Frequency Division Multiplexing For Spectra Efficiency

Key Takeaways

• Frequency division multiplexing allows for parallelizing data signals across an entire usable bandwidth.

• “Guardrail” frequencies are used as gaps between adjacent data signals to prevent distortion.

• Improving the orthogonality of the carrier signals can unlock more bandwidth for data transmission.

Frequency division multiplexing combines signals and modulates to a carrier frequency for physical and spectra efficiency.

The proliferation of high-speed data, especially in wireless settings, has grown expeditiously to meet the changing demands of consumer habits. Yet, as more and more data consumption occurs through mobile networks, the total throughput of these networks requires greater efficiency and technological breakthroughs. Frequency division multiplexing certainly is not a cutting-edge design theory. Still, there’s value in understanding its ability (alongside similar multiplexing techniques) to tackle issues related to transfer rates more efficiently in an increasingly data-hungry world.

Advantages and Disadvantages of Frequency Division Multiplexing

The Basics of Frequency Division Multiplexing

Frequency division multiplexing (FDM) subdivides the bandwidth into bands through modulation with a carrier signal. These carrier signals operate at a single frequency (also known as a channel) with considerable gaps in the bandwidth between signals to prevent overlap; for this reason, FDM is most applicable when the bandwidth of the transmission media is greater than the sum of the individual channels. The advantage of FDM is that a single medium can transport multiple signals simultaneously – these can be distinct data sources or a parallelization technique to increase the total transfer rate for large amounts of data. A receiver can then select between the data streams with an appropriately tuned filter.

FDM is an extension of baseline multiplexing, a collection of design implementations meant to carry signal streams from multiple sources for transmission over a shared medium. By coordinating each sender with a unique communication receiver, it’s possible for a demultiplexer or demux to extract the coded signal out of the combined data line. In addition to FDM, other multiplexing options include:

• Wavelength division multiplexing (WDM) - An analog multiplexing technique that combines optical signals. The primary advantage of this format is the high data-rate capacity of optical communication media, but it requires relatively complex components with greater up-front costs and worse reliability.
• Time division multiplexing (TDM) - The time-domain equivalent of FDM that divides up one communication line and carrier frequency with smaller time slots. Depending on the clocking method, TDM can perform both synchronously and asynchronously, where the former assures equal time-division frames and the latter uses frames that optimize bandwidth efficiency.
• Code division multiplexing (CDM) - The least restrictive multiplexing approach allows simultaneous transmission over an entire frequency spectrum. The basis of CDM is vector orthogonality: by assigning senders a unique code of orthogonal vectors, a single transmission can encode multiple targets in a single transmission.

FDM Attributes For Consideration

The individual inputs modulated to carrier signals sum together before transmission, but some restrictions apply. While inputs can be digital or analog, the method is natively analog; digital signals require conversion. Demultiplexing at the output occurs for each input (a filter for each output) from the composite broadband signal. A further modulation step can also shift the composite signal to a secondary carrier frequency, which can be helpful for systems needing to shift between two (or more) carrier frequencies, such as frequency shift-keying applications.

The primary noise concerns with FDM are insufficient spacing between channels, resulting in crosstalk and intermodulation noise inherent to amplifier nonlinearity in the transmission. Crosstalk introduced before or at transmission cannot be corrected later – the layout designer will employ their normal toolkit of providing adequate spacing and minimizing the length of parallel runs for single-ended traces while furnishing an uninterrupted return path with a solid ground plane. Unlike distortion arising solely from harmonics – integer multiples of the base frequency – intermodulation can be the sum or difference between signals or their harmonics. As a result, amplification in one channel can produce frequency components in another, hampering overall signal integrity.

The basic form of any multiplexing operation is pairing inputs and outputs over a shared medium, like a cable, to improve logistics and cost. More multiplexing options exist, including variants of frequency division like wavelength division and orthogonal frequency division multiplexing (OFDM), which use carrier frequencies that are 90° phase-shifted to prevent interference. Since there’s no need for bandgaps between orthogonal phases on the same carrier frequency, the density of the transmission media increases. 

Cadence Solutions Shifts Communication Possibilities

Frequency division multiplexing maximizes data density by wringing as much usable space available in the bandwidth for separate lines, allowing parallelized transmission with appropriate buffers in the frequency to prevent signal distortion. As demand for data throughput grows in mobile networks and other settings, designers must wring as much usable space from allocated bandwidths as possible without imperiling signal integrity. Cadence’s  PCB Design and Analysis Software suite gives design teams rapid simulation and modeling capabilities to accelerate project timelines. The results of these analysis tools are then easily folded into OrCAD PCB Designer for a DFM-led approach to board layout that minimizes costly and time-consuming revisions.

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