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LDM vs TDM vs FDM | Difference between LDM, TDM, FDM

This page compares LDM vs TDM vs FDM and mentions difference between LDM, TDM and FDM. LDM stands for Layered Division Multiplexing. Advantages of Layered Division Multiplexing are also mentioned.

LDM | Layered Division Multiplexing | Power Division Multiplexing

LDM is also known by another name Power Division Multiplexing. As shown in the figure-1 LDM uses spectrum overlay concept where in multiple data streams are trasmitted in one single RF channel with different robustness(i.e. modulation, code rate) and different data capacity. The robustness level is choosen based on various environmental or channel conditions and QoS requirements.

LDM-Layered Division Multiplexing

LDM uses complete available RF bandwidth all the time in order to transmit multi-layered signal.
Example: If there are say two streams A and B where in stream-A has higher power compare to stream-B. In this case, signal cancellation is applied to retrieve robust signal (i.e. stream-A) first and it has been cancelled from combined received signal. Now after cancellation only stream-B is left which will be decoded to retrieve the information data.

LDM Transmitter

The figure-2 depicts simple LDM transmitter with two layer path. At the LDM receiver, first upper layer signal is decoded. Later after signal cancellation for upper layer part is applied on combined signal, lower layer signal is decoded. The decoded bits of upper layer signal is used to generate ideal complex mapped data to be used in the signal cancellers.

TDM | FDM

TDM vs FDM

• TDM stands for Time Division Multiplexing. In this technique, informations are transmitted by modulating and up-converting on the same frequency but at different instants. Hence transmission of large packet occurs one after the other in round robin or in sequence using same transmitter frequency.
• FDM stands for Frequency Division Multiplexing. In this technique, informations are transmitted by modulating and up-converting on the different frequencies simultaneously at one instance in time. Hence transmission of large packet occurs simultaneously using set of transmitter frequencies.
Refer TDM versus FDM➤.

Advantages of LDM (Layered Division Multiplexing)

Following are the benefits or advantages of LDM over TDM/FDM based single layer system.
• LDM improves spectrum efficiency compare to single later TDM/FDM system which wastes channel capacity. LDM uses spectrum 100% of time and uses 100% of RF channel capacity.
• LDM provides gain of about 4 to 8 dB compare to TDM systems as per research conducted by University of the Basque Country, Spain.
• Signal cancellation error in LDM is same as channel estimation error of TDM/FDM based system. Hence same existing algorithms for channel estimation and equalization can be used here too.
• PAPR of LDM (Layered Division Multiplexing) signal is same as PAPR calculated for regular OFDM signal.
• LDM technique is used in ATSC 3.0 digital TV broadcasting technology.

RELATED LINKS

• This page covers difference between LDM and TDM/FDM.
• For difference between TDM and FDM refer TDM versus FDM➤.
• For difference between TDMA and FDMA refer FDMA vs TDMA vs CDMA➤.
• TDMA basics and types

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