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Whether intentional or unintentional, interfering signals reduce the fidelity of received information. We will discuss modulation and coding techniques to reduce the impact of interference.

2.4.1 Spreading the Transmitted Spectrum

Spread spectrum techniques will be discussed in detail in Chapter 5. This discussion is focused on the transfer of information versus bandwidth and the nature of the interference environment. The description of low probability of intercept (LPI) is also used to define these signals, but since this deals with only one advantage of the signals, we will talk about them as spread spectrum (SS) signals.

In general, these signals have a much wider transmission spectrum than that required to carry the transmitted information. The despreading of the signal at the receiver recovers the information transmitted while providing a processing gain that increases the ratio of the recovered information to the false outputs from received interference. Note that all of these types of systems trade noise/interference reduction for increased transmission bandwidth requirement. A simple way to think about this is to consider commercial frequency modulated (FM) broadcast signals.

2.4.2 Commercial FM Broadcast

The frequency modulated signal was the first widely used spread spectrum technique.

Figure 2.10 shows the modulation. Wideband FM improves signal quality by increasing the signal-to-noise ratio (SNR) and signal-to-interference ratio as a function of the square of the amount by which it spreads the transmission bandwidth. The spreading ratio is called the modulation index. It is the ratio between the maximum frequency offset from the carrier and the highest modulating frequency as shown in Figure 2.11. The cost of this SNR improvement is that transmission requires additional bandwidth. Commercial FM frequency assignments are 100 kHz apart and there must be multiple channel slots between occupied channels in a geographic area. With large modulation index, the transmission bandwidth is:

Figure 2.10 An FM signal carries information as variations in the transmitted frequency.

Figure 2.11 The transmitted FM signal carries its information in a bandwidth that is determined by the selected

modulation index. BW = 2f_mβ where BW is the transmitted bandwidth, f_m is the maximum frequency of the information signal, and β is the FM modulation index. The output signal to noise ratio improvement formula (in decibels) is: SNR = RFSNR + 5 + 20logβ where SNR is the output SNR in decibels and RFSNR is the predetection SNR in decibels. In order to achieve this SNR improvement, the RFSNR must be above a threshold level: either 4 or 12 dB, depending on the type of demodulator used in the receiver. For commercial FM broadcast signals, the maximum modulating frequency is 15 kHz, and the modulation index is 5. With the most common type of demodulator, the RFSNR threshold is 12 dB. Thus, the broadcast bandwidth is 150 kHz (which is 2 × 15 kHz × 5). With a minimum threshold signal out of the receiving antenna for the most common type of demodulator, the output SNR is 31 dB (which is 12 + 5 + 20 log 5 = 12 + 5 + 14). The frequency modulation improved the output SNR by 19 dB.

Note that pre-emphasis (increasing the power of higher modulating frequencies) in the transmitter and de-emphasis (decreasing the power of higher modulating frequencies) in the receiver can allow a few more decibels of SNR improvement, depending on the nature of the information being communicated.

Reduction in interference, either intentional or unintentional, depends on the nature of the interfering signals. If the interference is narrowband, the interference reduction will be similar to the SNR improvement. If the interference is noise-like, for example, noisy power lines, it will be reduced by something like the SNR improvement. However, if the interference is properly modulated jamming or is another similarly modulated FM signal, it will get the same processing gain as the desired signal (i.e., no improvement of performance against interference).

2.4.3 Military Spread Spectrum Signals

Communication in a high interference or hostile environment can profit from the use of special spectrum spreading techniques that are designed to overcome interference. These

special modulations include a pseudo-random function that assures that they are unique and sufficiently different from all interfering signals that the desired signal will have a significant processing gain relative to any other received signal.

The pseudo-random function is incorporated into the signal before transmission and all authorized receivers are synchronized so that they can use the same function to despread the received signal allowing recovery of the transmitted information (see Figure 2.12).

There are three types of modulation used in these military spread spectrum systems: frequency hopping, chirp, and direct sequence spread spectrum.

There are also hybrid systems that include multiple spreading modulations. These modulations are discussed in detail in Chapter 5, but our discussion here will focus on their information transfer implications.

There are specific reasons why each type of spread spectrum modulation must carry its information in digital form.

Digital information cannot be directly transmitted. It must be placed on some type of modulation compatible with radio transmission. Digital communication is covered in

Chapter 5, but we will go into additional detail here, again with an emphasis on information transfer. Figures 2.13 and 2.14 show the spectrum of a transmitted digital signal as it appears on a spectrum analyzer screen and in diagram form showing the power and frequency dimensions.

Figure 2.12 LPI communication systems spread their spectrum in response to a pseudorandom function which is

Figure 2.13 A spectrum analyzer display of a digital signal shows a main lobe pattern with clearly defined nulls on either side of the carrier frequency.

You will note that the transmission bandwidth required to carry digital signals is a function of the data clock rate, which is the number of bits per second in the transmitted signal. The required bit rate is a function of the bandwidth of the information carried and the required signal quality. With most digitizing schemes, the Nyquist rate is required. This requirement is that the sample rate be twice the bandwidth (in hertz) of the carried information. The captured signal quality is determined by the number of bits per sample. There are efficient coding approaches that can reduce the required bandwidth. The sample rate (hence the bit rate) can be greater to allow for higher fidelity capture of the information, and the transmitted signal will in almost all cases require additional bits for addressing, synchronization, and error detection/correction.

Figure 2.14 The digital signal spectrum includes a main lobe and side lobes with clearly defined nulls spaced at

2.5 Bandwidth Requirements for Information Transfer