LDM vs TDM vs FDM: A Detailed Comparison of Multiplexing Techniques

This article compares Layered Division Multiplexing (LDM), Time Division Multiplexing (TDM), and Frequency Division Multiplexing (FDM), outlining their differences and advantages.

LDM: Layered Division Multiplexing (Power Division Multiplexing)

LDM, also known as Power Division Multiplexing, employs a spectrum overlay concept. This involves transmitting multiple data streams within a single RF channel, each with varying levels of robustness (modulation, code rate) and data capacity.

The choice of robustness is determined by factors such as environmental conditions, channel characteristics, and Quality of Service (QoS) requirements.

LDM leverages the entire available RF bandwidth continuously to transmit this multi-layered signal.

Example: Consider two streams, A and B, where stream A has a higher power level than stream B. To retrieve the more robust signal (stream A), signal cancellation is applied. Stream A is then removed from the combined received signal. Following this cancellation, only stream B remains, which can then be decoded to extract the relevant data.

Figure 2 illustrates a simplified LDM transmitter featuring a two-layer path. At the receiver, the upper layer signal is decoded first. After signal cancellation of the upper layer component from the composite signal, the lower layer signal is decoded. The decoded bits from the upper layer are utilized to generate ideal complex mapped data, essential for the signal cancellers.

TDM vs. FDM

• TDM (Time Division Multiplexing): This technique transmits information by modulating and up-converting signals on the same frequency but at distinct time intervals. Large data packets are transmitted sequentially, one after the other, using the same transmitter frequency.

• FDM (Frequency Division Multiplexing): In FDM, information is transmitted by modulating and up-converting signals on different frequencies simultaneously. This allows for the concurrent transmission of large data packets using a set of transmitter frequencies.

Advantages of LDM (Layered Division Multiplexing)

LDM offers several benefits over traditional TDM/FDM-based single-layer systems:

• Improved Spectrum Efficiency: LDM enhances spectrum efficiency compared to single-layer TDM/FDM systems that may underutilize channel capacity. LDM utilizes the spectrum 100% of the time and leverages 100% of the RF channel capacity.

• Gain Improvement: Research from the University of the Basque Country in Spain suggests that LDM can provide a gain of approximately 4 to 8 dB compared to TDM systems.

• Comparable Signal Cancellation Error: The signal cancellation error in LDM is similar to the channel estimation error found in TDM/FDM-based systems. This means that existing algorithms for channel estimation and equalization can be readily adapted for use with LDM.

• PAPR Characteristics: The Peak-to-Average Power Ratio (PAPR) of an LDM signal is comparable to that of a regular OFDM signal.

• Application: LDM technology is implemented in ATSC 3.0 digital TV broadcasting.