Winter 2007 Issue

Tradeoffs in Designing
Digital Communication Systems

There are a number of tradeoffs to be made in the design of
wireless digital communication systems. This document provides
a short overview of the problems and some solutions.

By John B. Stephensen,* KD6OZH

With analog voice communication, we can use the speech recognition capabilities of our brains to compensate for signal degradation in the transmission channel. For digital communication, the processing power of a computer must be used. This requires making tradeoffs to minimize both the amount of energy that must be radiated by the transmitter and the complexity of the signal processing required at the receiver. These tradeoffs tend to fall into four categories:

1. Power versus Bandwidth
2. Transmission Channel Limitations
3. Frequency Selection
4. Limited Spectrum Availability

Claude Shannon enumerated the power versus bandwidth tradeoff in 1948. For a given bandwidth, using more power can increase the rate of information transmission. Conversely, for a given information rate, using a wider bandwidth can decrease the amount of power required. Table 1 shows the theoretical signal-to-noise ratio (SNR) required with a perfect demodulator for various forms of modulation for a 105 symbol error rate in an additive white Gaussian noise (AWGN) channel. This is a channel where the only source of interference is the thermal noise generated by the random motions of charged particles in atoms and molecules when they are at a temperature above 0K. The amount of electromagnetic energy radiated is proportional to the temperature and has constant power spectral density over a wide frequency range. Thus, when it is viewed at optical frequencies, it appears white.

As the data rate (in bits/Hz) increases, the amount of power required increases dramatically. Doubling the data rate from 1 to 2 bits per Hertz requires doubling the transmitter power. However, doubling the data rate gets progressively harder; going from 4 to 8 bits per Hertz requires increasing transmitter power by a factor of 16. This is because the power required increases with the number of states of the symbol (the signal constellation size) rather than the number of bits transmitted. Increasing the number of states in a phase-shift-keyed (PSK) or amplitude-shift-keyed (ASK) signal uses only one dimension and ultimately requires almost four times more power for each doubling. If both phase and amplitude information can be utilized, as in quadrature amplitude modulation (QAM), the power increase can be limited to a factor of 2, as two dimensions are utilized as shown in figure 1.

Note that the QAM signal can be arranged in different ways with each point in the constellation representing one possible value of the symbol being transmitted. AMPM (combined amplitude modulation and phase modulation) and QASK (quadrature amplitude shift keying) are two possibilities. The value being transmitted can also be represented as the difference between adjacent symbols as in differential phase shift keying (DPSK) or differential quadrature amplitude modulation (DQAM). Differential signaling requires more power but is more immune to variations in signal propagation, such as fading.

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