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Frequency-division multiple access
Frequency-division multiple access
Frequency-division multiple access
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Frequency-division multiple access (FDMA) is a channel access method used in some multiple-access protocols. FDMA allows multiple users to send data through a single communication channel, such as a coaxial cable or microwave beam, by dividing the bandwidth of the channel into separate non-overlapping frequency sub-channels and allocating each sub-channel to a separate user. Users can send data through a subchannel by modulating it on a carrier wave at the subchannel's frequency. It is used in satellite communication systems and telephone trunklines.

FDMA splits the total bandwidth into multiple channels. Each ground station on the earth is allocated a particular frequency group (or a range of frequencies). Within each group, the ground station can allocate different frequencies to individual channels, which are used by different stations connected to that ground station. Before the transmission begins, the transmitting ground station looks for an empty channel within the frequency range that is allocated to it and once it finds an empty channel, it allocates it to the particular transmitting station.

Method

[edit]

Alternatives include time-division multiple access (TDMA), code-division multiple access (CDMA), or space-division multiple access (SDMA). These protocols are utilized differently, at different levels of the theoretical OSI model.

Disadvantage: Crosstalk may cause interference among frequencies and disrupt the transmission.

  • In FDMA, all users share the satellite transponder or frequency channel simultaneously but each user transmits at single frequency.
  • FDMA can be used with both analog and digital signal but it generally used with analog signal.
  • FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA.
  • FDMA is not vulnerable to the timing problems that TDMA has. Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA.
  • Due to the frequency filtering, FDMA is not sensitive to near–far problem which is pronounced for CDMA.
  • Each user transmits and receives at different frequencies as each user gets a unique frequency slot.

FDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple users simultaneous access to a transmission system, FDD refers to how the radio channel is shared between the uplink and downlink (for instance, the traffic going back and forth between a mobile-phone and a mobile phone base station). Frequency-division multiplexing (FDM) is also distinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel, like in a car radio. FDMA, on the other hand, is an access method in the data link layer.

FDMA also supports demand assignment in addition to fixed assignment. Demand assignment allows all users apparently continuous access of the radio spectrum by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by COMSAT for use on the Intelsat series IVA and V satellites.

There are two main techniques:

  • Multi-channel per-carrier (MCPC)
  • Single-channel per-carrier (SCPC)

See also

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References

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Frequency-division multiple access

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Frequency-division multiple access (FDMA) is a utilized in systems to enable multiple users to share a common band by subdividing the available bandwidth into distinct, non-overlapping frequency sub-bands, with each sub-band assigned to a specific user or signal. This technique ensures in the , allowing simultaneous transmissions from different users without mutual interference, as long as the allocated channels remain sufficiently separated. The core principle of FDMA involves assigning unique carrier frequencies to each user within the total spectrum, often incorporating guard bands between channels to mitigate and maintain signal integrity. Originating as one of the foundational multiple access technologies in communications, FDMA was prominently featured in first-generation () analog cellular systems during the early 1980s, including the Advanced Mobile Phone Service (AMPS) in the United States, which operated in the 800 MHz band with 30 kHz channels for voice calls. These systems divided the spectrum into fixed channels, supporting approximately 56 simultaneous voice users per cell in a typical 7-cell configuration. FDMA finds applications in various radiocommunication services, including terrestrial mobile networks, systems, and access, where it facilitates efficient sharing for analog and early digital transmissions. In communications, for instance, FDMA allows multiple ground stations to access transponders via dedicated frequency slots, enhancing capacity in geostationary and low-Earth orbit configurations. Compared to (TDMA), which multiplexes users by allocating time slots on shared frequencies, FDMA offers simpler implementation for continuous analog signals but is less flexible for high-data-rate digital services due to its rigid frequency partitioning. Similarly, it contrasts with (CDMA) by avoiding code-based separation, instead relying solely on frequency isolation, which can lead to higher susceptibility to in multipath environments without additional mitigation. Modern evolutions of FDMA, such as orthogonal FDMA (OFDMA) and (SC-FDMA), build on these principles to support standards like , LTE, and , where subcarriers are dynamically allocated for improved efficiency and reduced peak-to-average power ratios in uplink transmissions. Despite the shift toward hybrid multiple access schemes in contemporary networks, FDMA remains integral to legacy systems and niche applications requiring straightforward frequency management.

Fundamentals

Definition

Frequency-division multiple access (FDMA) is a channel access technique in telecommunications systems that divides the total available radio frequency bandwidth into multiple non-overlapping sub-bands or channels, with each channel allocated to a distinct user or signal to enable simultaneous transmissions without mutual interference. This approach ensures that users operate on orthogonal frequency allocations, preventing while maximizing the shared use of the spectrum. Multiple access techniques, including FDMA, facilitate the sharing of a common communication medium among several users or devices, allowing efficient resource utilization in scenarios such as wireless networks where demand exceeds single-user capacity. In FDMA specifically, frequency separation serves as the primary mechanism for isolation, contrasting with alternatives like (TDMA), which segments the medium temporally, or (CDMA), which employs unique codes for distinction. To illustrate, FDMA can be likened to a multi-lane highway where each vehicle (user) travels in its own dedicated lane (frequency channel), permitting all to proceed concurrently without collision, thereby maintaining orderly and interference-free flow.

Core Principles

Frequency-division multiple access (FDMA) relies on the principle of frequency orthogonality, where signals occupying disjoint frequency bands exhibit no mutual interference when properly isolated, a property rooted in the Fourier transform's decomposition of signals into orthogonal frequency components. This orthogonality ensures that the inner product between signals in non-overlapping spectral regions is zero, allowing simultaneous transmission without cross-talk, as derived from Parseval's theorem in Fourier analysis. The total available bandwidth BB is divided into NN non-overlapping channels, each assigned exclusively to a user, with the channel bandwidth given by b=BNb = \frac{B}{N}. The channel spacing, which determines the separation between adjacent channel centers, is typically equal to bb plus any minimal guard band to account for practical filtering limitations, though ideal division assumes tight packing. This division enables multiple users to share the efficiently by allocating fixed frequency slots during call setup. Bandpass filters play a critical role in isolating each channel by passing signals within the assigned band while attenuating those outside, thereby preventing between adjacent channels. An ideal bandpass filter features a rectangular with infinite at the band edges, perfectly confining the signal energy and eliminating interference. In practice, however, filters exhibit transition bands with gradual , finite rates, and ripple in the and , necessitating guard bands to mitigate residual leakage and ensure reliable channel separation. Spectral efficiency in FDMA, quantified in bits per second per hertz (bps/Hz), represents the data rate achievable per unit bandwidth and is realized through the exclusive assignment of channels to users, maximizing utilization within each allocated band. This approach yields efficiency proportional to the modulation scheme's rate within bb, but overall system efficiency is reduced by unused channels and guard bands, often resulting in values below 1 bps/Hz in analog implementations.

Historical Development

Origins

The origins of frequency-division multiple access (FDMA) can be traced to early 20th-century advancements in , where (FDM) enabled multiple voice channels to be transmitted simultaneously over single wire pairs. In the 1920s, Bell Laboratories developed carrier systems that superposed additional circuits using frequencies above the standard voice range of approximately 300–3,000 Hz, assigning each channel a distinct band of about 3,000 Hz separated by electrical filters to prevent . By the early 1930s, these systems had evolved into standardized long-haul and short-haul configurations, with long-haul setups supporting up to three circuits per open-wire pair on transcontinental lines spanning over 5,000 km, such as the New York-to-Los Angeles route, while short-haul variants simplified deployment for regional use. Commercial implementation began around for high-fidelity voice transmission over widened bands (100–5,000 Hz), incorporating amplifiers and equalizers spaced every 150 miles to maintain across a 30-decibel volume range for both speech and music. Parallel developments in radio technology during the 1930s further advanced frequency-based signal separation through experiments in (FM). Edwin H. Armstrong's work on wideband FM, patented in 1933, demonstrated how modulating the carrier frequency could convey signals with reduced noise and interference, inherently relying on precise to isolate transmissions from amplitude-modulated (AM) signals. These experiments, conducted from 1931 onward, highlighted the potential for dividing the into discrete bands to support multiple simultaneous signals without overlap, building on earlier tuned-circuit principles that allowed selective reception at specific frequencies. Armstrong's demonstrations, including a 1935 presentation to the Institute of Radio Engineers showcasing FM operation over 17 miles, underscored the practical advantages of frequency separation for clearer communication in crowded airwaves. The first practical FDMA-like systems emerged in and early , facilitated by regulatory frequency allocations that assigned distinct bands to users and services to enable concurrent operations. In the late 1920s, the (predecessor to the FCC) defined bands starting in 1928, allocating segments such as 1.5–2 MHz for experimentation, which allowed hams to transmit on assigned frequencies without mutual interference. By , following the FCC's establishment in , regulations expanded allocations to include new VHF bands like 112–116 MHz (2.5 meters) and shorter wavelengths, promoting frequency-specific operations for voice and amid growing radio activity. For , 1930s FCC rules formalized the AM band (550–1,600 kHz) with channel spacing to accommodate multiple stations, as detailed in service bulletins from 1930 onward, ensuring nationwide coverage by segmenting the spectrum for simultaneous transmissions. These early allocations laid the groundwork for later mobile communications by establishing principles for multi-user access.

Evolution in Telecommunications

Following World War II, frequency-division multiple access (FDMA) saw significant adoption in military systems and civilian (FM) during the 1940s and 1950s, enabling efficient spectrum sharing for multiple signals through discrete frequency allocations. In applications, early public telephone services launched in utilized FDMA principles to support limited simultaneous users via central transmitters operating on assigned frequencies, addressing public safety needs and driving technological advancements in low-power transmission. Similarly, the post-war expansion of leveraged FDMA to allocate distinct channels within the 88–108 MHz band, allowing multiple stations to operate without interference while benefiting from FM's superior noise rejection compared to . A pivotal milestone occurred in 1983 with the deployment of the (AMPS), the first-generation analog cellular standard in the United States, which employed FDMA to assign 30 kHz voice channels across 832 frequencies in the 800 MHz band, supporting up to 42 simultaneous calls per cell through frequency reuse patterns. This approach marked FDMA's transition from basic to structured cellular architecture, revolutionizing wide-area voice communications by dividing the available spectrum into narrowband slots for individual users. AMPS's FDMA implementation facilitated nationwide rollout, though it faced capacity limitations due to analog signaling and fixed channel assignments. The 1990s brought digital enhancements to FDMA in second-generation () Global System for Mobile Communications () networks, where it combined with (TDMA) to form a hybrid framework, allocating 200 kHz carriers divided into eight time slots per frame in the 900/1800 MHz bands. An optional slow frequency-hopping extension, changing carriers up to 217 times per second across 64 frequencies, was introduced to mitigate multipath fading and improve , enabling global digital voice and basic data services for millions of subscribers. This digital FDMA evolution addressed AMPS's inefficiencies, paving the way for higher capacity and security in mobile networks. In third-generation (3G) Universal Mobile Telecommunications System (UMTS) and fourth-generation (4G) Long-Term Evolution (LTE) standards, FDMA integrated as a hybrid component, with UMTS using wideband code-division multiple access (W-CDMA) overlaid on FDMA carriers for downlink spectrum division, while LTE adopted orthogonal frequency-division multiple access (OFDMA) for downlink and single-carrier FDMA (SC-FDMA) for uplink to reduce peak-to-average power ratio and enhance power efficiency in user equipment. By 2025, FDMA maintains relevance in satellite communications, where FDMA-based multi-beam systems support geostationary and low-Earth orbit constellations for global coverage in interference-prone environments. In Internet of Things (IoT) standards like Narrowband IoT (NB-IoT), an LTE derivative, SC-FDMA persists for low-power uplink transmissions, with recent AI-driven optimizations—such as reinforcement learning algorithms for dynamic resource allocation—improving energy efficiency and latency in dense deployments.

Technical Implementation

Frequency Division and Guard Bands

In frequency-division multiple access (FDMA) systems, the total available spectrum bandwidth BtotalB_{\text{total}} is partitioned into NN non-overlapping bands, each assigned to a distinct user or for simultaneous transmission. The center fc,if_{c,i} for the ii-th channel is calculated as fc,i=f0+(i−1)×(B+G)+B2f_{c,i} = f_0 + (i-1) \times (B + G) + \frac{B}{2}, where f0f_0 is the starting of the spectrum, BB is the bandwidth allocated to each channel (typically narrow, such as 30 kHz in early cellular systems), and GG is the width of the between channels; this ensures the bands remain separated while maximizing spectrum utilization. Guard bands serve as unused frequency gaps inserted between adjacent channels to mitigate interference arising from the imperfect frequency selectivity of practical filters, particularly their characteristics where the transition from to is not abrupt. These bands are typically small relative to the channel bandwidth to accommodate filter imperfections without excessively wasting . The minimum guard band width Δf\Delta f must cover the sum of the adjacent channel excesses to ensure sufficient of from neighboring signals; for example, in systems using raised-cosine filters, this relates to the roll-off factor α\alpha, where the transition bandwidth is approximately αB\alpha B. Channel allocation in FDMA can be static or dynamic. In static allocation, prevalent in early systems like the Advanced Mobile Phone Service (AMPS), channels are pre-assigned to specific s or users on a fixed basis, limiting flexibility but simplifying . Dynamic allocation, used in modern FDMA variants, allows adaptive assignment controlled by the base station, which monitors traffic demands and reallocates channels in real-time to optimize resource use and reduce blocking probability. Non-ideal filters in FDMA transceivers, with finite and imperfect , cause that results in (ACI), where power from one channel spills into neighboring ones. This degrades the (SIR), defined as SIR=PsPi\text{SIR} = \frac{P_s}{P_i} where PsP_s is the desired signal power and PiP_i is the interfering power, potentially falling below required thresholds (e.g., 17-18 dB in early cellular standards like AMPS). Guard bands and directly influence ACI suppression, with distance between users providing additional path-loss mitigation to maintain adequate SIR.

Signal Processing Requirements

In FDMA transceivers, the transmitter relies on frequency synthesizers to generate precise carrier frequencies for each user channel, ensuring minimal overlap with adjacent channels. These synthesizers must achieve high stability, typically on the order of 1-3 ppm, to maintain channel integrity under temperature variations and aging effects. For instance, in the Narrowband Advanced Mobile Phone System (NAMPS), which employs FDMA, transmitter frequency stability is specified at ±1.0 ppm to support narrow 10 kHz channels. performance is equally critical, as excessive noise can cause spectral regrowth and inter-channel interference; requirements often limit to levels suitable for the system offset in RF transceivers for wireless systems. Receiver demands in FDMA include tunable bandpass filters to isolate the assigned frequency channel from out-of-band signals and demodulators to recover the modulated information. These filters must exhibit sharp roll-off characteristics to suppress in dense frequency allocations. Automatic gain control (AGC) is integral to handle dynamic signal variations across channels, arising from differing distances or multipath effects; AGC adjusts gains to maintain consistent signal levels at the demodulator input, typically achieving a of around 70-80 dB. Modern FDMA implementations in software-defined radios incorporate (DSP) for efficient channel management, particularly through FFT-based channelization to demultiplex multiple narrowband signals from a input. This approach transforms the received signal into the , allowing parallel filtering and extraction of individual FDMA channels with low latency. The scales as O(Nlog⁡N)O(N \log N) for NN channels, making it suitable for real-time processing on field-programmable gate arrays (FPGAs) or DSP chips. To combat impairments like and frequency-selective —where causes unequal across channels—FDMA integrates (FEC) coding schemes such as convolutional codes or Reed-Solomon codes. These codes add redundancy to detect and correct errors, with interleaving spreading burst errors over multiple frequency channels to leverage the diversity inherent in FDMA's frequency separation. In frequency-selective environments, this pairing enhances performance by 3-6 dB compared to uncoded systems, as demonstrated in analyses of FDMA over multipath channels.

Applications

Cellular and Mobile Networks

Frequency-division multiple access (FDMA) formed the foundational multiple access technique in first-generation () cellular systems, enabling analog voice calls by assigning dedicated frequency channels to each user within a cell. In the (AMPS), deployed in the United States starting in 1983, FDMA utilized 30 kHz wide channels in the 800 MHz band, with uplink frequencies from 824–849 MHz and downlink from 869–894 MHz, supporting up to 790 channels divided between two carriers (A and B) for full-duplex operation. Similarly, the Total Access Communications System (TACS), introduced in the in 1985 as an extended AMPS variant, operated in the 900 MHz band with comparable 25 kHz channels to accommodate voice traffic in urban environments. These systems relied on frequency division to avoid intra-cell interference, with guard bands separating channels to mitigate , though overall spectrum efficiency was limited by the analog nature and lack of digital modulation. In second-generation () and third-generation () networks, FDMA evolved into hybrid schemes combining division with time or code division to enhance capacity while managing interference. The Global System for Mobile Communications (), a 2G standard, employed a hybrid FDMA/TDMA approach where the available —such as 890–915 MHz uplink and 935–960 MHz downlink—was divided into 200 kHz wide FDMA carriers, each further subdivided into eight time slots for TDMA. reuse patterns, such as the common 7-cell cluster ( factor K=7), allowed the same frequencies to be reused in non-adjacent cells, with the co-channel reuse D/R ≈ 4.6 (where D is the and R the cell radius) to minimize ; this interference was managed through , antenna tilting, and site planning to maintain a carrier-to-interference (C/I) above 9 dB. In 3G Universal Mobile Telecommunications System (), based on wideband CDMA (W-CDMA), FDMA was used at the carrier level with 5 MHz bandwidth channels (e.g., in the 1920–1980 MHz uplink and 2110–2170 MHz downlink bands), while CDMA handled intra-carrier ; reuse was universal (K=1), but FDMA carriers were selectively deployed across cells to control inter-carrier interference. In modern fourth-generation (4G) Long-Term Evolution (LTE) and fifth-generation (5G) New Radio (NR) systems, FDMA principles underpin orthogonal frequency-division multiple access (OFDMA) as an advanced multi-carrier variant for downlink transmissions, dividing the spectrum into numerous narrow orthogonal subcarriers to combat multipath fading and enable flexible resource allocation. LTE employs OFDMA with a fixed 15 kHz subcarrier spacing in bandwidths up to 20 MHz (e.g., 1.4–20 MHz channels in bands like 700 MHz or 2.6 GHz), assigning groups of 12 subcarriers (one physical resource block, 180 kHz) to users for high data rates. 5G NR extends this with scalable numerologies, supporting subcarrier spacings of 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz to adapt to diverse scenarios—lower spacings for coverage in sub-6 GHz bands and higher for low-latency millimeter-wave applications—while maintaining orthogonality to avoid inter-carrier interference. Uplink in both uses single-carrier FDMA (SC-FDMA) to reduce peak-to-average power ratio, but OFDMA's FDMA roots enable dynamic spectrum sharing and beamforming for massive connectivity. Capacity in FDMA-based cellular systems is quantified using the Erlang B formula, which models voice traffic as a Poisson arrival process with exponentially distributed call durations to estimate the maximum offered load (in Erlangs) per cell before exceeding a target blocking probability. For c available channels and offered traffic a (calls per hour times average holding time in hours), the blocking probability B(c, a) is given by: B(c,a)=acc!∑k=0cakk!B(c, a) = \frac{\frac{a^c}{c!}}{\sum_{k=0}^{c} \frac{a^k}{k!}} This recursive formula determines traffic capacity; for example, in a 1G AMPS cell with approximately 56 channels and 2% blocking probability, capacity approximates 35–40 Erlangs, scaling with reuse patterns in multi-cell deployments. In hybrid 2G/3G systems, Erlang B is applied per carrier or time slot, adjusted for reuse efficiency (e.g., 1/7 of total spectrum per cell in a 7-cell pattern), yielding higher per-cell Erlangs (e.g., 20–40) through digital encoding, while OFDMA in 4G/5G adapts it for packet data by considering resource block utilization rather than fixed channels.

Broadcasting and Satellite Systems

In frequency-division multiple access (FDMA) systems for broadcasting, radio stations are assigned distinct frequency bands within the very high frequency (VHF) range of 88–108 MHz for FM radio, with channel spacing standardized at 200 kHz in the United States to prevent interference between adjacent stations. This allocation allows multiple broadcasters to operate simultaneously in the same geographic area, each transmitting on a dedicated carrier frequency modulated with audio signals for one-to-many distribution. In Europe, FM channel spacing is typically 100 kHz as per international planning standards, enabling denser packing of stations while maintaining signal integrity through guard bands. Similarly, television broadcasting employs FDMA in the VHF (54–216 MHz) and ultra high frequency (UHF, 470–890 MHz) bands, where channels are spaced at 6 MHz each to accommodate analog or digital video and audio signals, with the Federal Communications Commission (FCC) regulating assignments to ensure nationwide coverage without overlap. Satellite systems leverage FDMA extensively for global broadcasting and point-to-multipoint communications, particularly in geostationary orbits. In the network, which supports international and , transponders typically operate with a bandwidth of 36 MHz, divided into narrower FDMA channels to allocate for multiple users transmitting voice or video traffic across continents. This division facilitates efficient sharing for international calls, where uplink and downlink frequencies are assigned separately to earth stations, enabling seamless global connectivity without time-based coordination. Advanced multi-beam architectures enhance FDMA's utility through spot-beam , directing narrow beams to specific regions for targeted coverage. In these systems, frequency reuse is achieved by spatially separating beams, allowing the same frequency bands to be reused in non-adjacent coverage areas, which increases overall capacity for regional broadcasting services like direct-to-home television. For instance, inter-beam isolation of at least 20 dB ensures minimal , supporting FDMA's division of bandwidth into independent channels for diverse content delivery. Power efficiency in FDMA-based satellite broadcasting is optimized by careful of nonlinear effects in high-power amplifiers, where multiple carriers sharing a can generate . By incorporating guard bands and operating amplifiers below saturation—often with output backoff of several decibels—FDMA reduces products, preserving signal quality and maximizing for reliable one-to-many transmission. This approach contrasts with denser methods but aligns with broadcasting's emphasis on constant-envelope signals, which inherently limit in dedicated assignments.

Comparisons with Other Methods

Versus Time-Division Multiple Access

Frequency-division multiple access (FDMA) and (TDMA) represent two fundamental approaches to resource allocation in multiple access systems, with FDMA partitioning the available bandwidth into distinct channels for parallel transmission by multiple users, while TDMA divides a single channel into sequential time slots for users to transmit in turn. This core difference makes FDMA particularly suitable for applications requiring continuous streams, such as analog voice communications, where each user maintains a dedicated band for uninterrupted transmission. In contrast, TDMA excels in handling bursty or variable-rate , as time slots can be dynamically allocated or left idle based on demand, promoting flexibility in digital systems. Efficiency trade-offs between the two methods arise primarily from their handling of unused resources and overhead mechanisms. FDMA incurs bandwidth inefficiency due to fixed channel assignments and the necessity of guard bands to prevent , which wastes some of the . TDMA, however, avoids such persistent waste by reusing the full bandwidth across time slots but introduces overhead, including guard time intervals to account for delays and clock drifts, which reduce effective throughput. Overall, studies in digital cellular contexts show TDMA achieving up to three times the of FDMA under similar conditions, though this advantage diminishes in scenarios with low user activity where FDMA's dedicated channels minimize idle . Interference management further distinguishes the techniques, with FDMA relying on bandpass filters and guard bands to isolate frequency channels and mitigate crosstalk or co-channel interference, enabling robust performance in frequency-selective fading environments. TDMA, by comparison, depends on precise timing synchronization across users to avoid slot overlaps, using guard times to tolerate minor timing errors, but this can lead to higher peak power requirements since users must transmit bursts at elevated power levels to achieve equivalent average signal strength over short durations. As a result, FDMA systems typically exhibit lower power demands and simpler transmitter designs for continuous operation, whereas TDMA's bursty nature necessitates more complex to manage interference from timing inaccuracies. In practice, many modern systems adopt hybrid approaches to leverage strengths from both, such as the , which employs TDMA to subdivide FDMA-assigned frequency carriers into eight time slots per frame, allowing multiple users per channel while retaining frequency separation for interference control. This TDMA-over-FDMA structure in balances the continuous access of FDMA with TDMA's capacity gains, supporting up to eight voice channels per 200 kHz carrier. While offers a third alternative using orthogonal codes for simultaneous access, the FDMA-TDMA contrast highlights trade-offs in parallelism versus sequencing that persist in hybrid designs.

Versus Code-Division Multiple Access

Frequency-division multiple access (FDMA) and (CDMA) represent two distinct approaches to enabling multiple users to share a , primarily differing in their handling of allocation and interference management. In FDMA, each user is assigned an exclusive frequency band within the total available bandwidth, ensuring no spectral overlap and thus orthogonal separation without additional processing. Conversely, CDMA permits all users to transmit simultaneously across the entire bandwidth, relying on orthogonal spreading codes—such as Walsh codes, which are a set of binary sequences with zero —to distinguish user signals at the receiver. This code-based separation in CDMA allows for overlapping spectra, transforming multi-user interference into manageable self-noise rather than complete avoidance as in FDMA. Regarding capacity and robustness, FDMA provides predictable and stable capacity limited by the number of available frequency channels and guard bands, with minimal vulnerability to multi-user interference since users operate on disjoint bands. In contrast, CDMA's capacity can exceed FDMA's in multipath environments due to the , which combines delayed signal replicas from multiple paths to achieve diversity gain and improve . However, CDMA suffers from the near-far problem, where a strong signal from a nearby transmitter overwhelms weaker signals from distant users, necessitating mechanisms to maintain equity— a challenge absent in FDMA's isolated bands. Spectrum efficiency further highlights their differences: in FDMA, the bandwidth per user is given by b=BNb = \frac{B}{N}, where BB is the total bandwidth and NN is the number of users, leading to underutilization due to guard bands and fixed allocations. CDMA enhances efficiency through its processing gain G=TbTcG = \frac{T_b}{T_c}, where TbT_b is the bit duration and TcT_c is the chip duration, allowing multiple users to share the while codes suppress interference below the . This gain enables CDMA to support higher user densities, particularly in interference-limited scenarios. The evolution of mobile standards illustrates the shift from FDMA-dominant first-generation (1G) systems, such as AMPS, to CDMA in third-generation () standards like IS-95, driven by CDMA's superior voice capacity—up to 10-15 times that of FDMA/TDMA predecessors—through efficient spectrum reuse and interference mitigation. This transition prioritized CDMA's ability to handle growing demand in cellular networks while leveraging digital processing for enhanced quality.

Advantages and Limitations

Key Benefits

One of the primary advantages of frequency-division multiple access (FDMA) lies in its simplicity of implementation, as it requires no complex timing or coding schemes, allowing users to operate independently once frequency bands are assigned. This straightforward approach, relying primarily on bandpass filters for channel separation, makes FDMA particularly cost-effective for analog systems, where hardware demands are minimal and no equalization is needed to mitigate inter-symbol interference. FDMA minimizes interference through the allocation of exclusive, nonoverlapping frequency bands separated by guard bands, which effectively reduce and . This design is especially beneficial in environments with minimal mobility, such as fixed or low-speed applications, where stable channel conditions enhance reliability without the complications of dynamic frequency reuse. The technique supports continuous transmission, enabling simultaneous and uninterrupted data streams across assigned bands, which avoids the bursty nature of alternatives like . This characteristic makes FDMA well-suited for real-time applications, particularly voice communications, where low latency and steady connectivity are essential for maintaining call quality. FDMA's design facilitates backward compatibility with legacy systems, as demonstrated by its foundational role in early analog cellular networks like the (AMPS), which used FDMA for channel access and allowed seamless integration of proven equipment in transitioning infrastructures.

Principal Drawbacks

One of the primary limitations of frequency-division multiple access (FDMA) is its inefficient use of the , primarily due to the necessity of guard bands between allocated frequency channels to prevent . These guard bands remain unused and lead to underutilization, particularly in scenarios with low traffic where assigned channels lie idle. Fixed channel allocations exacerbate this issue, as bandwidth is dedicated to users regardless of varying , resulting in wasted resources during periods of inactivity. FDMA also exhibits limited , struggling to accommodate bursty data traffic or high user densities without significant spectrum expansion. In environments with intermittent or variable data flows, such as modern mobile applications, fixed assignments cannot dynamically reallocate resources efficiently, leading to bottlenecks and the need for additional bandwidth to support growth. This inflexibility makes it challenging to scale systems for large numbers of users, as adding capacity often requires reconfiguring the entire frequency band division. Furthermore, FDMA systems are vulnerable to frequency-selective fading caused by multipath propagation effects, where different frequency bands experience independent due to varying path lengths and delays. Each user's narrowband channel may undergo deep flat fades independently, degrading signal quality and necessitating additional diversity techniques, such as frequency hopping or , to maintain reliability. In contemporary high-speed networks, pure FDMA has become largely obsolete, having been supplanted by more flexible schemes like (OFDMA) in standards for improved adaptability and efficiency. Deployments of traditional FDMA have declined significantly, confined mostly to legacy systems, as evidenced by the adoption of OFDMA in New Radio (NR) specifications.

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