Spread Spectrum: Definition refers to the expansion of signal bandwidth, by several orders of magnitude in some cases, which occurs when a key is attached to the communication channel an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency signal 1
Transmission Using Spread Spectrum The corresponding SS code somewhere in the transmitting chain before the antenna 2
Bandwidth Effect of Spreading Operation Baseband signal is modulated first before spread spectrum modulation 3
Advantages Resistance to Interference and Anti-jamming Effects Intentional or un-intentional interference and jamming signals are rejected because they do not contain the SS key. 4
Advantages Resistance to interception Because non-authorized listeners do not have the key used to spread the original signal, they cannot decode it 5
Advantages Resistance to Fading (Multipath Effects) The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading. Because the despreading process synchronizes to signal D, signal R is rejected even though it contains the same key. 6
Spread Spectrum and (De)coding "Keys" The main SS characteristic is the presence of a code or key codes are digital sequences that must be as long and as random as possible to appear as "noiselike" as possible pseudo-random number (PRN) or sequence 7
Direct Sequence Spread Spectrum (DSSS) the PRN is applied directly to data entering the carrier modulator based on the multiplying of the baseband signal data with a broadband spreading code (chip rate). The characteristics of the broadband spreading code are that of pseudorandom noise. Consequently the receiver synchronized to the code will obtain the narrowband signal. 8
Direct Sequence Spread Spectrum (DSSS) continued All other receivers will see the spread signal as white or coloured noise phase modulation or a derivative of phase modulation is used. The scrambling process is achieved by mixing the actual data with the output of a PN coder. The resultant scrambled data is then modulated in a binary phase shift key (BPSK) or quadrature phase shift key (QPSK) modulator. The output of the BPSK modulator is then transmitted. 9
Direct Sequence Spread Spectrum (DSSS) 10
Direct Sequence Spread Spectrum (DSSS) 11
Frequency Hopping Spread Spectrum (FHSS) It causes the carrier to hop from frequency to frequency over a wide band according to a sequence defined by the PRN The speed at which the hops are executed depends on the data rate of the original information Fast Frequency Hopping (FFHSS) and Low Frequency Hopping (LFHSS) frequency shift keying (FSK) is used 12
Frequency Hopping Spread Spectrum (FHSS) Hop set: this is the number of channels that are used by the system (i.e. the number of different frequencies utilised). Dwell time: this is the length of time that the system transmits on an individual channel (i.e., the length of time spent on one frequency). Hop rate: this is the rate at which the hopping takes place (i.e. how fast the system changes from one channel to another or from one frequency to another). 13
Frequency Hopping Spread Spectrum (FHSS) 14
Time Hopping Spread Spectrum (THSS) Time hopping is where a short pulse, called a chirp, is transmitted having either a pseudorandom pulse duration or transmitted in a random position relative to the input bit period. 15
Time Hopping Spread Spectrum (THSS) Time Hopping Method 1 with Equal Chirp Durations 16
Time Hopping Spread Spectrum (THSS) Time Hopping Method 2 with Varying Chirp Durations 17
Disadvantages of Spread Spectrum Techniques Complex circuitry Expensive to develop Very large bandwidths Easily jammed and hence is not generally used in its true form (Time Hopping) 18
Access Technologies FDMA: Frequency Division Multiple Access most common analog system a technique whereby spectrum is divided up into frequencies and then assigned to users only one subscriber at any given time is assigned to a channel. the channel therefore is closed to other conversations until the initial call is finished, or until it is handed-off to a different channel a full-duplex FDMA transmission requires two channels, one for transmitting and the other for receiving FDMA has been used for first generation analog systems. 19
Access Technologies FDMA: Frequency Division Multiple Access 20
Access Technologies TDMA: Time Division Multiple Access (TDMA) improves spectrum capacity by splitting each frequency into time slots allows each user to access the entire radio frequency channel for the short period of a call. Other users share this same frequency channel at different time slots. The base station continually switches from user to user on the channel. TDMA is the dominant technology for the second generation mobile cellular networks 21
Access Technologies TDMA: Time Division Multiple Access (TDMA) 22
Access Technologies CDMA: Code Division Multiple Access based on spread spectrum technology. Since it is suitable for encrypted transmissions, it has long been used for military purposes. CDMA increases spectrum capacity by allowing all users to occupy all channels at the same time. Transmissions are spread over the whole radio band, and each voice or data call are assigned a unique code to differentiate from the other calls carried over the same spectrum. CDMA allows for a soft hand-off, which means that terminals can communicate with several base stations at the same time. The dominant radio interface for third-generation mobile, or IMT-2000, will be a wideband version of CDMA with three modes (IMT-DS, IMT-MC and IMT-TC). 23
Access Technologies CDMA: Code Division Multiple Access 24