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Diversity scheme

In telecommunications, particularly wireless communications, a diversity scheme is a technique that improves the reliability of a transmitted signal by sending multiple independent replicas of the information across different propagation paths, thereby mitigating the destructive effects of multipath fading and reducing error rates.^[1] These schemes exploit the statistical independence of fading channels to ensure that while one path may experience a deep fade, others remain viable, effectively averaging out amplitude variations and enhancing overall link performance.^[2] Multipath fading arises in wireless environments where signals reflect off obstacles, arriving at the receiver with varying delays and phases that cause constructive or destructive interference, leading to rapid signal fluctuations and high bit error probabilities, especially in mobile scenarios.^[1] Diversity schemes address this by providing redundancy, where the receiver processes signals from multiple branches—each corresponding to an independent fade realization—to achieve a diversity order L, defined as the negative exponent in the error probability's asymptotic decay with signal-to-noise ratio (SNR), such that P_e ≈ c · SNR<sup>-L</sup> at high SNR.^[1] This results in a steeper slope in the error rate curve, significantly lowering outage probabilities without requiring excessive transmit power. Common types of diversity include time diversity, frequency diversity, and space diversity, each leveraging different channel characteristics for independence. Time diversity spreads coded symbols across time intervals exceeding the channel's coherence time, using interleaving to decorrelate fades; frequency diversity transmits over a bandwidth broader than the coherence bandwidth, as in OFDM systems where subcarriers experience independent flat fading; and space diversity employs multiple antennas at the transmitter, receiver, or both to capture signals from spatially separated paths.^[1] Additional variants, such as polarization and angle diversity, further exploit signal orthogonality in propagation.^[3] Receiver-side implementation often involves combining techniques like selection combining (picking the strongest signal), maximal ratio combining (weighting by channel gain), or equal gain combining, while transmit diversity schemes, such as the seminal Alamouti code, encode signals across antennas and time slots to achieve full diversity gain equivalent to receiver diversity without channel feedback.^[1] Introduced in 1998, the Alamouti scheme uses two transmit antennas and one receiver antenna to provide a diversity order of 2, generalizable to M receivers for order 2M, while maintaining bandwidth efficiency and low complexity.^[4] These methods form the foundation for modern standards like LTE and 5G, enabling robust high-data-rate communications in fading-prone environments.^[1]

Fundamentals

Definition and Principles

A diversity scheme is a technique employed in wireless communications to enhance the reliability of signal transmission by utilizing two or more independent channels to send redundant copies of the same information, thereby exploiting variations in channel conditions to improve reception quality.^[1] This approach addresses the challenges posed by fading in wireless channels, where signal strength fluctuates due to multipath propagation.^[1] The core principle of diversity schemes lies in providing multiple uncorrelated replicas of the signal, known as diversity branches, which experience independent fading.^[1] By combining these branches at the receiver, the scheme mitigates the effects of deep fades that might corrupt a single path, effectively reducing the overall error probability and aiming to maximize the signal-to-noise ratio (SNR).^[1] The greater the number of independent branches, the higher the diversity order, leading to more robust performance against fading.^[1] The origins of diversity schemes trace back to the early 1920s, when engineers at the Radio Corporation of America (RCA), including Harold H. Beverage and H. O. Peterson, began investigating signal fading in high-frequency radio systems to overcome losses from ionospheric and multipath effects. Their experiments demonstrated that signals received at spatially separated antennas faded independently, leading to the development of space diversity reception by 1926. First practical implementations emerged in the mid-20th century, with RCA deploying triple-diversity receivers in the late 1920s and commercial systems like the Diversity Receiving System by 1933.^[5] For diversity schemes to yield significant gains, the fading across branches must be statistically independent, typically achieved through sufficient separation in space, time, or frequency to decorrelate the channel responses.^[1] Without this independence, the benefits diminish, as correlated fades would not provide the necessary redundancy.^[1]

Fading in Wireless Channels

In wireless communications, fading refers to the variation in signal amplitude and phase experienced by a propagating electromagnetic wave due to interactions with the environment. This phenomenon arises primarily from multipath propagation, where the signal arrives at the receiver via multiple paths, leading to constructive or destructive interference that causes rapid fluctuations in the received signal strength. Additionally, mobility introduces Doppler shifts, which alter the phase and frequency of the signal components, further contributing to time-varying fading effects on both amplitude and phase.^[6]^[7] Fading is broadly classified into large-scale and small-scale categories. Large-scale fading encompasses path loss, which describes the signal attenuation due to distance and obstacles, and shadowing, a log-normal variation caused by large obstructions like buildings or terrain, resulting in slower changes over distances of tens to hundreds of meters. Small-scale fading, in contrast, involves rapid signal variations over short distances (fractions of a wavelength) due to multipath interference, and it is further subdivided into flat fading, where the entire signal bandwidth experiences similar attenuation and the channel impulse response is shorter than the symbol duration, and frequency-selective fading, where different frequency components of the signal are affected differently due to a longer channel impulse response exceeding the symbol duration. Small-scale fading often follows statistical distributions such as Rayleigh for non-line-of-sight (NLOS) scenarios or Rician for line-of-sight (LOS) conditions.^[8]^[9]^[10] A key statistical model for small-scale fading in NLOS environments is the Rayleigh fading model, which assumes the signal envelope follows a Rayleigh distribution derived from the sum of two independent Gaussian random variables representing in-phase and quadrature components with zero mean and equal variance \sigma^2. The probability density function (PDF) of the envelope r is given by

f(r) = \frac{r}{\sigma^2} \exp\left(-\frac{r^2}{2\sigma^2}\right), \quad r \geq 0,

where \sigma^2 represents the power per dimension, and the average power is $2\sigma^2. This model captures the severe amplitude fluctuations typical in urban or indoor settings without a dominant path. In LOS scenarios, the Rician model extends this by incorporating a deterministic LOS component, resulting in a distribution with a non-zero mean Gaussian term, leading to less severe fading compared to Rayleigh.^[11]^[6] Fading significantly impairs communication reliability by creating deep signal nulls, where the envelope approaches zero due to destructive interference, thereby increasing bit error rates (BER) in uncoded systems, often by orders of magnitude compared to additive white Gaussian noise (AWGN) channels alone. For instance, in Rayleigh fading, the probability of deep fades is higher than in AWGN, exacerbating errors in digital modulation schemes without mitigation.^[12]^[13]

Types of Diversity

Spatial Diversity

Spatial diversity exploits the spatial separation of multiple antennas to create independent signal paths, thereby mitigating the effects of multipath fading in wireless channels. This technique involves placing antennas at the transmitter or receiver (or both) such that the distance between them exceeds the coherence distance, typically around half the signal wavelength (λ/2) in environments with rich scattering, ensuring that the fading experienced by each antenna is uncorrelated.^[14] In receive diversity, multiple antennas at the receiver capture signals arriving via different paths, while transmit diversity uses multiple antennas at the transmitter to send redundant or coded signals that decorrelate in space. A foundational demonstration of spatial diversity's impact showed that even modest numbers of antennas can substantially improve link reliability by providing multiple independent fading branches. Common antenna configurations for spatial diversity include single-input multiple-output (SIMO) systems, where a single transmit antenna feeds multiple receive antennas, and multiple-input single-output (MISO) systems, which reverse this setup for transmit diversity. For instance, a basic SIMO configuration with two receive antennas separated by λ/2 allows the receiver to select or combine signals from paths that fade independently, enhancing reliability without altering the transmission scheme.^[15] These setups form the basis for more advanced multiple-input multiple-output (MIMO) systems, though spatial diversity focuses on the independence gained from physical separation rather than multiplexing.^[16] The primary advantages of spatial diversity lie in its effectiveness within multipath-rich environments, where it combats deep fades without requiring additional bandwidth or spectrum resources, thus improving signal reliability and overall system capacity.^[17] However, implementation challenges include the need for sufficient physical space to achieve the required antenna spacing, which can be prohibitive in compact devices like mobiles, leading to increased hardware complexity and cost.^[15] Insufficient spacing may result in correlated fading between antennas, diminishing the diversity benefits and potentially introducing mutual coupling effects.^[14]

Frequency Diversity

Frequency diversity is a technique in wireless communications that exploits variations in the channel frequency response to improve signal reliability by transmitting the same information over multiple frequency-separated channels that experience independent fading. In frequency-selective fading environments, the channel impulse response leads to a non-flat frequency response, allowing signals on frequencies spaced apart to fade independently. This independence arises when the frequency separation exceeds the coherence bandwidth of the channel, typically approximated as B_c \approx \frac{1}{\tau}, where \tau is the delay spread representing the multipath propagation time dispersion.^[1]^[18] Implementations of frequency diversity often involve spread spectrum or multicarrier modulation schemes to achieve the required frequency separation. Frequency hopping spread spectrum (FHSS) rapidly switches the carrier frequency among a set of channels wider than the coherence bandwidth, ensuring each hop encounters uncorrelated fading. In the Global System for Mobile Communications (GSM), slow frequency hopping (SFH) is employed, where the transmitter changes frequency every burst (approximately 217 hops per second across up to 64 frequencies), providing diversity gains against both fast fading and co-channel interference.^[19]^[20] Orthogonal frequency-division multiplexing (OFDM), another key method, divides the signal into multiple closely spaced subcarriers, each experiencing potentially independent fading if the subcarrier spacing is sufficiently large relative to the coherence bandwidth; this is inherent in Wi-Fi standards like IEEE 802.11a/g/n/ac, where OFDM subcarriers (spaced at 312.5 kHz) leverage frequency selectivity for robustness in indoor multipath environments.^[21]^[22] The primary advantages of frequency diversity include its effectiveness in bandwidth-abundant systems, where it combats deep fades by relying on at least one robust frequency path, while simultaneously reducing inter-symbol interference through signal spreading across the spectrum.^[23] In OFDM-based systems like Wi-Fi, this also enables efficient equalization of frequency-selective channels without excessive complexity. However, a notable drawback is the additional spectrum consumption required for multiple channels or subcarriers, which reduces overall bandwidth efficiency compared to single-carrier approaches in spectrum-constrained scenarios.^[21]^[24]

Time Diversity

Time diversity leverages the inherent temporal variations in wireless channels, caused by Doppler-induced fading, to mitigate the effects of signal fading by transmitting redundant copies of the information-bearing signal at different time instants. These transmissions are spaced apart by intervals exceeding the channel's coherence time, which ensures that each copy encounters an independent fading realization, thereby providing multiple uncorrelated channel samples for improved reliability. The coherence time T_c is generally approximated as T_c \approx \frac{1}{f_d}, where f_d represents the Doppler frequency resulting from relative motion between the transmitter and receiver.^[1]^[25] Key techniques for implementing time diversity include repetition coding, where the identical signal is retransmitted after a suitable delay; time interleaving, which rearranges the order of coded bits or symbols over time to spread burst errors from deep fades; and cyclic delay diversity, which applies cyclic shifts to the signal across multiple transmissions to simulate multipath effects and enhance diversity. In practice, time interleaving is widely used in conjunction with forward error correction (FEC) schemes, such as in digital video broadcasting standards like DVB-T2, where it combats fading in mobile reception by allowing flexible trade-offs between diversity gain, latency, and power efficiency.^[1]^[18] This approach offers significant advantages, including the absence of requirements for multiple antennas or additional spectrum, making it particularly suitable for resource-constrained, low-mobility scenarios such as fixed or pedestrian wireless links where channel variations occur gradually. However, time diversity introduces inherent latency from the buffering needed for interleaving or repetition delays, and it becomes less effective in high-mobility, fast-fading environments if the time separation fails to exceed the shortened coherence time.^[25]

Polarization Diversity

Polarization diversity leverages orthogonal polarization states of electromagnetic waves to combat fading in wireless channels by providing multiple independent signal paths at the receiver. Signals transmitted with a specific polarization, such as linear horizontal or vertical, undergo changes due to multipath scattering, resulting in components arriving with mixed polarizations at the receiver. By deploying antennas tuned to orthogonal polarizations—typically horizontal/vertical linear or left/right circular—the system captures these differently faded replicas, exploiting cross-polarization discrimination where the isolation between polarization branches arises from varying reflection coefficients in scattering environments. This mechanism is particularly pronounced in environments with rich multipath propagation, where the scattering alters polarization orthogonality, leading to low correlation between the received signals.^[26] Implementation of polarization diversity commonly involves dual-polarized antennas at base stations, which integrate two orthogonal ports into a single physical structure to transmit and receive signals simultaneously. These antennas, often featuring crossed dipoles or patch elements, are mounted with minimal spacing, such as 0.3λ vertical separation, and are shielded under a common radome to maintain isolation greater than 30 dB between ports and cross-polarization discrimination exceeding 20 dB. The technique proves effective in urban settings with mixed polarization scattering from buildings and obstacles, where wide angular spreads of multipath components enhance branch independence without needing frequency or time variations.^[26]^[27] A key advantage of polarization diversity is its space-efficient design, which reuses the same antenna aperture without requiring lateral separation, thereby minimizing infrastructure footprint and costs compared to spatial alternatives. In diverse propagation conditions, it achieves low signal correlation (ranging from -0.013 to 0.34) and delivers substantial diversity gains, such as 6-11 dB at 99% reliability levels, making it suitable for capacity-limited base stations in high-multipath areas. This compactness also simplifies deployment in dense urban networks, where it performs comparably to other diversity methods while easing site acquisition challenges.^[26]^[27] Despite these benefits, polarization diversity faces challenges from polarization mismatch losses, which can degrade signal-to-noise ratio by up to 12 dB due to unpredictable handset orientations that misalign with the base station's polarizations. Depolarization effects in channels with limited scattering, such as suburban or line-of-sight scenarios, further reduce branch independence, limiting gains to as low as 2-3 dB. Additionally, in personal communication systems operating at higher frequencies, potential intermodulation products exceeding -100 dBm may arise from imperfect port isolation, complicating compliance with interference standards.^[26]^[27]

Combining Techniques

Selection Combining

Selection combining (SC) is a fundamental diversity combining technique employed in wireless receivers to mitigate fading effects by selecting the single strongest signal among multiple diversity branches. At the receiver, the instantaneous signal-to-noise ratios (SNRs) from each branch—derived from various diversity types such as spatial or frequency separation—are compared, and the branch exhibiting the highest SNR is chosen for demodulation and further processing. Unlike more complex methods, SC requires no phase synchronization or amplitude weighting, relying solely on continuous SNR monitoring and a simple switching mechanism to route the selected signal.^[28] The mathematical foundation of SC centers on maximizing the effective SNR across L independent branches. The output SNR is expressed as \gamma_{SC} = \max(\gamma_1, \gamma_2, \dots, \gamma_L), where \gamma_i denotes the instantaneous SNR of the i-th branch. For Rayleigh fading channels with identical average SNR \bar{\gamma} per branch, the cumulative distribution function (CDF) of each \gamma_i is F_{\gamma_i}(\gamma) = 1 - \exp(-\gamma / \bar{\gamma}), leading to the outage probability P_{out}(\gamma_{th}) = [F_{\gamma_i}(\gamma_{th})]^L = [1 - \exp(-\gamma_{th} / \bar{\gamma})]^L, where \gamma_{th} is the required SNR threshold. This derivation underscores SC's ability to reduce outage by exploiting the statistical independence of branch fades.^[28]^[29] SC offers significant advantages in terms of simplicity and resource efficiency, as it demands minimal hardware—typically just comparators for SNR estimation and a switch—making it well-suited for power- and size-limited systems like early mobile devices.^[30] However, its performance is inherently suboptimal because it discards contributions from weaker branches, resulting in lower overall SNR gains compared to techniques that coherently integrate all signals; for example, with two branches under Rayleigh fading, SC yields about a 10 dB SNR improvement at 1% outage probability relative to a single branch, but additional branches provide diminishing returns due to the logarithmic scaling of gains.^[30]^[28]

Maximal Ratio Combining

Maximal ratio combining (MRC) is a linear combining technique that optimally processes signals from multiple diversity branches to maximize the instantaneous signal-to-noise ratio (SNR) at the receiver output. In this method, the signal from each branch is weighted by the complex conjugate of its channel gain, ensuring that branches with stronger signals contribute more to the combined output while those with weaker signals or higher noise are down-weighted accordingly. This weighting aligns the phases of the signals for constructive addition and scales their amplitudes proportional to their individual SNRs, resulting in the highest possible output SNR among linear coherent combiners.^[31] The derivation of MRC stems from matched filtering principles, where each branch's receiver acts as a matched filter to its respective channel impulse response, maximizing the SNR for that isolated branch before combination. For L independent diversity branches, the combined output signal is given by

y = \sum_{i=1}^{L} h_i^* r_i,

where r_i = h_i s + n_i is the received signal on the i-th branch, h_i is the complex channel gain, s is the transmitted symbol, n_i is additive white Gaussian noise with variance \sigma^2 (assumed equal across branches), and h_i^* is the complex conjugate of h_i. The resulting output SNR, after normalization, is the sum of the individual branch SNRs:

\gamma_{\text{MRC}} = \sum_{i=1}^{L} \gamma_i,

where \gamma_i = |h_i|^2 E_s / \sigma^2 is the SNR of the i-th branch and E_s is the symbol energy. This additive SNR property holds under the assumptions of independent fading branches and known channel gains, directly following from the orthogonality of the signal and noise components in the weighted sum.^[29]^[31] Implementing MRC requires accurate channel state information (CSI) at the receiver to compute the weights h_i^*, typically obtained through pilot symbols or training sequences. Additionally, knowledge of the noise variance \sigma^2 is necessary for precise scaling, although in many practical systems with equal noise power across branches, this simplifies to using only the channel conjugates. Without perfect CSI, performance degrades due to estimation errors, but MRC remains robust when channel estimates are reasonably accurate over the coherence time of the fading channel.^[32] MRC delivers the maximum achievable diversity gain of order L for L branches, serving as the theoretical benchmark against which other combining techniques are evaluated, as it fully exploits the available diversity resources without sacrificing optimality in terms of SNR. This superior performance makes it particularly valuable in high-SNR regimes or systems limited by fading rather than noise, though it incurs higher complexity due to the need for per-branch weighting and summation.^[31]

Equal Gain Combining

Equal gain combining (EGC) is a coherent diversity technique that aligns the phases of the signals received from multiple branches using channel state information (CSI) and sums them with equal amplitude weights of unity. This approach simplifies the combining process by avoiding the need for amplitude-based weighting, while still achieving coherent addition to maximize the signal component. The method was analyzed in early work on linear diversity systems, demonstrating its effectiveness in practical receivers.^[33] The instantaneous signal-to-noise ratio (SNR) at the output of an L-branch EGC combiner is given by

\gamma_{\text{EGC}} = \frac{ \left( \sum_{i=1}^{L} \sqrt{\gamma_i} \right)^2 }{L},

where \gamma_i denotes the instantaneous SNR of the i-th branch. This expression arises from co-phasing the branch signals, resulting in a signal amplitude proportional to the sum of the individual branch amplitudes, divided by the increased noise variance due to equal weighting. When branch SNRs are similar, EGC approximates the performance of maximal ratio combining (MRC), though it generally incurs a small penalty.^[34] EGC provides a performance loss of approximately 1 dB compared to MRC in Rayleigh fading channels, depending on the number of branches and fading severity, while maintaining the same diversity order. This near-optimal behavior is particularly evident for large L, where the average output SNR approaches (\pi/4) L \bar{\gamma}, about 1 dB below the MRC value of L \bar{\gamma}. The primary advantage of EGC lies in its reduced complexity, as it requires only phase estimation from CSI rather than full amplitude and phase knowledge, making it easier to implement in hardware-constrained systems.^[35]^[33] EGC is well-suited for applications where branch SNRs are comparable, such as in balanced antenna arrays with low correlation, where the simplicity outweighs the minor performance gap to MRC. In such scenarios, it delivers substantial diversity gains without the overhead of precise amplitude tracking.^[36]

Performance Analysis

Diversity Gain and Order

Diversity gain, in this context referring to the array gain, quantifies the increase in effective average signal-to-noise ratio (SNR) from combining, typically expressed in decibels (dB). In ideal scenarios with L independent branches and maximal ratio combining (MRC), the array gain is exactly $10 \log_{10} L dB, reflecting the enhanced mean SNR from coherently averaging over multiple faded signals. However, the SNR improvement required to maintain a specified error rate in fading channels is larger than this array gain and depends on the target error rate; for example, at high SNR and BER = $10^{-5} in Rayleigh fading, it can be 10-20 dB for L=4 versus L=1.^[29] The diversity order d measures the steepness of the error probability P_e decay with increasing SNR at high values, formally defined as

d = -\lim_{\rho \to \infty} \frac{\log P_e(\rho)}{\log \rho},

where \rho denotes the SNR. For systems operating over Rayleigh fading channels with L independent branches, the diversity order achieves d = L, meaning P_e scales as \rho^{-L}, providing an exponential reliability boost. This full order is realized when branches experience uncorrelated fading, ensuring each contributes uniquely to combating outage events.^[37]^[1] Several factors influence the achievable diversity order and gain. Branch correlation, arising from insufficient antenna spacing or environmental scattering limitations, degrades the effective order below L, as correlated signals fail to provide independent fading realizations and thus diminish the slope of the P_e versus SNR curve. For instance, correlation coefficients exceeding 0.5 can significantly erode performance in Rayleigh fading. Moreover, the choice of combining technique affects realization of full order and gain: MRC optimally weights branches by their SNR to attain the maximum d = L with highest array gain, whereas simpler methods like selection combining achieve the full order d = L but provide lower array gain due to ignoring weaker branches.^[38]^[39] Simulation studies illustrate these concepts through performance plots, such as SNR requirements versus L for a target P_e = 10^{-5} in Rayleigh fading. For MRC with independent branches, the SNR gain grows beyond $10 \log_{10} L dB due to the steeper asymptotic slope, outperforming equal gain combining (which approaches but does not fully match MRC at high L) and selection combining (which provides less gain due to ignoring weaker branches). These curves underscore that increasing L from 1 to 4 can reduce required SNR by 10-20 dB, depending on the scheme, emphasizing MRC's superiority in harnessing full diversity potential.^[29]

Error Rate Metrics

Error rate metrics evaluate the reliability of diversity schemes by quantifying the probability of bit errors or system outages in fading environments. For uncoded systems employing binary phase-shift keying (BPSK) modulation over independent Rayleigh fading channels with maximal ratio combining (MRC) using L branches, the average bit error rate (BER) is given by

P_b = \left( \frac{1 - \mu}{2} \right)^L \sum_{k=0}^{L-1} \binom{L-1 + k}{k} \left( \frac{1 + \mu}{2} \right)^k,

where \mu = \sqrt{\frac{\bar{\gamma}}{1 + \bar{\gamma}}} and \bar{\gamma} is the average signal-to-noise ratio (SNR) per branch, assuming equal average SNRs across branches.^[40] This closed-form expression derives from averaging the conditional BER over the gamma-distributed total SNR resulting from MRC. At high SNR, this simplifies to an asymptotic form P_b \approx \frac{(L-1)!}{4^L \bar{\gamma}^L}, highlighting the diversity order of L that scales the error rate inversely with the L-th power of SNR.^[40] Outage probability, defined as the likelihood that the effective SNR falls below a required threshold \gamma_{\text{th}}, provides another key metric for assessing diversity performance. For MRC with L independent branches, the outage probability is

P_{\text{out}} = 1 - e^{-\gamma_{\text{th}} / \bar{\gamma}} \sum_{k=0}^{L-1} \frac{(\gamma_{\text{th}} / \bar{\gamma})^k}{k!},

which follows from the cumulative distribution function of the Erlang-distributed total SNR.^[40] In contrast, for selection combining (SC), where the strongest branch is selected, the outage probability is P_{\text{out}} = \left[1 - e^{-\gamma_{\text{th}} / \bar{\gamma}}\right]^L, reflecting the distribution of the maximum branch SNR.^[40] Comparisons across combining techniques reveal distinct error performance trade-offs. MRC achieves the lowest BER among linear combiners, outperforming SC by approximately 3 dB at a target outage of 1% for L=4 branches in Rayleigh fading, due to its optimal weighting of all branches.^[29] Equal gain combining (EGC) falls between MRC and SC in BER performance, offering simplicity at the cost of slightly higher error rates than MRC.^[40] Modulation choice also influences metrics; for quadrature phase-shift keying (QPSK), the average BER under MRC is roughly twice that of BPSK for the same symbol SNR (E_s/N_0), as QPSK effectively transmits two BPSK symbols per symbol, doubling the error vulnerability per transmitted energy unit.^[40] In coded diversity systems, forward error correction (FEC) such as convolutional or turbo codes integrates with combining techniques to further suppress BER. By exploiting redundancy across diversity branches, FEC provides additional coding gain, reducing the required SNR for a target BER by several dB compared to uncoded schemes, particularly in correlated fading scenarios.^[40]

Applications

In Mobile Communication Systems

Diversity schemes have been integral to mobile communication systems since the early generations, addressing the challenges of multipath fading and signal attenuation inherent in cellular environments. In second-generation (2G) Global System for Mobile Communications (GSM) networks, frequency hopping was introduced as a key technique to provide time and frequency diversity. By rapidly switching transmission frequencies across a set of channels, frequency hopping randomizes the impact of frequency-selective fading and interference, effectively averaging out channel impairments over time and improving overall link quality. This approach, often combined with slow frequency hopping, enhances interference diversity, leading to more uniform performance across users and reduced outage probability in urban and suburban deployments.^[41]^[42] The third-generation (3G) Universal Mobile Telecommunications System (UMTS), based on wideband code-division multiple access (WCDMA), advanced diversity implementation through rake receivers, which exploit multipath propagation for path diversity—a form of spatial and time diversity. Rake receivers correlate delayed versions of the signal arriving via different paths, combining them to capture energy from multiple resolvable multipaths and mitigate fading effects. This technique significantly improves bit error rate (BER) performance in frequency-selective channels typical of mobile scenarios, enabling higher reliability for voice and early data services. While primarily focused on path diversity, rake receivers in UMTS can integrate with multi-antenna setups to further leverage spatial diversity at base stations.^[43] In fourth-generation (4G) Long-Term Evolution (LTE) systems, diversity schemes evolved to incorporate spatial and frequency diversity within orthogonal frequency-division multiplexing (OFDM) frameworks, supporting higher data rates and robustness in dynamic channels. Transmit diversity modes, such as space-frequency block coding (SFBC), encode signals across multiple antennas and subcarriers to combat fading, particularly in non-line-of-sight conditions. For the uplink, antenna diversity in mobile handsets—often using two or more antennas—enhances signal reception at base stations by providing receive diversity gains, improving coverage and throughput for user equipment with limited transmit power. Base stations frequently apply combining techniques, like maximal ratio combining, to aggregate these diverse signals effectively.^[44]^[45]^[46] These diversity implementations yield substantial system benefits in mobile networks, particularly in challenging propagation environments. Diversity techniques extend coverage in fringe areas by boosting effective signal-to-noise ratio through constructive combining, allowing reliable connectivity in rural or building-edge scenarios where single-antenna links would fail. In fading channels, they enable higher data rates by reducing outage events and supporting adaptive modulation, with studies showing up to 3-5 dB gains in signal-to-interference-plus-noise ratio. Case studies in urban mobility highlight BER reductions of 20-50% with antenna diversity, as multipath-rich environments benefit from spatial and frequency averaging, minimizing errors during handoffs and high-speed travel.^[47]^[48]^[49] Despite these advantages, diversity schemes in mobile systems face notable challenges, especially as networks scale. In mobile devices, multiple antennas for diversity increase power consumption due to additional radio frequency chains and processing demands, potentially shortening battery life in handsets already constrained by size and heat dissipation. This is particularly acute in uplink scenarios, where transmit diversity requires coordinated antenna switching or beamforming, consuming 20-40% more energy compared to single-antenna operation. In dense deployments, such as urban small-cell networks, diversity can exacerbate inter-cell interference if not carefully managed, as correlated signals from nearby users amplify co-channel interference and degrade overall spectrum efficiency.^[50]^[51]^[52]

Integration with MIMO

Multiple-input multiple-output (MIMO) systems leverage multiple antennas at both transmitter and receiver to combat fading and increase data rates, with diversity schemes playing a key role in enhancing reliability through spatial redundancy. In MIMO, diversity is often realized via space-time block coding (STBC), which transmits coded symbols across multiple antennas and time slots to create virtual replicas of the signal, thereby achieving higher diversity orders without sacrificing bandwidth. A seminal example is the Alamouti scheme for a 2x1 MISO configuration, where two symbols s_1 and s_2 are encoded into a transmission matrix over two time slots and two transmit antennas:

\begin{bmatrix} s_1 & -s_2^* \\ s_2 & s_1^* \end{bmatrix}

This orthogonal design enables simple linear decoding at the receiver, providing full diversity order of 2 equivalent to two receive antennas, while maintaining a rate of 1 symbol per channel use.^[4] The integration of diversity with MIMO also involves navigating the fundamental diversity-multiplexing tradeoff, which characterizes the inherent tension between achieving high diversity gain (for reliability) and high multiplexing gain (for capacity) in fading channels. This tradeoff, formalized by Zheng and Tse, shows that the maximum diversity order d(r) decreases linearly with the multiplexing rate r, bounded by d(r) \leq (M - r)(N - r) for M transmit and N receive antennas, guiding the design of practical codes.^[37] The Bell Labs Layered Space-Time (BLAST) architecture exemplifies this balance, employing layered spatial multiplexing where independent data streams are transmitted over different antennas, combined with successive interference cancellation at the receiver to approach capacity while retaining configurable diversity through coding across layers.^[53] Advancements in 5G New Radio (NR) have integrated diversity schemes into massive MIMO systems, which deploy large antenna arrays (e.g., 64 or more elements) at base stations to exploit inherent spatial diversity from channel orthogonality and asymptotic properties. These systems achieve macro-diversity gains by serving multiple users simultaneously, mitigating inter-user interference through precoding. Hybrid schemes further combine massive MIMO diversity with beamforming, where narrow beams focus energy toward users, enhancing signal-to-noise ratios and diversity in multipath environments while reducing overhead compared to fully digital architectures.^[54] In high-mobility 5G scenarios, such as vehicular communications, diversity integration ensures robustness against rapid channel variations, with massive MIMO and low-complexity STBCs like Alamouti extensions achieving near-full diversity orders (e.g., up to 128 in large arrays) at decoding complexities scalable with antenna count. These approaches significantly reduce outage probabilities and improve error rates at velocities over 100 km/h by exploiting transmit diversity in beamforming.^[55]

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Nov 1, 2014 · A spacing of λ/2 between diversity branch antenna is considered as sufficient for minimising the effect of mutual coupling [3]. Mutual coupling ...
[15]
https://www.sciencedirect.com/science/article/pii/B9780124076822000120
[16]
Diversity Techniques in Wireless Communication Systems: A Review
Antenna diversity is an important technique to combat fading and reduce cochannel interference (CCI). In this paper, we present an analytical approach to derive ...
[17]
Spatial Diversity - an overview | ScienceDirect Topics
Spatial diversity is defined as the use of multiple antennas at the transmitter and/or receiver to exploit the random nature of signal propagation in ...
[18]
[PDF] ELEG 5693 Wireless Communications Ch. 6 Diversity
• Frequency diversity (multipath diversity). – Transmitting the same information ... spacing larger than the coherent bandwidth (frequency selective fading).
[19]
https://ieeexplore.ieee.org/document/503460
[20]
https://ieeexplore.ieee.org/document/504838
[21]
https://ieeexplore.ieee.org/document/6819850
[22]
[PDF] Harnessing Frequency Diversity in Wi-Fi Networks
The default OFDM im- plementation in this package includes a basic OFDM sender and receiver chain. It supports BPSK, QPSK and QAM modulation schemes on the OFDM ...
[23]
Diversity in Wireless Communication Systems
Nov 4, 2021 · We explain the idea of space diversity with the help of a simple scheme: Selection Combining (SC). Consider a wireless link with 1 Tx antenna ...
[24]
[PDF] Multiple Access Schemes for OFDM
Mar 21, 2006 · • CDMA: Provides frequency diversity and multi-user access scheme ... Why is the implementation of OFDM-FDMA the most straightforward?
[25]
[PDF] Mitigating the Degradation Effects of Fading Channels
Listed below are some of the ways in which diversity methods can be implemented: • Time diversity—Transmit the signal on L different time slots with time.<|control11|><|separator|>
[26]
[PDF] CHAPTER 8 SPATIAL, POLARIZATION, AND PATTERN DIVERSITY ...
Diversity antennas provide two major benefits. First, reliability is improved in multipath channels where interference from reflected signals causes fading of ...
[27]
A Comparison of Space Diversity and Polarization Diversity Receive ...
Jun 3, 1998 · A discussion of advantages and disadvantages of using polarization diversity antennas in cellular and PCS systems.Missing: wireless challenges
[28]
[PDF] WIRELESS COMMUNICATIONS
Page 1. WIRELESS COMMUNICATIONS. Andrea Goldsmith. Stanford University. Page 2 ... Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . 112. 4.2 ...
[29]
[PDF] Receive Diversity 1 1 Introduction
Within diversity combining (or diversity reception) are three common techniques: Selection. Combining, Maximal Ratio Combining (MRC) and Equal Gain Combining ( ...
[30]
[PDF] Diversity
Selection Combining: largest fading path chosen. Maximal Ratio Combining: all paths cophased and summed with optimal weighting to maximize SNR at combiner ...<|control11|><|separator|>
[31]
https://ieeexplore.ieee.org/document/4065786
[32]
https://www.researchgate.net/publication/3416655_Optimal_maximal_ratio_combining_with_correlated_diversity_branches
[33]
Linear Diversity Combining Techniques | IEEE Journals & Magazine
This paper provides analyses of three types of diversity combining systems in practical use. These are: selection diversity, maximal-ratio diversity, ...
[34]
https://ieeexplore.ieee.org/document/873065
[35]
[PDF] MRC and EGC Diversity. Transmit Diversity
Optimal weights are proportional to branch SNR. • Resulting combiner SNR γΣ is sum of branch SNRs. • Distribution obtained by characteristic function analysis ( ...
[36]
[PDF] The Impact of User Effects on the Performance of Dual Receive ...
With balanced branches, EGC is a sub-optimal but at- tractive solution because it does not require estimation of the fading amplitudes and thus is easier to ...
[37]
[PDF] a fundamental tradeoff in multiple-antenna channels
The definition of diversity gain here differs from the standard definition in the space–time coding literature (see, for example ,[7]) in two important ways. • ...
[38]
https://ieeexplore.ieee.org/document/490230
[39]
[PDF] Necessary and Sufficient Conditions for Full Diversity Order in ...
The proofs in this paper work for any modulation scheme with provable full diversity order in. MRT/MRC and SD systems that transmit over memoryless and ...Missing: seminal | Show results with:seminal
[40]
[PDF] Digital Communication over Fading Channels
Simon dedicates this book to his wife, Anita, whose devotion to him and this project never once faded during its preparation. Mohamed-Slim Alouini dedicates ...
[41]
Modeling interference diversity in GSM networks | IEEE Conference ...
The benefits of slow frequency hopping in GSM are frequency and interference diversity and result in better quality. Frequency diversity can easily be ...
[42]
https://ieeexplore.ieee.org/document/605883/
[43]
https://ieeexplore.ieee.org/document/5712287/
[44]
Performance analysis of transmission modes in 4G LTE/LTE-A ...
The performances of the different transmission modes like single antenna port, transmit diversity, open loop spatial multiplexing are analyzed under normal ...Missing: mobile | Show results with:mobile
[45]
[PDF] LTE SFBC MIMO Transmitter Modelling and Performance Evaluation
Space Frequency Block Code Scheme in LTE. Frequency Switched Transmit Diversity. When four transmit antennas have been used, the diversity scheme is called ...
[46]
Antenna configuration comparison in challenging NLOS locations
The objective of this paper is to compare uplink (UL) performance with different antenna technologies when receiving Non-Line of Sight (NLOS) signal in ...
[47]
Evaluation of the distributed MISO gain for mobile broadcast ...
This paper analyzes the impact produced by the use of distributed MISO diversity techniques on the network planning for mobile broadcasting services, ...
[48]
Performance evaluation of Adaptive MIMO Switching in Long Term ...
Adaptive MIMO switching works on the principle of switching among transmit diversity, receive diversity, and spatial multiplexing in accordance to SINR level.
[49]
Performance results with antenna diversity in urban environment for ...
Antenna diversity is a technique widely used in mobile radio systems to improve performance and reduce fading effects. In this paper, the up-link of a ...
[50]
[PDF] A Novel Resource-Efficient Diversity Technique for 6G and Beyond
May 6, 2022 · Besides, when using multiple antennas, transceivers should have multiple RF chains, which can be costly and consume a lot of power. For newer ...Missing: phones | Show results with:phones
[51]
[PDF] Beamsteering on Mobile Devices: Network Capacity and Client ...
Jun 23, 2010 · The results show that averagely BeamAdapt can reduce client power consumption by 40% and 55% with two and four antennas, respectively, while ...Missing: phones | Show results with:phones
[52]
[PDF] and Frequency- Spreading for Interference Minimization in Dense IoT
Dec 4, 2022 · Abstract—In this article, we propose a space spreading-assisted framework that leverages either time or frequency diversity or both.
[53]
[PDF] Layered Space-Time Architecture for Wireless Communication in a ...
This paper describes a new point-to-point com- munication architecture employing an equal number of antenna array elements at both the transmitter and receiver.Missing: BLAST | Show results with:BLAST
[54]
Massive MIMO, preferred option for large scale 5G - Ericsson
Massive MIMO enables state-of-the-art beamforming and MIMO techniques that are powerful tools for improving end-user experience, capacity, and coverage.
[55]
(PDF) Massive MIMO Beamforming With Transmit Diversity for High ...
In this paper, by exploiting the special characters in the high mobility scenario, we introduce an applicable low-complexity beamforming scheme with transmit ...