Multiplexing techniques: FDM, TDM, and WDM

• Multiplexing is a technique to combine multiple signals into one signal over a shared medium. The goal is to efficiently use the available bandwidth.

Frequency Division Multiplexing (FDM)

• FDM divides the bandwidth into non-overlapping frequency sub-bands. Each signal is modulated onto a different carrier frequency.
• Guard bands are used between sub-bands to prevent interference.
• FDM is commonly used in radio and television broadcasting.
• All signals transmit simultaneously.

Time Division Multiplexing (TDM)

• TDM divides the transmission time into slots. Each signal gets a time slot to transmit data.
• TDM is used in telephone networks and digital communication systems.
• Only one signal transmits at a time.
• TDM can be synchronous or asynchronous (statistical).

Wavelength Division Multiplexing (WDM)

• WDM is a type of FDM used for optical fibers. Different wavelengths of light carry different signals.
• WDM increases the capacity of optical fibers.
• Dense WDM (DWDM) uses closely spaced wavelengths for even higher capacity.
• Coarse WDM (CWDM) uses wider wavelength spacing, which is cheaper but supports fewer channels.

Code Division Multiplexing (CDM)

• CDM assigns a unique code to each signal. Signals are transmitted simultaneously over the same frequency band.
• Receivers use the codes to extract the desired signal.
• CDMA (Code Division Multiple Access) is a common example used in mobile communication.
• All signals transmit simultaneously and can overlap in both time and frequency.

Multiplexing vs. Multiple Access

• Multiplexing combines multiple signals into one for transmission over a single link.
• Multiple access allows multiple users to share a common channel.
• Multiplexing is typically done at the physical layer and data link layer, while multiple access is usually associated with the medium access control (MAC) sublayer of the data link layer.

Synchronous TDM

• In synchronous TDM, time slots are pre-assigned to each signal source, regardless of whether the source has data to transmit.
• If a source has no data, its time slot remains empty. Leads to inefficiency if some channels are often idle.
• Requires precise synchronization between the sender and receiver.
• Simpler to implement compared to asynchronous TDM
• Example: T1 lines in telephony.

Asynchronous TDM (Statistical TDM)

• In asynchronous TDM, time slots are allocated dynamically based on demand.
• If a source has no data, its time slot is assigned to another source that needs it.
• More efficient than synchronous TDM because it avoids wasting time slots. Requires addressing information to identify the source of each data packet.
• More complex to implement due to the dynamic allocation and addressing requirements.
• Example: Used in packet switching networks.

Inverse Multiplexing

• Inverse multiplexing splits a high-speed data stream into multiple lower-speed streams for transmission over multiple channels.
• At the receiving end, the lower-speed streams are recombined to reconstruct the original high-speed data stream.
• Used when a single high-bandwidth channel is not available or is too expensive.
• Example: Bonding multiple DSL lines to increase internet bandwidth.

Add-Drop Multiplexing

• Add-drop multiplexing (ADM) allows individual channels to be added or removed from a multiplexed signal stream without demultiplexing the entire signal.
• Used in telecommunications networks to selectively route traffic.
• Common in WDM systems where specific wavelengths can be added or dropped at intermediate nodes.

Polarization Division Multiplexing (PDM)

• PDM uses different polarization states of light to carry independent signals.
• Typically used in conjunction with WDM in optical fiber communication systems.
• Increases the capacity of optical fibers by a factor of two compared to using only WDM.
• Requires polarization maintaining fibers and sophisticated polarization control techniques.

Subcarrier Multiplexing (SCM)

• SCM is used in cable television (CATV) systems and wireless communication.
• Multiple signals are modulated onto different subcarriers within a larger frequency band.
• Similar to FDM, but the subcarriers can be closely spaced.
• Orthogonal Frequency Division Multiplexing (OFDM) is a special case of SCM where the subcarriers are orthogonal to each other, minimizing interference.

Space Division Multiplexing (SDM)

• SDM increases capacity by using multiple physical paths or spatial channels.
• In fiber optics, this involves using multi-core fibers or multiple fibers within a cable. In wireless, MIMO (Multiple-Input Multiple-Output) systems use multiple antennas at both the transmitter and receiver to create spatial channels.
• Offers significant capacity gains but requires more complex hardware and signal processing.

Code Division Multiple Access (CDMA) in Detail

• Each user is assigned a unique code sequence.
• Data is encoded by multiplying it with the user's code.
• All users transmit simultaneously over the same frequency band.
• At the receiver, the desired user's signal is extracted by correlating the received signal with the user's code.
• Other users' signals appear as noise due to their codes being orthogonal (uncorrelated) to the desired user's code.
• More resilient to interference and jamming compared to FDMA and TDMA.
• Soft handoff capabilities, where a mobile device can maintain connections with multiple base stations simultaneously, improving reliability.
• Near-far problem: Signals from users closer to the base station can overpower signals from users farther away. Power control mechanisms are needed to mitigate this.