Bit Rate Baud Comparison

Assuming that an FSK signal over voice-grade phone lines can send 1200 bits per second, it has a bit rate of l200. Each frequency shift represents a single bit so it requires 1200 signal elements to send 1200 bits. Its baud rate, therefore, is also 1200. Each signal variation in an 8-QAM system, however, represents three bits. So a bit rate of 1200, using 8-QAM, has a baud rate of only 400. As Figure 4.3-17 shows, a dibit system has a baud rate of one-half the bit rate. A tribit system has a baud rate of one-third the bit rate. And a quadbit system has a baud rate of one-fourth the bit rate.

Baud Rate Bit Rate
Fig. 4.3-17 Bit and baud

Table 4.1 shows the comparative bit and baud rates for the various methods of digital-to-analog encoding.

Table 4.1 Bit and baud rate comparison

Encoding

Units

Bits/Baud

Baud rate

Bit rate

ASK, FSK, 2PSK

Bit

1

N

N

4PSK, 4-QAM

Dibit

2

N

2N

8-PSK, 8-QAM

Tribit

3

N

3N

16- QAM

Quadbit

4

N

4N

32- QAM

Pentabit

5

N

5N

64- QAM

Hexabit

6

N

6N

128- QAM

Septabit

7

N

7N

256-QAM

Octabit

8

N

A constellation diagram consists of eight equally spaced points on a circle. If the bit rate is 4800

bit/s, what is the baud rate?

Solution

The constellation indicates 8-PSK encoding with the points 45 degrees apart. Since 23=8, three bits are transmitted with each signal element. Therefore, the baud rate is

4800/3=1600 baud

Example 4.14

Compute the bit rate for a 1000 baud 16-QAM signal. Solution

A 16-QAM signal means that there are four bits per signal element since T=16. Thus,

Example 4.15

Compute the baud rate for a 72,000 bit/s 64-QAM signal. Solution

A 64-QAM signal means that there are six bits per signal element since 26=64. Thus,

72,000/6=12,000 baud

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