One Question of my note is like this.
Consider a communication channel with a bandwidth of 2400Hz. If QPSK technique is used, what is the possible transmission rate? (Assume that rectangular pulses are used in the baseband signals, and that 90% energy preservation is required.) For a baseband system, a bipolar signal of bandwidth B can support a rate of B with 90% energy preservation (since bandwidth =1/τ in this case). For a bandpass system, a BPSK signal of bandwidth of B can support a rate of B/2. This is because, after modulation, the bandwidth is 2/τ. For QPSK, rate can be doubled using the same bandwidth. Therefore the supported rate is 2400bps.
Why the BPSK signal of bandwidth 2/τ can support a datarate of B/2? and what is the relationship between the bandwidth and the datarate? and why in the QPSK, the rate can be doubled using the same bandwidth?