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?