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

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

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

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

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

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

Direct Sequence Prepared By Ibrahim AL-OBIDA

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

Direct Sequence Spread Spectrum System Prepared By Ibrahim AL-OBIDA

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.

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.

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

Direct Sequence Processing Gain: = is the processing gain fc 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

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

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.

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 802.11 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.

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).

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.

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.

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).

Fast MFSK FHSS

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

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

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

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

Gold Sequences 4/27/2017

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

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

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

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

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

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