Page 74 - Complete Wireless Design

Modulation

Modulation 73

Figure 2.31 Peak and average amplitudes of a digital
signal.

states that the speed of information transfer is limited by the bandwidth and
the signal-to-noise ratio of a communications channel:

C W log (1 SNR)
2
where C capacity of the data link, bits per second (bps)
W bandwidth of the channel, Hz
SNR signal-to-noise ratio, dB
Table 2.2 shows the theoretical maximum amount of bits per symbol that
can be transmitted for the most common modulation schemes. In a real radio,
these maximum rates are never attained, because of hardware and transmis-
sion impairments. And, depending on error correction techniques, Table 2.3
lists the different SNRs required of the various modulation formats to sustain
a desired BER.
Since the symbol rate equals the bit rate divided by number of bits repre-
sented by each symbol, then a modulation format such as BPSK would trans-
mit at a symbol rate that is equal to its bit rate, or bit rate symbol rate.
Since symbol rate and baud rate have the same meaning, BPSK’s baud rate
equals its bit rate. However, in modulation schemes that encode more than one
bit per symbol, such as QPSK (2 bits/baud), the baud rate will be less than the
bit rate (in this case, half). This, as discussed above, allows more data to be
transmitted within a narrower bandwidth.
The modulation index (h, bits/symbol), also referred to as bandwidth effi-
ciency, is measured in bits per second per hertz (bits/s/Hz). The higher the
modulation efficiency, the higher the data rate that can be sent through a
certain fixed bandwidth. For instance, BPSK has an h of 1, while QAM-64
has an h of 6. However, a higher h comes at the expense of higher equip-
ment cost, complexity, and linearity, and an increased SNR to maintain the
same BER as the lower h systems. Table 2.4 displays the various common
modulation schemes and their h values, number of states, amplitudes, and
phases.

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