1. Code Division Multiple Access (CDMA) Transmission Technology
Chapter 5 of Hiroshi Harada Book 
Group 4.1

2. Outline Introduction Type of CDMA Spreading code
Averaging systems
Avoidance systems
Spreading code
M-seuence
Gold sequence
Ortogonal Gold sequence
Simulation and results
Prepared By Ibrahim AL-OBIDA

3. Type of Multiplexing: 1. Frequency-Division Multiple Access (FDMA).
2. Time-Division Multiple Access (TDMA).
3. Code-division Multiple-Access (CDMA)
Prepared By Ibrahim AL-OBIDA

4. Code Division Multiple Access (CDMA)
A digital method for simultaneously transmitting signals over a shared portion of the spectrum by coding each distinct signal with a unique code.
CDMA is a wireless communications technology that uses the principle of spread spectrum communication.
Advantages
Multiple access capability
Protection against multipath interference
Privacy
Interference rejection
Ant jamming capability
Low probability of interception
Prepared By Ibrahim AL-OBIDA

5. Code Division Multiple Access (CDMA)
There are different ways to spread the bandwidth of the signal:
Direct sequence
Frequency hopping
Time hopping
Chirp spread spectrum
Hybrid systems
Prepared By Ibrahim AL-OBIDA

6. Direct Sequence Features:
All users use same frequency and may transmit simultaneously
Narrowband message signal multiplied by wideband spreading signal, or codeword
Each user has its own pseudo-codeword (orthogonal to others).
Receivers detect only the desired codeword. All others appear as noise.
Receivers must know transmitter’s codeword.
Prepared By Ibrahim AL-OBIDA

7. Direct Sequence
Prepared By Ibrahim AL-OBIDA

8. Pseudo-Noise Spreading
Direct Sequence
Pseudo-Noise Spreading
Prepared By Ibrahim AL-OBIDA

9. Direct Sequence Spread Spectrum System
Prepared By Ibrahim AL-OBIDA

10. Direct Sequence Spread Spectrum Example
One technique with direct sequence spread spectrum is to combine the digital information stream with the spreading code bit stream using an exclusive-OR (XOR). Stallings DCC8e Figure 9.6 shows an example. Note that an information bit of one inverts the spreading code bits in the combination, while an information bit of zero causes the spreading code bits to be transmitted without inversion. The combination bit stream has the data rate of the original spreading code sequence, so it has a wider bandwidth than the information stream. In this example, the spreading code bit stream is clocked at four times the information rate.

11. Direct Sequence Spread Spectrum System
To see how this technique works out in practice, assume that a BPSK modulation scheme is to be used. Rather than represent binary data with 1 and 0, it is more convenient for our purposes to use +1 and –1 to represent the two binary digits. To produce the DSSS signal, we multiply the BPSK signal by c(t), which is the PN sequence taking on values of +1 and –1:
s(t) = A d(t)c(t) cos(2πfct) : Equation (9.5)
At the receiver, the incoming signal is multiplied again by c(t). But c(t)  c(t) = 1 and therefore the original signal is recovered. Equation (9.5) can be interpreted in two ways, leading to two different implementations. The first interpretation is to first multiply d(t) and c(t) together and then perform the BPSK modulation. That is the interpretation we have been discussing. Alternatively, we can first perform the BPSK modulation on the data stream d(t) to generate the data signal sd(t). This signal can then be multiplied by c(t). An implementation using the second interpretation is shown in Stallings DCC8e Figure 9.7 above.

12. DSSS Example Using BPSK
Stallings DCC8e Figure 9.8 is an example of the approach discussed on the previous slide.

13. Direct Sequence Processing Gain: = is the processing gainfc
is Chipping Frequency (the bit rate of the PN code). fi
is Information Frequency (the bit rate of the digital data).
Prepared By Ibrahim AL-OBIDA

14. Direct Sequence Disadvantages: Advantages: Increased capacity
Improved voice quality
Eliminating the audible effects of multipath fading
Enhanced privacy and security
Reduced average transmitted power
Reduced interference to other electronic devices
Disadvantages:
Wide bandwidth per user required
Precision code synchronization needed
Prepared By Ibrahim AL-OBIDA

15. Frequency Hopping Spread Spectrum (FHSS)
signal is broadcast over seemingly random series of frequencies
receiver hops between frequencies in sync with transmitter
jamming on one frequency affects only a few bits
With frequency-hopping spread spectrum (FHSS), the signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at fixed intervals. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message. Would-be eavesdroppers hear only unintelligible blips. Attempts to jam the signal on one frequency succeed only at knocking out a few bits of it.

16. Frequency Hopping Example
Stallings DCC8e Figure 9.2 shows an example of a frequency-hopping signal. A number of channels are allocated for the FH signal. Typically, there are 2k carrier frequencies forming 2k channels. The spacing between carrier frequencies and hence the width of each channel usually corresponds to the bandwidth of the input signal. The transmitter operates in one channel at a time for a fixed interval; for example, the IEEE standard uses a 300-ms interval. During that interval, some number of bits (possibly a fraction of a bit, as discussed subsequently) is transmitted using some encoding scheme. A spreading code dictates the sequence of channels used. Both transmitter and receiver use the same code to tune into a sequence of channels in synchronization.

17. FHSS (Transmitter)
Stallings DCC8e Figure 9.3 shows a typical block diagram for a frequency-hopping system. For transmission, binary data are fed into a modulator using some digital-to-analog encoding scheme, such as frequency shift keying (FSK) or binary phase shift keying (BPSK). The resulting signal sd(t) is centered on some base frequency. A pseudonoise (PN), or pseudorandom number, source serves as an index into a table of frequencies; this is the spreading code referred to previously. Each k bits of the PN source specifies one of the 2k carrier frequencies. At each successive interval (each k PN bits), a new carrier frequency is selected. The frequency synthesizer generates a constant-frequency tone whose frequency hops among a set of 2k frequencies, with the hopping pattern determined by k bits from the PN sequence. This is known as the spreading or chipping signal c(t). This is then modulated by the signal produced from the initial modulator to produce a new signal with the same shape but now centered on the selected carrier frequency. A bandpass filter is used to block the difference frequency and pass the sum frequency, yielding the final FHSS signal s(t).

18. Frequency Hopping Spread Spectrum System (Receiver)
On reception, Stallings DCC8e Figure 9.3 shows that the spread spectrum signal is demodulated using the same sequence of PN-derived frequencies and then demodulated to produce the output data. At the receiver, a signal of the form s(t) defined on the previous slide, will be received. This is multiplied by a replica of the spreading signal to yield a product signal. A bandpass filter is used to block the sum frequency and pass the difference frequency, which is then demodulated to recover the binary data.

19. Slow and Fast FHSS commonly use multiple FSK (MFSK)
have frequency shifted every Tc seconds
duration of signal element is Ts seconds
Slow FHSS has Tc  Ts
Fast FHSS has Tc < Ts
FHSS quite resistant to noise or jamming
with fast FHSS giving better performance
A common modulation technique used in conjunction with FHSS is multiple FSK (MFSK), which uses M = 2L different frequencies to encode the digital input L bits at a time (see Chapter 5). For FHSS, the MFSK signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signal. The effect is to translate the MFSK signal into the appropriate FHSS channel. For a data rate of R, the duration of a bit is T = 1/R seconds and the duration of a signal element is Ts = LT seconds. If Tc is greater than or equal to Ts, the spreading modulation is referred to as slow-frequency-hop spread spectrum; otherwise it is known as fast-frequency-hop spread spectrum.
Typically, a large number of frequencies is used in FHSS so that bandwidth of the FHSS signal is much larger than that of the original MFSK signal. One benefit of this is that a large value of k results in a system that is quite resistant to jamming. If frequency hopping is used, the jammer must jam all 2k frequencies. With a fixed power, this reduces the jamming power in any one frequency band to Sj/2k. In general, fast FHSS provides improved performance compared to slow FHSS in the face of noise or jamming, as we will discuss shortly.

20. Slow MFSK FHSS
Stallings DCC8e Figure 9.4 shows an example of slow FHSS, using the MFSK example from Stallings DCC8e Figure 5.9. Here we have M = 4, which means that four different frequencies are used to encode the data input 2 bits at a time. Each signal element is a discrete frequency tone, and the total MFSK bandwidth is Wd = Mfd. We use an FHSS scheme with k = 2. That is, there are 4 = 2k different channels, each of width Wd. The total FHSS bandwidth is Ws = 2kWd. Each 2 bits of the PN sequence is used to select one of the four channels. That channel is held for a duration of two signal elements, or four bits (Tc = 2Ts = 4T).

21. Fast MFSK FHSS

22. Linear Feedback Shift Register Implementation of PN Generator
Output is periodic with max-period N=2n-1;
LFSR can always give a period N sequence -> resulting in m-sequences.
Different Ai allow generation of different m-sequences
4/27/2017

23. Properties of M-Sequences
Property 1:
Has 2n-1 ones and 2n-1-1 zeros
Property 2:
For a window of length n slid along output for N (=2n-1) shifts, each n-tuple appears once, except for the all zeros Sequence
Property 3:
Sequence contains one run of ones of length n
One run of zeros of length n-1
One run of ones and one run of zeros of length n-2
Two runs of ones and two runs of zeros of length n-3
2n-3 runs of ones and 2n-3 runs of zeros of length 1
4/27/2017

24. Advantages of Cross Correlation
The cross correlation between an m-sequence and noise is low
This property is useful to the receiver in filtering out Noise
The cross correlation between two different msequences is low
This property is useful for CDMA applications
Enables a receiver to discriminate among spread spectrum signals generated by different m-sequences
4/27/2017

25. Gold Sequences
Gold sequences constructed by the XOR of two m-sequences with the same clocking
Codes have well-defined cross correlation Properties
Only simple circuitry needed to generate large number of unique codes
In following example two shift registers generate the two m-sequences and these are then bitwise XORed
4/27/2017

26. Gold Sequences
4/27/2017

27. Orthogonal Codes Orthogonal codes
All pairwise cross correlations are zero
Fixed- and variable-length codes used in CDMA Systems
For CDMA application, each mobile user uses one sequence in the set as a spreading code
Provides zero cross correlation among all users
4/27/2017

28. BER performance of DS CDMA with m-sequence in AWGN
4/27/2017

29. BER performance of DS CDMA with Gold sequence in AWGN
4/27/2017

30. BER performance of DS CDMA with orthogonal Gold sequence in AWGN
4/27/2017

31. BER performance of DS CDMA with m-sequence in Rayleigh fading
4/27/2017

32. BER performance of DS CDMA with orthogonal Gold sequence in Rayleigh fading
4/27/2017