Achievable Bit Research Articles

Rate-Adaptive Modulation and Low-Density Parity-Check Coding for Optical Fiber Transmission Systems

We propose a rate-adaptive optical transmission scheme using variable-size constellations at a fixed symbol rate and variable-rate forward error correction (FEC) codes with soft-decision decoding (SDD), quantifying how achievable bit rates vary with transmission distance. The scheme uses outer Reed–Solomon codes and inner extended irregular repeat-accumulate low-density parity-check (LDPC) codes to vary the code rate, combined with single-carrier polarization-multiplexed M-ary quadrature amplitude modulation with variable M and digital coherent detection. LDPC codes are decoded iteratively using belief propagation. Employing M=4,8,16, the scheme achieves a maximum bit rate of 200 Gbit/s in a nominal 50-GHz channel bandwidth. A rate adaptation algorithm uses the signal-to-noise ratio (SNR) or the FEC decoder input bit-error ratio (BER) estimated by a receiver to determine the FEC code rate and constellation size that maximize the information bit rate while yielding a target FEC decoder output BER and a specified SNR margin. We simulate single-channel transmission through long-haul fiber systems with or without inline chromatic dispersion compensation, incorporating numerous optical switches, evaluating the impact of fiber nonlinearity and bandwidth narrowing. With zero SNR margin, we achieve bit rates of 200/100/50/20 Gbit/s over distances of 960/2800/4400/9680 km and 1920/4960/8160/19,360 km in dispersion-compensated and -uncompensated systems, respectively, corresponding to an increase of about 50% in reach compared to a reference system that uses a hard-decision FEC scheme. Compared to an ideal coding scheme, the proposed scheme exhibits a performance gap ranging from about 4.0 dB at 960 km to 2.7 dB at 9680 km in compensated systems, and from about 3.9 dB at 1920 km to 2.9 dB at 19,360 km in uncompensated systems. Observed performance gaps are about 2.5 dB smaller than for the reference hard-decision FEC scheme, close to the improvement expected when using SDD.

A Framework to Optimize the OFDM-Based Multiuser Dynamic Spectrum Access Networks with Frequency Reuse

This paper presents a novel framework to optimally solve the spectrum access problem at the central spectrum moderator (CSM) for multiple radio pairs. In the system model considered, the available spectrum is partitioned into N subcarriers, and the proposed CSM assigns subcarriers to multiple radio pairs with the objective to maximize the total achievable bit rate in the network. Each transceiver adopts the OFDM frontend, and transmits only on those subcarriers assigned to it. The proposed algorithm jointly considers the effect of interference from multiple co-channel radios, while at the same time supports all transmitters at their minimum guaranteed bit rate, under individual total transmit power constraint. Our proposed method converts the original NP-hard mixed integer non-linear programming problem into a mixed binary linear programming problem, which can be solved optimally using standard algorithms.

Performance Bounds for Maximum Likelihood Detection of Single Carrier FDMA

Single Carrier FDMA (SC-FDMA) plays an important role in modern wireless communications as an alternative to OFDM mainly since it exhibits low peak-to-average power ratio. Accordingly, SC-FDMA is employed as the uplink scheme for the 3GPP long term evolution (LTE). In the presence of multipath, the SC-FDMA signal may arrive at the receiver perturbed by inter-symbol-interference, which renders the single-tap frequency-domain equalization suboptimal. Optimum maximum likelihood detection for SC-FDMA is often prohibitively complex, and hence useful mostly as a performance bound for suboptimal detection schemes. In this work, the optimum-detector performance is analyzed in detail; the analysis can be easily applied to any SC frequency-domain equalization scheme. Closed form bounds on the achievable bit error rate are provided for low and high SNR regimes in correlated and uncorrelated Rayleigh fading channels. The bounds reveal that the diversity order at high SNR is significantly smaller than in the low SNR regime. Moreover, error rate flaring behavior is demonstrated under optimum detection of SC-FDMA. The analytical results are verified by simulations.

Performance Bounds for Maximum Likelihood Detection of Single Carrier FDMA

Single Carrier FDMA (SC-FDMA) plays an important role in modern wireless communications as an alternative to OFDM mainly since it exhibits low peak-to-average power ratio. Accordingly, SC-FDMA is employed as the uplink scheme for the 3GPP long term evolution (LTE). In the presence of multipath, the SC-FDMA signal may arrive at the receiver perturbed by inter-symbol-interference, which renders the single-tap frequency-domain equalization suboptimal. Optimum maximum likelihood detection for SC-FDMA is often prohibitively complex, and hence useful mostly as a performance bound for suboptimal detection schemes. In this work, the optimum-detector performance is analyzed in detail; the analysis can be easily applied to any SC frequency-domain equalization scheme. Closed form bounds on the achievable bit error rate are provided for low and high SNR regimes in correlated and uncorrelated Rayleigh fading channels. The bounds reveal that the diversity order at high SNR is significantly smaller than in the low SNR regime. Moreover, error rate flaring behavior is demonstrated under optimum detection of SC-FDMA. The analytical results are verified by simulations.

Rate-Adaptive Modulation and Coding for Optical Fiber Transmission Systems

We propose a rate-adaptive optical transmission scheme using variable-rate forward error correction (FEC) codes and variable-size constellations at a fixed symbol rate, quantifying how achievable bit rates vary with distance. The scheme uses serially concatenated Reed-Solomon codes and an inner repetition code to vary the code rate, combined with single-carrier polarization-multiplexed -ary quadrature amplitude modulation with variable and digital coherent detection. Employing , the scheme achieves a maximum bit rate of 200 Gb/s in a nominal 50-GHz channel bandwidth. A rate adaptation algorithm uses the signal-to-noise ratio (SNR) or the FEC decoder input bit-error ratio (BER) estimated by a receiver to determine the FEC code rate and constellation size that maximizes the information bit rate while yielding a target FEC decoder output BER and a specified SNR margin. We simulate single-channel transmission through long-haul fiber systems with or without inline chromatic dispersion compensation, incorporating numerous optical switches, evaluating the impact of fiber nonlinearity and bandwidth narrowing. With zero SNR margin, we achieve bit rates of 200/100/50 Gb/s over distances of 640/2080/3040 km and 1120/3760/5440 km in dispersion-compensated and dispersion-uncompensated systems, respectively. Compared to an ideal coding scheme, the proposed scheme exhibits a performance gap ranging from about 6.4 dB at 640 km to 7.6 dB at 5040 km in compensated systems, and from about 6.6 dB at 1120 km to 7.5 dB at 7600 km in uncompensated systems. We present limited simulations of three-channel transmission, showing that interchannel nonlinearities decrease achievable distances by about 10% and 7% for dispersion-compensated and dispersion-uncompensated systems, respectively.

Increased Bit Rate Direct Modulation AMO-OFDM Transmission by Optical Injection Using Monolithically Integrated Lasers

Experimental and simulation work, presented in this letter, demonstrates for the first time how the monolithic integration of two single-mode lasers in a master-slave configuration, can substantially increase the achievable bit rate of a direct modulation adaptively modulated optical orthogonal frequency-division multiplexing (AMO-OFDM) system. The Levin-Campello algorithm is applied to select the OFDM bit and power loading scheme used for each system configuration. Improvement in terms of data throughput due to injection is measured for several transmission distances with the improvement in performance presented in terms of error vector magnitude per OFDM subcarrier.

Communication-theoretic analysis of capture-based networks

Under the power-based capture model, a transmission is successfully received at the destination, even in the presence of other transmissions and background noise, if the received signal-to-interference-plus-noise ratio exceeds a capture threshold. We evaluate the spectral efficiency of simple multi-user channels by combining the basic capture model with a communication-theoretic model. The result is a more refined capture model that incorporates key system design parameters (such as achievable bit rate, target bit error rate, channel bandwidth, and modulation signal constellations) that are absent from the basic capture model. The relationships among these parameters can serve as a tool for optimizing the network performance.

Multi-user detection in ultra-wideband multiple-access communications systems using an efficient heuristic algorithm

This paper investigates learning and achievable bit error rate (BER) performance of ultra-wideband (UWB) systems that use intelligent multiuser detector (MUD) when communicating over UWB channels that experience both multiuser interference (MUI) and intersymbol interference (ISI), in addition to multipath fading. Multiple access interference (MAI) degrades performance of conventional single user detector in UWB systems. Due to high complexity of the optimum multiuser detector, suboptimal multiuser detectors with less complexity and reasonable performance have drawn considerable attention. By taking advantage of heuristic values and collective intelligence of tabu search with Hopfield neural networks (TAHNN), the proposed detector offers almost the same BER performance as a full-search-based optimum multiuser detector does, while greatly reducing computational complexity. To evaluate performance and robustness of our proposed TAHNN based MUD, we experiment with a number of test problems. Computational results show that our proposed TAHNN in almost all cases outperforms other foregoing heuristics applied to this paper. Key words: Ultra-wideband, Tabu search method, Hopfield neural networks, multiuser detection.

Multi-user detection in ultra-wideband multiple-access communications systems using an efficient heuristic algorithm

This paper investigates learning and achievable bit error rate (BER) performance of ultra-wideband (UWB) systems that use intelligent multiuser detector (MUD) when communicating over UWB channels that experience both multiuser interference (MUI) and intersymbol interference (ISI), in addition to multipath fading. Multiple access interference (MAI) degrades performance of conventional single user detector in UWB systems. Due to high complexity of the optimum multiuser detector, suboptimal multiuser detectors with less complexity and reasonable performance have drawn considerable attention. By taking advantage of heuristic values and collective intelligence of tabu search with Hopfield neural networks (TAHNN), the proposed detector offers almost the same BER performance as a full-search-based optimum multiuser detector does, while greatly reducing computational complexity. To evaluate performance and robustness of our proposed TAHNN based MUD, we experiment with a number of test problems. Computational results show that our proposed TAHNN in almost all cases outperforms other foregoing heuristics applied to this paper. Key words: Ultra-wideband, Tabu search method, Hopfield neural networks, multiuser detection.

Approximate MIMO Iterative Processing With Adjustable Complexity Requirements

Targeting only the best achievable bit error rate (BER) performance in iterative receivers operating over multiple-input-multiple-output (MIMO) channels may result in a significant waste of resources, particularly when the achievable BER is considerably better than the target performance [e.g., under good channel conditions and at high signal-to-noise ratio (SNR)]. In contrast to the typical iterative schemes, a practical iterative decoding framework that approximates the soft information exchange is proposed, allowing reduced complexity sphere and channel decoding that is adjustable to the transmission conditions and the required BER. With the proposed approximate soft information exchange, the performance of the exact soft information can still be reached with significant complexity reduction.

Non-Peaky Signals in Wideband Fading Channels: Achievable Bit Rates and Optimal Bandwidth

In the context of fading channels it is well established that, with a constrained transmit power, the bit rates achievable by signals that are not peaky vanish as the bandwidth grows without bound. Stepping back from the limit, we characterize the highest bit rate achievable by such non-peaky signals and the approximate bandwidth where that apex occurs. As it turns out, the gap between the highest rate achievable without peakedness and the infinite-bandwidth capacity (with unconstrained peakedness) is small for virtually all settings of interest to wireless communications. Thus, although strictly achieving capacity in wideband fading channels does require signal peakedness, bit rates not far from capacity can be achieved with conventional signaling formats that do not exhibit the serious practical drawbacks associated with peakedness. In addition, we show that the asymptotic decay of bit rate in the absence of peakedness usually takes hold at bandwidths so large that wideband fading models are called into question. Rather, ultrawideband models ought to be used.

Joint Adaptive Modulation and Distributed Switch-and-Stay for Partial Relay Selection Networks

In this letter, we propose a distributed switch-and-stay combining network with partial relay selection and show that the system spectral efficiency can be improved via adaptive modulation. Analytical expressions for the achievable spectral efficiency and average bit error rate of the proposed system over Rayleigh fading channels are derived for an arbitrary switching threshold. Numerical results are gathered to substantiate the analytical derivation showing that in terms of spectral efficiency, the system with single relay outperforms that with more than one relay at high signal-to-noise ratios (SNRs) and the optimal switching threshold can significantly improve the system performance at medium SNRs.

Downlink performance of distributed antenna systems in multicell environment

In this study, the authors investigate the downlink performance of distributed antenna systems (DAS) in multicell environment with blanket transmission. In most existing works, the interference plus noise is treated as Gaussian random variable with fixed variance by the central limit theorem. However, doing this will ignore the effect of the short-term fading on interference that is called as restrictive condition. To avoid the restrictive condition, the authors consider the variance of interference plus noise as a random variable with changeable variance that is influenced by the short-term fading when propagation pathloss and transmit power are given. From the perspective of information theoretic, the closed-form and approximate analytical expressions of downlink achievable throughput, outage probability and average bit error rate in the cellular system are derived for no-shadowing and shadowing scenarios, respectively. Extensive simulation results validate the theoretical analysis and demonstrate that the system performances can be significantly improved for cell-edge users. Moreover, the proposed analytical method can obtain more accurate system performances.

Variable Tap-Length Mixed-Tone RLS-based Per-Tone Equalisation with Adaptive Implementation

In this paper, a methodology of mixed-tone recursive least squares algorithm development based on an orthogonal projection approach and a new variable tap-length mechanism is presented for per-tone equalisation (PTEQ) in discrete multitone systems. A mixed-tone cost function described as the sum of weight estimated errors is minimised to achieve the solutions for different per-tone equalisers simultaneously. We describe about the inverse square-root recursive least squares algorithm based upon the QRdecomposition which preserves the Hermitian symmetry of the inverse autocorrelation matrix by means of the product of square-root matrix and its Hermitian transpose. Such symmetrical property lends the benefit to the parallel implementation. In order to reduce the computational complexity, a new variable tap-length algorithm based on the sense of mean square mixted-tone errors is introduced to search a proper choice of tap-length of PTEQ. Simulation results show the improvement of achievable bit rate and signal to noise ratio performance as compared to the PTEQ exploiting conventional recursive least squares algorithm.

Experimental Demonstrations and Extensive Comparisons of End-to-End Real-Time Optical OFDM Transceivers With Adaptive Bit and/or Power Loading

Experimental demonstrations are reported for end-to-end real-time optical orthogonal frequency division multiplexing (OOFDM) transceivers incorporating three widely adopted adaptive loading techniques, namely, power loading (PL), bit loading (BL), and bit-and-power loading (BPL). In directly modulated distributed-feedback (DFB) laser-based, intensity-modulation, and direct-detection (IMDD) transmission systems consisting of up to 35-km single-mode fibers (SMFs), extensive experimental comparisons between these adaptive loading techniques are made in terms of maximum achievable signal bit rate, optical power budget, and digital signal processing (DSP) resource usage. It is shown that BPL is capable of supporting end-to-end real-time OOFDM transmission of 11.75 Gb/s over 25-km SMFs in the aforementioned systems at sampling speeds as low as 4 GS/s. In addition, experimental measurements also show that BPL (PL) offers the highest (lowest) signal bit rate, and their optical power budgets are similar. The observed signal bit rate difference between BPL and PL is almost independent of sampling speed and transmission distance. All the aforementioned key features agree very well with numerical simulations. On the other hand, BPL-consumed DSP resources are approximately three times higher than those required by PL. The results indicate that PL is a preferred choice for cost-effective OOFDM transceiver design.

Achievable bit rates of cognitive user with Vandermonde precoder in cognitive radio network

In cognitive radio network (CRN) where primary user (PU) and secondary user (SU) wish to communicate with their corresponding receivers simultaneously over frequency selective fading channels, the interference between PU and SU should be eliminated in order to realize spectrum sharing. Precoder division multiple access (PDMA) that implements Vandermonde precoder in cognitive orthogonal frequency division multiplexing (OFDM) is proposed, which benefits from the frequency selectivity of the cognitive channel to form a frequency beam-former. Vandermonde precoder enables SUs to achieve higher bit rate with optimal power allocation strategy under the condition of PU interference constraint. Numerical and simulation results are presented to prove theoretical derivations. It is shown that, achievable bit rates (ABR) of SU could be achieved with optimal power allocation scheme in CRN. Meanwhile, it is also verified that, with Vandermonde precoder to form PDMA, SU’s signal-to-noise ratio (SNR) and the target rate of PU could impact the ABR of SU significantly in CRN.

Real-time experimental demonstration of low-cost VCSEL intensity-modulated 1125Gb/s optical OFDM signal transmission over 25km PON systems

The feasibility of utilising low-cost, un-cooled vertical cavity surface-emitting lasers (VCSELs) as intensity modulators in real-time optical OFDM (OOFDM) transceivers is experimentally explored, for the first time, in terms of achievable signal bit rates, physical mechanisms limiting the transceiver performance and performance robustness. End-to-end real-time transmission of 11.25 Gb/s 64-QAM-encoded OOFDM signals over simple intensity modulation and direct detection, 25 km SSMF PON systems is experimentally demonstrated with a power penalty of 0.5 dB. The low extinction ratio of the VCSEL intensity-modulated OOFDM signal is identified to be the dominant factor determining the maximum obtainable transmission performance. Experimental investigations indicate that, in addition to the enhanced transceiver performance, adaptive power loading can also significantly improve the system performance robustness to variations in VCSEL operating conditions. As a direct result, the aforementioned capacity versus reach performance is still retained over a wide VCSEL bias (driving) current (voltage) range of 4.5 mA to 9 mA (275 mVpp to 320 mVpp). This work is of great value as it demonstrates the possibility of future mass production of cost-effective OOFDM transceivers for PON applications.

Performance analysis of cooperative multi‐carrier relay‐based UAV networks over generalized fading channels

AbstractThe outage probability in a network of cooperative unmanned airborne vehicles (UAVs) over generalized fading channels is studied analytically using finite mixture with expectation maximization technique. A relay‐based topology with one ground control unit (GCU) is considered, where the cooperative UAVs can communicate with the GCU directly or through relay. The application the UAV is assigned for specifies the minimum required transmission rate the UAV should achieve. The outage probability of the system is defined as the probability that either the transmission rate over any of the links drops below a predefined minimum threshold for that link or the Relay‐GCU link is not able to transmit the aggregate data from all relayed UAVs and the minimum rate required by the relay UAV itself. Throughout the paper, expressions for the outage probability and the average achievable bit rate of a cooperative multi‐carrier system are derived over generalized fading channels. Finite mixture with expectation maximization algorithm is utilized to derive a simple approximate expression for the probability density function (pdf) of the achievable bit rate assuming adaptive M‐ary quadrature amplitude modulation (M‐QAM). This pdf is used to derive closed‐form expressions for the outage probability and the average bit rate. Copyright © 2011 John Wiley & Sons, Ltd.

Rate-Adaptive Coding for Optical Fiber Transmission Systems

We propose a rate-adaptive transmission scheme using variable-rate forward error correction (FEC) codes with a fixed signal constellation and a fixed symbol rate, quantifying how achievable bit rates vary with distance in a long-haul fiber system. The FEC scheme uses serially concatenated Reed-Solomon (RS) codes with hard-decision decoding, using shortening and puncturing to vary the code rate. An inner repetition code with soft combining provides further rate variation. While suboptimal, repetition coding allows operation at very low signal-to-noise ratio (SNR) with minimal increase in complexity. A rate adaptation algorithm uses the SNR or the FEC decoder input bit-error ratio (BER) estimated by a receiver to determine the combination of RS-RS and repetition codes that maximizes the information bit rate while satisfying a target FEC decoder output BER and providing a specified SNR margin. This FEC scheme is combined here with single-carrier polarization-multiplexed quadrature phase-shift keying (PM-QPSK) and digital coherent detection, achieving 100-Gbit/s peak information bit rate in a nominal 50-GHz channel bandwidth. We simulate variable-rate single-channel transmission through a long-haul system incorporating numerous optical switches, evaluating the impact of fiber nonlinearity and bandwidth narrowing. With zero SNR margin, achievable information bit rates vary from 100 Gbit/s at 2000 km, to about 60 Gbit/s at 3000 km, to about 35 Gbit/s at 4000 km. Compared to an ideal coding scheme achieving information-theoretic limits on an AWGN channel, the proposed coding scheme exhibits a performance gap ranging from about 5.9 dB at 2000 km to about 7.5 dB at 5000 km. Much of the increase in the gap arises from the inefficiency of the repetition coding used beyond 3280 km. Rate-adaptive transmission can extend reach when regeneration sites are not available, helping networks adapt to changing traffic demands. It is likely to become more important with the continued evolution toward optically switched mesh networks, which make signal quality more variable.

Investigation of Upstream Near-Far Problem in VDSL Systems via Complete Adaptive Iterative Water Filling Algorithm

Crosstalk is the main degrading factor in Digital Subscriber Line (DSL) systems which are the result of electromagnetic coupling between two adjacent twisted pairs in a cable. Very-high bit-rate Digital Subscriber Line (VDSL) systems which use higher frequencies for data transmission than the other DSL systems, this effect is more considerable in Bit Error Rate (BER) degradation. This paper considers a complete adaptive iterative water-filling (IWF) algorithm for Resolving Upstream Near-Far Problem in VDSL Systems. The new distributed dynamic spectrum management algorithm is proposed, which improve achievable bit rate of iterative water-filling algorithm. The paper proffers a new power back-off strategy of the spectral mask at the near-end users, in order to protect the far-end users. Simulation of the proposed algorithm indicates that the bit rate is increased considerably rather the IWF and adaptive water-filling (AIWF) algorithms by keeping their low complexity. Furthermore, by adding the number of users in network, the new algorithm achieves performance gains over the AIWF, completely adaptive.