimester 2
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