2.9: Summary

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All modern modulation methods impress information on a sinusoidal carrier that is at a high enough frequency that it can be easily transmitted. There are many modulation techniques with the choice of which to use based on the technology available to implement the modulation scheme, the tolerance of the modulation scheme to interference, how efficiently the modulation scheme uses the EM spectrum, and the amount of DC power consumed. In military communications it is also important that a modulation scheme produce a noise-like signal that is difficult to detect and intercept.

The first widely adopted modulation schemes produced simple pulses as used in wireless telegraphy. The tolerance to interference was achieved through the relatively slow transmission of bits, and hence redundancy. More information was transmitted when amplitude modulation was introduced to superimpose voice on a carrier. With this scheme, interference was always a problem, and once interference appeared on a signal it could not be removed nor suppressed. The first significant advance in modulation techniques was the invention of frequency modulation. In this scheme a narrowband analog modulating signal (e.g. voice) became a relatively wideband frequency-modulated RF signal. When the modulated signal was received and demodulated, the wide bandwidth of the modulated signal was collapsed to the original relatively narrowband modulating signal. The demodulation process combined correlated components of the modulated signal and the uncorrelated components, noise and interference, were suppressed.

The introduction of digital modulation was a significant advance in the suppression of interference. Digital information was now being transmitted, and errors in the data caused by interference and noise would be completely unacceptable. The solution was to embed error-correcting codes in the data so that if a manageable number of bits were lost in the transmitted signal, the original data could still be fully recovered. As a result, digital radio (using digital modulation) could be used in situations with even more distortion than was acceptable in analog radio (using analog modulation). If interference is low, then today’s wireless systems use high-order modulation switching to lower-order (and less spectrally efficient) modulation when necessary to cope with higher interference.

Several important metrics are used to provide a measure of the signal characteristics. The crest factor, peak-to-average ratio, and peak-to-mean envelope power ratio (PMEPR) are all indicators of how much care must be given to nonlinear circuit design, especially to amplifier and mixer design. Amplifiers must operate so that the peak signal is amplified with minimal distortion. It is the peak signal that determines the DC power drawn by an amplifier. However, the average RF output power of an RF front end is determined by the mean of the envelope. So a high PMEPR signal will result in lower amplifier efficiency.

Many of the techniques described in this chapter for modulating and demodulating RF signals were presented as circuit techniques. However many modern phones support multiple standards and hardware implementation would require multiple copies of similar versions of analog circuits. Today it is more cost effective to perform most of the operations in a DSP unit. Most of the time the DSP realization is close to the hardware implementation. An example is carrier recovery. For narrow-band communication signals in wireless communicators, carrier recovery can be performed using a digital implementation of the concepts described for the hardware carrier recovery circuits. While it is more power efficient to implement many of the techniques in hardware, the need to support multiple standards has necessitated the software reconfigurability available with a DSP unit. Which approach is used is the decision of the RF system designer— an experienced engineer with a rich background in wireless technologies. It is therefore important that the aspiring and practicing RF engineer have a broad perspective of RF circuits and of communications theory. Hence the emphasis of this book on a systems approach to RF and microwave design.

Frequency modulation, and the similar PM modulation method were used in the 1G analog cellular radio. With the addition of AM, the three schemes are the bases of all analog radio. Digital cellular radio began with 2G and there were two types of 2G cellular radios with the GSM system using GMSK modulation, a type of FSK modulation, and the NADC system using \(π/4\)-DQPSK modulation. The two 2G systems were incompatible. The 3G cellular radio used two types of QPSK modulation, one for the up-link from handset to basestation, and one from the basestation to a handset. The 1G–3G systems implemented most of the modulation and demodulation functions in analog hardware. With 4G and 5G cellular radio a large number of modulation schemes are supported choosing as high an order of modulation as allowed by the channel conditions. Most of the modulation and demodulation in 4G and 5G are implemented in DSP with just the translation to and from the radio frequency signal implemented in analog hardware.