Question E8B11:
What is digital time division multiplexing?  

Choices:  
A: Two or more data streams are assigned to discrete sub-carriers on an FM transmitter  
B: Two or more signals are arranged to share discrete time slots of a data transmission  
C: Two or more data streams share the same channel by transmitting time of transmission as the sub-carrier  
D: Two or more signals are quadrature modulated to increase bandwidth efficiency    

Correct Answer is  B: Two or more signals are arranged to share discrete time slots of a data transmission  

Correct answer: B — Two or more signals are arranged to share discrete time slots of a data transmission.

1) Why B is correct
- Digital Time Division Multiplexing (TDM) splits the channel into repeating time slots within a frame. Each signal (user/stream) transmits only in its assigned slot, taking turns on the same frequency.
- This is exactly what the statement describes: sharing the channel in time.

2) Why the other choices are incorrect
- A: Assigning streams to discrete sub-carriers is frequency division multiplexing (FDM), or in digital systems, OFDM. That’s frequency-based, not time-based.
- C: “Transmitting time of transmission as the sub-carrier” is nonsensical. TDM doesn’t use sub-carriers; it uses time slots. Sub-carriers belong to FDM/OFDM.
- D: Quadrature modulation (QPSK/QAM) uses in-phase and quadrature components to increase spectral efficiency. It is a modulation method, not a time-division multiplexing scheme.

3) Useful formulas and rules of thumb
- For N equal time slots per frame:
  - Frame time: $$T_f = N \, t_s$$
  - Per-channel duty cycle: $$D = \frac{t_s}{T_f} = \frac{1}{N}$$
  - If the raw channel bit rate is $R_b$ and overhead is negligible, the per-user payload rate is approximately:
    $$R_{\text{user}} \approx \frac{R_b}{N}$$
- With overhead (sync and guard times):
  - $$T_f = \sum_i t_{s,i} + t_{\text{sync}} + t_{\text{guard}}$$
  - $$R_{\text{user}} \approx R_b \cdot \frac{t_{s}}{T_f}$$
- Rule of thumb: TDM requires timing synchronization; users do not overlap because they transmit in different time slots. DMR (amateur radio) is a practical example: 2 time slots share one 12.5 kHz channel.