Which of the following statements are correct?

A. M‐ary modulation scheme is preferable where the bandwidth requirement is important.

B. M‐ary PSK system considers 'M' different phases in the range 'π/2'.

C. In M‐ary modulation scheme, only coherent detection is possible.

D. M‐ary QAM scheme uses 'm²' carrier signals having the same frequency.

Choose the correct answer from the options given below: 

Concept:

• An M-ary modulation scheme transmits log₂M bits are transmitted at a time. 
• The bandwidth requirements of the M-ary modulation scheme is reduced by a factor of log₂M.
• In M-ary PSK, the information is stored in the phase of the symbol. It has M possible symbols with the phases given by ϕ_i = 2π/M. 
• In coherent detection, the local carrier wave generated at the receiver is phase locked with the carrier at the transmitter. Hence, the carriers at the transmitter and receiver are synchronized in both frequency and phase. In non-coherent detection, there is no need for the two carriers at transmitter and receiver to be phase locked.
• Quadrature Amplitude Modulation (QAM) utilises both amplitude and phase components for the modulation.

Solution:

A. Since, the bandwidth requirement of the M-ary modulation scheme is reduced, hence it is preferred where the bandwidth requirement is important.

B. Since M > 4, hence for M-ary PSK, ϕi = 2π/M , π/2. Hence, M-ary PSK system has M different phases in the range 'π/2'.

C. For M-ary modulation scheme, both coherent and incoherent detection can be used. However, the probability of error differs.

D. M-ary QAM stores the information in the phase and amplitude of the signal, hence only 1 carrier frequency is required. The constellation diagram of 16-QAM is shown below.

Hence, only statements A and B are correct.