This paper presents the first experimental demonstration of Spectrally Efficient Frequency Division Multiplexing (SEFDM) transmission in the D-band (110–170 GHz). This is implemented using a low complexity IFFT / FFT-based transceiver with LDPC coding, employing QPSK and 8PSK modulations to achieve error-free transmission at 10Gbps and 15Gbps, respectively. Compared to Orthogonal Frequency Division Multiplexing (OFDM), this system improves spectral efficiency by 50% at α = 0.67 (QPSK) and 25% at α = 0.80 (8PSK), where α denotes the subcarrier compression factor (α=1 for OFDM). This is achieved without adding receiver complexity. Performance is evaluated in a transmit power range extending over 25 dB, from −35.7 dBm to −10 dBm, ensuring operation above the noise floor of the system but below its saturation. The results confirm the viability of SEFDM for high-efficiency millimeter-wave communications under practical hardware constraints.

This paper presents the development and implementation of an IoT-enabled environmental sensing system designed for crop yield modelling and optimisation across diverse agro-ecological zones in Nigeria. The system integrates in-situ soil and atmospheric sensors, drone-based observations, and farm-level data using a multi-layered communication architecture, including cellular, Long Range Wide Area Network (LoRaWAN), and novel UAV-based mesh networks. Preliminary analysis of the collected data reveals strong correlations between cassava fresh yield and environmental factors such as soil water content, temperature, and electrical conductivity. The system demonstrates the potential of data-driven approaches to inform site-specific agronomic decisions and breeding strategies. While the system is still in the early phase of data collection, future work will involve expanded data integration, AI-based predictive modelling, and enhanced decision support for climate-smart agriculture.

The work reports, for the first time, the use of Linvill’s negative impedance converter (NIC) to generate negative resistance and inductance for the purpose of extending the bandwidth of optical receivers used in visible light communication (VLC). The work shows that the bandwidth-limiting series resistance, inherent in large-area photodiodes, can be compensated by using a simple NIC circuitry generating a compensating negative resistance in series with a negative inductive reactance. Mathematical derivations and simulations are verified by experimental results showing a bandwidth extension of up to 400% when used with only minimal additional circuit elements.

This paper explores the application of radio frequency (RF) return loss measurements to estimate starch content in cassava, from the theoretical framework to the development of a portable test instrument utilising this method. The design evolution of the portable instrument for starch content estimation, employing RF reflectometry, is described, together with the calibration of the instrument against known starch content samples in the field. This design progression, from a basic hardware platform for proof of concept to versions with wireless connectivity, user interface enhancements, rechargeable batteries, and a focus on cost reduction and mass production suitability, has the potential to benefit farmers and further advancements in food quality assessment. Uploading of measured data from this instrument to the cloud, by means of a custom smartphone app, is also described.

This work proposes a system that optimises multidimensional signal transmission, utilising signals with probabilistic shaping designed with the aid of end-to-end learning of an autoencoder-based architecture. For the first time, this work reports bit mapping optimisation for multidimensional signals and applied the newly derived optimised signals to the probabilistic shaping system. The autoencoder employs two neural networks for the transceiver, separated by the embedded channel. The optimisation of the autoencoder configuration is implemented for probabilistic shaping for n-dimensional signals. Specifically, We investigate a 4-dimensional (4D) signal employing 2 successive time slots that has better noise immunity relative to regular 2-dimensional quadrature amplitude modulation (QAM) signals. We propose a new application of autoencoders in communication systems based on 4D signals and apply machine learning to optimise the 4D probabilistic shaping on the basis of receiver signal-to-noise-ratio (SNR). The performance of the optimised probabilistically shaped 4D signals is evaluated in terms of the bit error rate (BER) and mutual information. Simulation results show that the proposed probabilistically shaped 4D signal achieves better BER performance relative to the unshaped 4D and regular 2D QAM. We demonstrate the mutual information of the proposed signal with varying SNR, showing its improved capacity in comparison with other constellations.

Spectral efficiency improvement is a key focus in most wireless communication systems and achieved by various means such as using large antenna arrays and/or advanced modulation schemes and signal formats. This work proposes to further improve spectral efficiency through combining non-orthogonal spectrally efficient frequency division multiplexing (SEFDM) systems with index modulation (IM), which can efficiently make use of the indices of activated subcarriers as communication information. Recent research has verified that IM may be used with SEFDM to alleviate inter-carrier interference (ICI) and improve error performance. This work proposes new SEFDM signal formats based on novel activation pattern designs, which limit the locations of activated subcarriers and enable a variable number of activated subcarriers in each SEFDM subblock. SEFDM-IM system designs are developed by jointly considering activation patterns, modulation schemes and signal waveform formats, with a set of solutions evaluated under different spectral efficiency scenarios. Detailed modelling of coded systems and simulation studies reveal that the proposed designs not only lead to better bit error rate (BER) but also lower peak-to-average power ratio (PAPR) and reduced computational complexity relative to other reported index-modulated systems.

This work proposes and explores the use of probabilistic shaping for the non-orthogonal multicarrier spectrally efficient frequency division multiplexing (SEFDM) system. The system design considers the reverse concatenation architecture which cascades the constant composition distribution matching (CCDM) algorithm together with soft-decision forward error correction (SD-FEC)-LDPC code for the probabilistic shaping scheme. The non-orthogonal signalling is implemented by discrete Fourier transform (DFT)-based SEFDM modulation with matched filtering demodulation and advanced interference cancellation detection. The high achievable spectral efficiency, low computation complexity and reliability make SEFDM a good candidate for multicarrier signalling for beyond 5G communications. By adding extra shaping gain and flexibility of rate adaptation, the combination of two capacity-achieving techniques provides significant insight of further performance improvement. In this paper, we investigate the performance of the proposed probabilistically shaped-SEFDM (PS-SEFDM) system with regular QAM constellations. The presented results of the proposed system show less required power and bandwidth saving compared to OFDM when achieving the same error performance and same spectral efficiency.

This work proposes and demonstrates the utility of a negative impedance converter (NIC) circuit, based on a common collector (CC) amplifier, for the generation of negative capacitance. The design principle of the proposed NIC is introduced, then a negative capacitance equals -200 pF is demonstrated using discrete devices constructed on a printed circuit board (PCB). The designed NIC is applied for the bandwidth extension of LEDs to enhance the achievable data rates in visible light communication (VLC) systems. The paper includes analytical derivations of the obtained negative capacitance as a function of circuit parameters and verifies this by both simulation and experimentally. Measurements show significant bandwidth extension by neutralising the bandwidth-limiting effect of the LED diffusion capacitance through the introduction of a parallel negative capacitance.

In this paper, we derive new models that describe the noise voltage and equivalent input noise current spectral densities for multiplicative distributed amplifiers. Based on the derived models, design optimisation techniques to minimise the noise contribution of transimpedance amplifiers based on the multiplicative DA topologies are proposed.

The problem of simply and reliably estimating starch content of cassava roots in the field is addressed by the development of a low cost test instrument that measures return loss at radio frequencies using a coaxial probe. A clear relationship between starch content of cassava roots and the measured return loss of root samples at a specific frequency of 30 MHz is first verified experimentally. A prototype test instrument is then designed with goals of portability, low cost and simplicity of use. The test instrument displays starch content in 5 categories, from “low” to “high” using an array of 5 LEDs. The performance of the test instrument is experimentally verified in the field and a reliable correlation between cassava root starch content and LED indication is demonstrated.

This paper proposes a cyclic prefix (CP) free spectral efficiency frequency division multiplexing (SEFDM) wireless signal transmission and reception method based on alignment signal (AS). The method needs the cooperation of both the transmission and reception sides. At the transmitter, time-domain AS is designed to prevent inter-symbol interference (ISI) caused by multipath propagation in the received signal. At the receiver, the channel circularity convolution providing processing is used to enable high accurate frequency domain one-tap equaliser. The extensive computer simulation results under the 5G new radio (5G-NR) channel model, TDL-D, show that the AS aided SEFDM has similar bit error rate performance as CP-SEFDM without energy and latency cost on CP. The AS-SEFDM capability to mitigate the ISI and to enable the one-tap equaliser in SEFDM systems makes it a promising technique for future wireless communication systems.

This paper proposes a frequency-domain multiple user access scheme termed non-orthogonal frequency division multiple access (NoFDMA), which maintains the same data rate per user while allowing more users to access via non-orthogonal user overlapping in a given spectral band. User side signal processing follows existing standards with minor modifications. Receiver side operation can jointly process signals from all the users. Computational complexity is investigated for NoFDMA, which shows slightly increased operations than the typical orthogonal frequency division multiple access (OFDMA). Nevertheless, effective spectral efficiency of NoFDMA, considering both raw spectral efficiency and computational complexity, is higher than that of OFDMA. The scalability of the multiple access scheme is flexible via tuning the user overlapping ratio. Simulation reveals that the number of accessed users is doubled using the NoFDMA strategy when compared with the traditional OFDMA scheme over the same spectral resource utilization.

Non-cooperative communications, where a receiver can automatically distinguish and classify transmitted signal formats prior to detection, are desirable for low-cost and low-latency systems. This work focuses on the deep learning enabled blind classification of multi-carrier signals covering their orthogonal and non-orthogonal varieties. We define Type-I signals with large feature diversity and Type-II signals with strong feature similarity. We evaluate time-domain and frequency-domain convolutional neural network (CNN) models with wireless channel/hardware impairments. Experimental systems are designed and tested, using software defined radio (SDR) devices, operated for different signal formats in line-of-sight and non-line-of-sight communication link scenarios. Testing, using four different time-domain CNN models, showed the pre-trained CNN models to have limited efficiency and utility due to the mismatch between the analytical/simulation and practical/real-world environments. Transfer learning, which is an approach to fine-tune learnt signal features, is applied based on measured over-the-air time-domain signal samples. Experimental results indicate that transfer learning based CNN can efficiently distinguish different signal formats for Type-I in both line-of-sight and non-line-of-sight scenarios relative to the non-transfer-learning approaches. Type-II signals are not identified correctly in the experiment even with the transfer learning assistance leading to potential applications in secure communications.

To enable user diversity and multiplexing gains, a fully digital precoding multiple-input-multiple-output (MIMO) architecture is typically applied. However, a large number of radio frequency (RF) chains make the system unrealistic to low-cost communications. Therefore, a practical three-stage hybrid analog-digital precoding architecture, occupying fewer RF chains, is proposed aiming for a nonorthogonal Internet of Things (IoT) signal in low-cost multiuser MIMO systems. The nonorthogonal waveform can flexibly save spectral resources for massive devices connections or improve data rate without consuming extra spectral resources. The hybrid precoding is divided into three stages, including analog domain, digital domain, and waveform domain. A codebook-based beam selection simplifies the analog-domain beamforming via phase-only tuning. Digital-domain precoding can fine-tune the codebook shaped beam and resolve multiuser interference in terms of both signal amplitude and phase. In the end, the waveform-domain precoding manages the self-created intercarrier interference (ICI) of the nonorthogonal signal. This article designs over-the-air signal transmission experiments for fully digital and hybrid precoding systems on software-defined radio (SDR) devices. Results reveal that waveform precoding accuracy can be enhanced by hybrid precoding. Compared to a transmitter with the same RF chain resources, hybrid precoding significantly outperforms fully digital precoding by up to 15.6 dB error vector magnitude (EVM) gain. A fully digital system with the same number of antennas clearly requires more RF chains and, therefore, is low power, space-efficient, and cost-efficient. Therefore, the proposed three-stage hybrid precoding is a quite suitable solution to nonorthogonal IoT applications.

This work reports a modified regulated cascode (RGC) amplifier which, when used as a transimpedance amplifier (TIA), improves tolerance to ultra-high photodiode capacitances encountered in some optical communications applications. When a conventional RGC is used as a TIA the Miller capacitance, introduced by the base-collector capacitance of its common base stage, limits bandwidth and compromises stability. Such undesirable characteristics are simply eliminated by introducing an additional cascode stage that nullifies the Miller capacitance. We show that exceptional improvement in bandwidth can be made, which is important for visible light communication systems, where using large-area (large capacitance) photodiodes is desirable.

This paper analyses the noise performance of cascaded and matrix single stage distributed amplifiers (C-SSDA and M-SSDA), together termed multiplicative distributed amplifiers. The analytical expressions derived are verified and applied in predicting the noise figure of a two- and three-tiered M-SSDA based on a full foundry model of an InP double heterojunction bipolar transistor (DHBT). Based on observations from the analytical study, we provide design considerations that optimise noise, gain and bandwidth performance for this class of distributed amplifiers, for improved utility in ultra-wideband applications.

Visible light communication systems often suffer from high frequency attenuation when transmitting out-of-band. This effect has been ameliorated by multi-band modulations such as multi-band carrier-less amplitude and phase (m-CAP), which minimises the effect of decreased high frequency magnitude and maximises signal-to-noise ratio-per-sub-band. On the other hand, in the pass-band region, super-Nyquist CAP (SCAP) can offer throughput improvements with no additional complexity at the receiver, at the cost of bit error rate. We propose, for the first time, a new hybrid SCAP modulation format that takes advantageous of both SCAP (i.e. overlapped sub-bands within the modulation bandwidth) and conventional m-CAP (orthogonally spaced bands outside the modulation bandwidth) while maintaining isolation between noise sources. We show higher baud rates within the passband region whilst supporting out-of-band transmission at lower error vector magnitudes.

In this paper, we propose a time precoding scheme for cancelling inter-carrier interference in non-orthogonal frequency division multiplexing for the first time. Achieving high spectral efficiencies is a recurring and key challenge in wireless communications systems and researchers generally use high order and advanced modulation formats to approach this problem, in particular, non-orthogonal modulation formats are a topic of particular interest. Fast orthogonal frequency division multiplexing (F-OFDM) doubles the throughput of conventional OFDM by violating orthogonality of the quadrature carrier, causing interference in the real and imaginary domains. Here, we propose a precoding scheme that enables self-interference cancellation without the need of the interference level calculation. The proposed scheme is implemented in the context of a narrowband internet-of-things (NB-IoT) system and verified on a software define radio (SDR) testbed with realistic AWGN and multiple path channels from channel emulator for concept proving. By comparing with the standard OFDM transmission, the time precoded F-OFDM outperforms around 3dB by BER with same signal-to-ratio (SNR) level.

Massive connectivity is one of the main research directions for beyond 5G. The cellular based narrowband IoT (NB-IoT), enabled by the orthogonal frequency division multiplexing (OFDM) signal, is an important technique. To evolve into the beyond 5G era, non-orthogonal concepts are preferred to re-shape the NB-IoT to provide higher spectral efficiency, wider coverage and lower power consumed services. This work investigates a non-orthogonal waveform in next generation IoT (NG-IoT) scenarios. Previous work has verified the advantages of zero forcing (ZF) precoding in interference mitigation but with some limitations. This work proposes a singular value reconstruction (SVR) precoding method, which can improve the precoding reliability and greatly reduce noise sensitivity. Simulations show significant spectral efficiency gain when compared with the previous work. An experiment platform is then configured in an over-the-air multiuser multiple input multiple output (MIMO) scenario to verify the practical feasibility of the precoding algorithm.

This paper investigates, for the first time, a new scheme for increasing the data rates of 5G new radio (NR), through replacing the orthogonal frequency division multiplexing (OFDM) signal format with a multi-carrier signal resulting in more than 20% improved spectral efficiency. Such signal format is known as spectrally efficient frequency division multiplexing (SEFDM), where the subcarriers' orthogonality is intentionally violated to increase spectral efficiency compared to OFDM. A new method to generate continuous cyclic prefix (CP) for SEFDM signals is reported. Furthermore, the work details a newly proposed system structure capable of generating both continuous CP and special pilot signals as part of the SEFDM frame. The behaviour of such system is investigated by comparing its error rates and throughput to 5G OFDM-based systems. Results show that the use of SEFDM signals can increase the throughput by at least 22.8% relative to OFDM, at the expense of power penalty below 1 dB.

This work proposes a novel signalling method for Narrowband Internet of Things (NB-IoT) mobile system where a quadrupled data rate can be achieved by combining two orthogonal techniques; the frequency orthogonal Fast-Orthogonal Frequency Division Multiplexing (Fast-OFDM) scheme coupled with the time orthogonal Hilbert transform (HT) pair. In this paper, the orthogonality features of one dimension in Fast-OFDM and of the HT pair are explored. The newly designed system structure is presented where the HT pulse pair is generated by square root raised cosine (SRRC) filter with a matched configuration at the receiver. System performance is investigated in terms of bit error rate (BER), effective data rate and spectral efficiency. Simulation results show that the HT-Fast-OFDM system provides 4 times the data rate relative to an OFDM system employing the same modulation scheme and occupying the same bandwidth. Furthermore, the proposed system has compelling advantages over 16- QAM OFDM for it achieves a better BER performance at the same spectral efficiency.

This work presents preliminary investigations into the use of power allocation for the multi-carrier non-orthogonal spectrally efficient frequency division multiplexing (SEFDM) signalling format. SEFDM is utilized to improve the spectral efficiency compared to conventional orthogonal frequency division multiplexing (OFDM), by violating the orthogonality condition and getting the sub-carriers closer to each other. In this paper, subcarriers within the same SEFDM symbol are allocated different power levels. Results show that such power allocation is beneficial to SEFDM from several perspectives: i) Overall system stability enhancement; ii) a drastic complexity reduction in SEFDM detector; iii) peak to average power ratio (PAPR) performance improvement.

The non-orthogonal IoT signal, following the bandwidth compression spectrally efficient frequency division multiplexing (SEFDM) characteristics, can bring benefits in enhanced massive device connections, signal coverage extension and data rate increase, but at the cost of computational complexity. Resource-constrained IoT devices have limited memory storage and complex signal processing is not allowed. Machine learning can simplify signal detection by training a general data-driven signal detection model. However, fully connected neural networks would introduce processing latency and extra power consumption. Therefore, the motivation of this work is to investigate different neural network compression schemes for system simplification. Three compression strategies are studied including topology compression, weight compression and quantization compression. These methods show efficient neural network compression with trade-offs between computational complexity and bit error rate (BER) performance. Practical neural network prototyping is evaluated as well on a software defined radio (SDR) platform. Results show that the practical weight compression neural network can achieve similar performance as the fully connected neural network but with great resource saving.

Single carrier frequency division multiple access (SC-FDMA) has been comprehensively investigated and standardized in 4th generation (4G) and 5th generation (5G) mobile systems. Its significant advantage is low peak-to-average power ratio (PAPR), which makes it suitable for uplink channel communications. However, over a long time period, SC-FDMA has not made breakthrough especially in data rate enhancement, which may not catch up with the next generation evolution in communications. This work proposes to use a non-orthogonal waveform in SC-FDMA to promote a new concept, termed single carrier spectrally efficient frequency division multiple access (SC-SEFDMA). This non-orthogonal single carrier access technique maintains essentially similar complexity as SC-FDMA but advantageously can either achieve higher data rate for the same amount of power consumption or the same data rate with less power consumption.

This paper investigates duobinary signals and their applications in non-orthogonal multi-carrier systems to enhance spectral efficiency. In duobinary transmission schemes, the signal spectrum is reshaped by introducing controlled correlation, which can be eliminated at the duobinary decoder. For the first time, we propose the idea of combining duobinary transmission technique and the fast orthogonal frequency division multiplexing (Fast-OFDM) system with three subcarriers, with experimental results presented. The proposed system is capable of achieving three times the data rate of single-carrier ASK scheme with the same bandwidth. Results show that bit error rate (BER) performance of the proposed duobinary-based Fast-OFDM system is slightly worse than the ASK system. In addition, we also tested various 3-PSK constellation patterns designed for the duobinary signal to achieve performance improvement.

Narrowband Internet of Things (NB-IoT) was introduced by 3GPP in low power wide area network to support low power and wide coverage applications. Since it follows long term evolution standard, its signal quality is guaranteed and its deployment is straightforward via reusing existing infrastructures. Current NB-IoT supports low data rate services via using low order modulation formats for the purpose of power saving. However, with the increase of data rate driven applications, next generation NB-IoT would require data rate enhancement techniques without consuming extra battery power. In this paper, a downlink framework, using a nonorthogonal signal waveform for next generation enhanced NB-IoT (eNB-IoT), is proposed and experimentally tested in both single-antenna and multiantenna systems. In the single-antenna scenario, waveform precoding is used to pre-equalize the self-created inter carrier interference distorted signal waveform. For the multiantenna multiuser scenario, both waveform and antenna space precoding have to be used. Measured results show that in both single-antenna and multiantenna systems, the proposed signal waveform in eNB-IoT can increase data rate by ~11% compared with NB-IoT occupying the same spectral resource in similar receiver computational complexity.

Spectrally efficient frequency division multiplexing (SEFDM) is a multi-carrier signalling format, which has a higher spectral efficiency than conventional orthogonal frequency division multiplexing (OFDM) signals. This work presents preliminary investigations into the use of power allocation for SEFDM. In this method, different subcarriers within the same SEFDM symbol are allocated different power levels. Results show that such power allocation is beneficial to SEFDM detection, with a particular case studied here of the suboptimal fixed sphere decoder (FSD) detector. The investigated method results in a drastic complexity reduction compared to FSD without power allocation for the same error performance.

Narrowband Internet of Things (NB-IoT) is widely used in low power wide area network (LPWAN) applications due to its long distance coverage and low power consumption. According to the 3GPP NB-IoT standard, the maximum modulation format that NB-IoT can support is QPSK, which limits the data rate sensitive IoT applications. To overcome this limitation either higher order modulation formats such as 8PSK or advanced non-orthogonal signal waveforms can be used. In this work, we propose an enhanced NB-IoT framework, eNB-IoT, which applies a non-orthogonal spectrally efficient frequency division multiplexing (SEFDM) signal waveform beyond the typical orthogonal frequency division multiplexing (OFDM). Since the uplink signal recovery is at base station (BS), in this work we use an efficient and sophisticated minimum Euclidean norm search detector, termed sphere decoding (SD) detector. Simulation results indicate that to achieve the same data rate improvement (i.e. 50% improvement) and BER performance, eNB-IoT, employing the non-orthogonal signal waveform, requires 3 dB less transmission power than typical NB-IoT using the dense modulation approach at BER=10-5. The saved power can either extend the battery life of IoT devices or extend the signal transmission distance. This work also proposes an overlapped SD (OSD) detector to simplify the BS signal processing. Its parallel architecture, employing multiple small size SD kernels, can speed up the signal detection. Results show that by using the OSD detector, eNB-IoT still outperforms NB-IoT but with one order of magnitude complexity reduction.

Narrowband Internet of Things (NB-IoT) is a cellular based IoT technique, which can send messages at a long distance using repetitive transmission and single tone frequency hopping. However, retuning of the RF frontend for each narrowband hop could cause frequency offset. Since each tone is shaped by a sinc pulse, when combining tones into a complete signal at the receiver, the side lobe of the sinc pulse would result in significant signal interference in the frequency offset condition. In this work, we propose a half-sinc (HS) waveform for uplink channels via cutting half band using the Hilbert transform to intentionally reserve a frequency offset protection gap. It is verified that the half-sinc waveform can tolerate up to 100% frequency offset via simulation and can practically remove the half side signal band in a software defined IoT platform.

This paper proposes and explores a novel channel estimation scheme for non-orthogonal multi-carrier signals and systems; spectrally efficient frequency division multiplexing (SEFDM), in which higher spectral efficiency is achieved by violating the orthogonality of its subcarriers. The proposed scheme is distinguished by its simplicity, low computational complexity, high accuracy and performance independent of the number of subcarriers and compression factor. The presented results demonstrate the efficacy of the proposed scheme by comparing its complexity and performance to other estimation schemes.

In spectrally efficient frequency division multiplexing (SEFDM), the separation between subcarriers is reduced below the Nyquist criteria, enhancing bandwidth utilisation in comparison to orthogonal frequency division multiplexing (OFDM). This leads to self-induced inter-carrier interference (ICI) in the SEFDM signal, which requires more sophisticated detectors to retrieve the transmitted data. In previous work, iterative detectors (IDs) have been used to recover the SEFDM signal after processing a certain number of iterations, however, the sequential iterative process increases the processing time with the number of iterations, leading to throughput reduction. In this work, ID pipelining is designed and implemented in software defined radio (SDR) to reduce the overall system detection latency and improve the throughput. Thus, symbols are allocated into parallel IDs that have no waiting time as they are received. Our experimental findings show that throughput will improve linearly with the number of the paralleled ID elements, however, hardware complexity also increases linearly with the number of ID elements.

Spectral efficiency can be improved in multicarrier systems through the employment of non-orthogonal overlapping sub-carriers, termed spectrally efficient frequency division multiplexing (SEFDM), but with self-created interference. Previous work has focused on signal detection development. The trade-off between performance and complexity is challenging. This work investigates a self interference cancellation scheme for SEFDM to make use of ICI information at the transmitter and simplify the design of receiver. Repetition codes are used in the system where the same symbol with opposite signs are modulated onto adjacent sub-carriers. Therefore, ICI caused by adjacent sub-carriers would be cancelled mutually. However, the spectral efficiency is reduced. In order to maintain the same spectral efficiency and mutual interference cancellation benefits, the optimal combination of various modulation formats and bandwidth compression factors have to be studied jointly to derive maximum achievable spectral efficiency. Both simulation and experiment are reported and results validate the performance of the proposed self interference cancellation scheme.

For high data rates satellite systems, where multiple carriers are frequency division multiplexed with a slight overlap, the overall spectral efficiency is limited. This work applies highly overlapped carriers for satellite broadcast and broadband scenarios to achieve higher spectral efficiency. Spectrally efficient frequency division multiplexing (SEFDM) compresses subcarrier spacing to increase the spectral efficiency at the expense of orthogonality violation. SEFDM systems performance degrades compared to orthogonal signals, unless efficient interference cancellation is used. Turbo equalisation with interference cancellation is implemented to improve receiver performance for variable coding, compression and modulation/constellation proposals that may be applied in satellite communications settings. Such parameters may be set to satisfy pre‐defined spectral efficiency values for a given quality index or associated application. Assuming low‐density parity check coded data, the work proposes two approaches to receiver design: a simple matched filter approach and an approach utilising an iterative interference cancellation structure specially designed for SEFDM. Mathematical models and simulations studies are presented indicating promising gains to be achieved for SEFDM transmission with advanced transceiver architectures at the cost of increased complexity at the receiver.

Narrowband Internet of Things (NB-IoT) is a low power wide area network (LPWAN) technique introduced in 3GPP release 13. The narrowband transmission scheme enables high capacity, wide coverage, and low power consumption communications. With the increasing demand for services over the air, wireless spectrum is becoming scarce and new techniques are required to boost the number of connected devices within a limited spectral resource to meet the service requirements. This paper provides a compressed signal waveform solution, termed fast-orthogonal frequency division multiplexing (FastOFDM), to double potentially the number of connected devices by compressing occupied bandwidth of each device without compromising data rate and bit error rate performance. Simulation is first evaluated for the Fast-OFDM with comparisons to singlecarrier-frequency division multiple access (SC-FDMA). Results indicate the same performance for both systems in additive white Gaussian noise channel. Experimental measurements are also presented to show the bandwidth saving benefits of Fast-OFDM. It is shown that in a line-of-sight scenario, Fast-OFDM has similar performance as SC-FDMA but with 50% bandwidth saving. This research paves the way for extended coverage, enhanced capacity and improved data rate of NB-IoT in fifth generation new radio networks.

This work investigates several experimental validations for the bandwidth compressed multicarrier signal termed spectrally efficient frequency division multiplexing (SEFDM). The signal compresses bandwidth, therefore improved spectral efficiency, by packing sub-carriers closer. Unlike typical orthogonal frequency division multiplexing (OFDM) signals, SEFDM violates the orthogonality criterion, therefore self-created inter carrier interference (ICI) is introduced. In this work, to ameliorate the effect of interference, a method based on sub-carrier pulse shaping, targeting massive machine-type communication (mMTC), is developed and tested experimentally. Practical over-the-air testing of the proposal is operated on commercially developed software defined radio platforms. Results show that in the condition of coexistence scenario SEFDM can significantly reduce interference when used with existing long term evolution (LTE) signals leading to improved quality of service. The throughput of LTE signals is therefore improved from 49.92 Mbps to 63.21 Mbps. Additionally, the proposed pulse shaping Nyquist-SEFDM performs well in scenarios where the spectrum is limited and in fact it outperforms pulse shaped OFDM significantly, both in terms of bandwidth saving and throughput, which is boosted from 4.35 Mbps to 43.36 Mbps.

Much work has been done on a non-orthogonal signal termed spectrally efficient frequency division multiplexing (SEFDM). Due to its self-created inter carrier interference (ICI), signal detection is complicated. A linear detector named iterative detection (ID) detector shows better bit error rate (BER) performance and complexity trade-off than other linear detectors. Therefore, this work shows the first time hardware modelling of the ID detector at the register transfer level (RTL) stage. The impact of bit precision on the system performance is studied at the beginning. Then, an RTL model is designed with results showing competitive fixed-point performance which are comparable to Matlab floating-point results. Verification work is operated in a co-simulation environment through comparison between fixed-point Matlab results and ISim (a hardware modelling software from Xilinx Inc.) simulation results. Their results are consistent indicating the hardware model is correct.

In the spectrally efficient frequency division multiplexing (SEFDM) non-orthogonal multicarrier signal, higher spectral efficiency can be achieved at the expense of self-created inter carrier interference (ICI). The effective interference, which is contributed by all sub-carriers, has to be minimized and this results in a receiver of significant complexity. In order to mitigate the interference and simplify the receiver design, in this work, a precoding technique, based on eigenvalue decomposition of the sub-carrier correlation matrix, is utilised. Briefly, the technique is based on modifying the data sent on individual sub-carriers according to the signal quality of each, which is based on the sub-carrier to interference ratio (ScIR) of such sub-carrier as estimated from eigenvalue decomposition. A full system model is presented in this paper and simulations show that the precoding of SEFDM results in either better bit error rate (BER) performance compared to that of an orthogonal frequency division multiplexing (OFDM) system of the same spectral efficiency or in higher effective bit rate relative to an OFDM system with the same BER performance. Modelling is done in simple Gaussian noise channels and in a static frequency selective channel and for different modulation formats. Results show that for the same bandwidth a 128QAM precoded SEFDM system outperforms a 16QAM OFDM one by offering 75% bit rate increase. Furthermore, Turbo coding assisted BER performance comparisons are investigated in this work. Using 64QAM modulated symbols, the precoded SEFDM outperforms the typical OFDM by several dBs.

A non-orthogonal multicarrier signal, termed spectrally efficient frequency division multiplexing (SEFDM), is investigated in this work. It improves spectral efficiency by compressing sub-carrier spacing below the symbol rate at the cost of self-created inter carrier interference (ICI). Sphere decoding (SD) is an efficient method to recover signals approaching maximum likelihood (ML) performance. However, the complexity of the SD approaches that of ML with the increase of system size. Studies in this work show that for a small number of sub-carriers, SEFDM signals with low order modulation formats outperform spectral efficiency equivalent OFDM signals modulated by higher order modulation symbols. A key achievement is that 16QAM SEFDM signal outperforms 64QAM OFDM signal at high Eb/N0of the same spectral efficiency. In order to maintain the performance benefit and reduce the complexity of SD for large size SEFDM signals, multi-sphere decoding for a multi-block architecture is applied. For a large number of sub-carriers, the multi-sphere architecture works well for SEFDM signals modulated by 4QAM symbols. Whilst for 16QAM symbols, the performance is related to the number of sub-carriers. This work offers an efficient detection solution for SEFDM signals and reveals challenges. It paves the way for future study of signal detection of large size interfered multicarrier signals.

We comprehensively summarize experimental validations 1 of bandwidth compressed multicarrier waveforms for future 5th generation (5G) applications. The proposed waveforms are derived from an existing non-orthogonal multicarrier concept termed spectrally efficient frequency division multiplexing (SEFDM) where sub-carriers are non-orthogonally packed at frequencies below the symbol rate. This improves the spectral efficiency at the cost of self-created inter carrier interference (ICI). In this work, experiments are reported and testing is carried out in three scenarios including long term evolution (LTE)-like wireless link; millimeter wave radio-over-fiber (RoF) link and optical fiber link. In the first scenario, for a given 25 MHz bandwidth, the SEFDM testbed can provide 70 Mbit/s gross data rate while only 50 Mbit/s can be achieved for an OFDM system occupying the same bandwidth. For the millimeter wave experiment, occupying a 1.125 GHz bandwidth, the gross bit rate for OFDM is 2.25 Gbit/s and with 40% bandwidth compression, 3.75 Gbit/s can be achieved for SEFDM. Two experimental optical fiber links are described in this work; a 10 Gbit/s direct detection optical SEFDM system and a 24 Gbit/s coherent detection SEFDM system. The LTE-like signals and millimeter wave technologies are well suited to provide last mile communications to end users as both can support mobility in wireless environments. The lightwave signals delivered by optical fibers would offer higher data rates and support long-haul communications. The reported techniques, used individually or combined, would be of interest to future wireless system designers, where bandwidth saving is of importance, such as in 5G networks, aiming to provide high capacity and high mobility, simultaneously while saving spectrum.

The complementary cumulative distribution function (CCDF) of delay in a wireless communication system can be approximated by the effective capacity model. However, it is still unknown if such a distribution function can be approximated analytically and accurately. In this paper, we consider a system model with a Nakagami-m fading channel and an exponential arrival process, and derive a simple approximation formula for the CCDF of delay. Simulation results show that our formula is accurate in approximating the CCDF of delay in the system model considered above.

A newly proposed optical-spectrally efficient frequency division multiplexing (O-SEFDM) system reduces the required communication spectrum by employing non-orthogonal and overlapping sub-carriers. This results in higher spectral efficiency relative to an equivalent optical-orthogonal frequency division multiplexing (O-OFDM) delivering the same data rate. O-SEFDM technique can save spectrums in both the electrical and optical domains. However, due to the loss of orthogonality, detection of O-SEFDM signals becomes more complicated. In this work, we employ a hybrid soft Iterative Detection (ID) together with fixed sphere decoder (FSD), concurrently optimizing performance and complexity. We show that for Bandwidth Compression Factor (BCF) of up to 25 percent, we can achieve the same performance as O-OFDM. This verifies Mazo's rates of transmission 25 percent faster than the Nyquist rate. We report a 4QAM system occupying approximately the same bandwidth as that of an 8QAM with 1.6 dB improved error performance for the same transmission rate. The same system shows only minor power penalty (1 dB) relative to its 4QAM OFDM bit rate equivalent but with the advantage of 30% bandwidth saving. This study reports1optical and electrical bandwidth saving with minor error performance degradation and paves the way for practical.

End-to-end delay is an important consideration of the design of wireless tele-ultrasonography systems because such systems support delay-sensitive applications and require critical delay constraints. This paper addresses the characterisation and estimation of end-to-end delay distributions by using the Effective Capacity (EC) technique. A cross-layer simulation platform is built to 1) represent a scenario of remote medical ultrasound video streaming over fixed WiMAX networks, 2) transmit a real ultrasound medical video and 3) verify the accuracy of the EC-based delay distribution estimation for video frames. The results show that in most cases, the simulation results and estimation results are in good agreement.

Previous work has shown that Spectrally Efficient Frequency Division Multiplexing (SEFDM) systems yield up to 40% bandwidth savings at the expense of receiver complexity. Maximum Likelihood (ML) and Sphere Decoding (SD) suffer from an impractical computational complexity. Hybrid detectors, such as Truncated Singular Value Decomposition (TSVD) with fixed complexity SD offer a notable reduction in complexity while maintaining an acceptable error performance. Yet, for high-dimensional systems, even these reduced complexity detectors present challenges for implementation. Hence, in this work, we apply Sort-Free (SF) and Modified Real Valued Decomposition (MRVD) techniques to obtain targeted reduction in computational complexity and improved error performance. Our findings are validated via simulation and practical experimentation with the aid of a Digital Signal Processor (DSP) chip.

It is suggested that packet-based wireless digital communication systems should be represented by discrete-time queueing models. On the basis of the Effective Capacity model, we characterise the probability distribution of a delay process in a digital system by two parameters, namely, the probability of non-zero delay and the success probability of a connection. An estimator that estimates these two parameters is mathematically derived. We consider non-ARQ traffic over the IEEE 802.16-2004 WiMAX systems as an example and establish a cross-layer simulation platform for such an example. The estimator is tested through simulation and results show that the empirical results from the simulation platform are close to estimation results.

In this paper, we present a novel circuit-level timing error mitigation technique, which aims to increase energy-efficiency of digital signal processing datapaths without loss of robustness. Timing errors are detected using razor flip-flops on critical-paths, and the error-rate feedback is used to control a dynamic voltage scaling control loop. In place of conventional razor error correction by replay, we propose a new approach to bound the magnitude of intermittent timing errors at the circuit level. A timing guard-band is created by shaping the path delay distribution such that the critical paths correspond to a group of least-significant bit registers. These end-points are ensured to be critical by modifying the topology of the final stage carry-merge adder, and by using tool-based device sizing. Hence, timing violations lead to weakly correlated logical errors of small magnitude in a mean-squared-error sense. We examine this approach in an finite-impulse response (FIR) filter and a 2-D discrete cosine transform implementation, in 32-nm CMOS. Power saving compared to a conventional design at iso-frequency is 21%-23% at the typical corner, while retaining a voltage guard-band to protect against fast transient changes in switching activity and supply noise. The impact on minimum clock period is small (16%-20%), as it does not necessitate the use of ripple-carry adders and also requires only a bare minimum of additional design effort.

The Razor dynamic voltage scaling approach uses in situ error-detection and correction of timing errors to reclaim safety margins for improved energy-efficiency in digital circuits. In this paper, we propose the use of a time borrowing window on critical logic paths, over which timing errors can resolve safely without an explicit replay mechanism. We demonstrate that time borrowing can be incorporated into DSP pipelines without increasing the minimum clock period, while removing the metastability risk associated with many previously published approaches to replay-free timing error tolerance. A novel hybrid control approach is used to ensure timing violations do not exceed the safe borrowing window. Implementation and back-end simulation of FIR and FFT pipelines demonstrate a significant power reduction. Simulation of the hybrid control loop demonstrates robustness of the proposed approach.

Simulation, using a model of a system or process, is widely accepted as a method of qualification and evaluation of complex systems. The success of a simulation depends on the accuracy of user defined values used to model parameters, which are gathered by analytical or empirical methods. In situations where some parameters are unknown or difficult to quantify, it is required to perform those operations in a real-world situation. Often, moving from a simulation environment to a hardware environment can result in considerable development and implementation challenges. Hence, simulation in conjunction with hardware, so called in the loop validation, can provide a fast method for verification and a reduction in development time. This paper concerns the successful signal transmission and reception of a recently proposed communication scheme, termed as Spectrally Efficient Frequency Division Multiplexing (SEFDM). Signal transmission is verified using in the loop methodology. Experimentally generated signals from a bespoke reconfigurable Field Programmable Gate Array (FPGA) are sampled, using an oscilloscope and formatted for detection using a model of the FPGA architecture together with an analytical model of the SEFDM transmitter. Subsequently, such signals form the input to an analytical receiver model which is used to confirm the experimental signals and hence, exemplifies the in the loop methodology to form an SEFDM pseudo-transceiver.

Spectrally Efficient Frequency Division Multiplexing (SEFDM) systems offer significant bandwidth gains at the expense of receiver complexity. While Maximum Likelihood (ML) and Sphere Decoding (SD) yield optimum performance, these techniques suffer from an impractical computational complexity. Previous work has shown that hybrid detectors combining Truncated Singular Value Decomposition (TSVD) with Fixed SD (FSD) offer a targeted reduction in complexity with an acceptable error performance. This work describes a modified FSD adopting a Sort-Free (SF) approach to make the algorithm better-suited for application in the real world. It further presents for the first time the hardware implementation of a TSVD-FSD using Field Programmable Gate Arrays (FPGAs) and Digital Signal Processors (DSPs). The TSVD detector is realized on an FPGA with a flexible and reconfigurable design supporting different system sizes, modulation orders and levels of bandwidth compression while providing a data rate of up to 136.8 Mbps. The modified FSD is implemented on a DSP and is shown to provide up to six times greater speed when compared to the conventional FSD. The error performance, computational complexity and resource utilization of the system are examined.

Current trends in low-cost and low power consumption transmitters in state-of-the-art wireless systems is to use the direct-conversion principle. Such transmitters, however, suffer severely from the I/Q imbalance effect, which introduces mirror-frequency interference and degrades data detection. This paper proposes a novel method for frequency-dependent I/Q imbalance estimation and compensation. The I/Q imbalance is estimated jointly in the frequency domain with very low complexity and minimum error variance, by relying on a specially designed pilot and its optimization. The performance and complexity of the algorithm are evaluated through both theoretical analysis and computer simulations. The simulation results are compared with those of existing techniques and show considerable advantages of the proposed algorithm. Moreover, a new time domain I/Q imbalance compensator structure is developed which delivers ideal compensation performance as well as the low complexity benefit. Both of the proposed estimation and compensation algorithms are implemented and their respective operation is verified on a test bed utilizing a commercial wideband direct-conversion based signal generator. Experimental results show excellent performance improvement in image rejection in the imbalanced modulator and a resultant very low EVM values below 0.5% across a frequency band of 90 MHz.

Spectral efficiency is a key design issue for all wireless communication systems. Orthogonal frequency division multiplexing (OFDM) is a very well-known technique for efficient data transmission over many carriers overlapped in frequency. Recently, several studies have appeared that describe spectrally efficient variations of multi-carrier systems where the condition of orthogonality is dropped. Proposed techniques suffer from two weaknesses: firstly, the complexity of generating the signal is increased. Secondly, the signal detection is computationally demanding. Known methods suffer either unusably high complexity or high error rates because of the inter-carrier interference. This study addresses both problems by proposing new transmitter and receiver architectures whose design is based on using the simplification that a rational spectrally efficient frequency division multiplexing (SEFDM) system can be treated as a set of overlapped and interleaving OFDM systems. The efficacy of the proposed designs is shown through detailed simulation of systems with different signal types and carrier dimensions. The decoder is heuristic but in practice produces very good results that are close to the theoretical best performance in a variety of settings. The system is able to produce efficiency gains of up to 20% with negligible impact on the required signal-to-noise ratio.

In this paper, we present a novel circuit-level timing error mitigation technique, which aims to increase energy-efficiency when applying a known in situ error-detection and correction technique, called Razor, to DSP datapaths. Timing errors are detected using Razor flip-flops at critical-path endpoints and the error-rate feedback is used to control a dynamic voltage scaling (DVS) control loop. We propose a new approach to bound the magnitude of intermittent timing errors at the circuit level by introducing a guard-band over which timing errors are safely mitigated. The guard-band is achieved by shaping the path delay distribution such that the critical paths correspond to a group of LSB result registers. These end-points are ensured to be critical by modifying the topology of the final stage carry-merge adder and by using tool-based device sizing. Hence, timing violations lead to weakly correlated logical errors of small magnitude in a mean-squared-error sense. We applied this approach to a digital filter in 32nm CMOS. Power saving compared to a conventional design was 23%, over worst-case process and temperature corners.

Spectrally Efficient FDM (SEFDM) systems employ non-orthogonal overlapped carriers to improve spectral efficiency for future communication systems. One of the challenges for SEFDM systems is to demonstrate efficient hardware implementations for transmitters and receivers. This paper presents the first VLSI digital baseband transmitter architecture for SEFDM. The transmitter is reconfigurable between three bandwidth compression ratios, including OFDM and Fast OFDM, therefore supporting operation with current OFDM systems. Complexity analysis is presented of the proposed architecture, along with an area and power efficient hardware mapping, implemented using a 65nm CMOS cell library to provide analysis of area and power compared to a baseline OFDM transmitter.

This work presents the hardware design of a novel algorithm using Field Programmable Gate Arrays (FPGAs) for the detection of Spectrally Efficient Frequency Division Multiplexing (SEFDM) signals. Previous work has shown that a sub-optimal Truncated Singular Value Decomposition (TSVD) approach is well-suited for use in SEFDM systems. TSVD offers a targeted reduction in complexity while outperforming linear detectors, such as Zero Forcing (ZF) and Minimum Mean Squared Error (MMSE), in terms of Bit Error Rate (BER). This is the first time a hardware design for the TSVD algorithm has been devised for implementation on an FPGA device using Very high speed integrated circuit Hardware Description Language (VHDL). Results show excellent fixed-point performance which are comparable to existing floating-point computer-based simulations. The optimal parameters required to achieve this outcome combined with their effect on system performance are identified. The impact of finite FPGA resources against performance gain is also examined.

Spectrally Efficient FDM (SEFDM) signals employs non-orthogonal and overlapping carriers to provide higher spectrum utilization relative to Orthogonal FDM signals (OFDM). Complex detectors are employed to extract the signal from the intercarrier interference (ICI) created by the loss of orthogonality. Sphere Decoder (SD) is proposed for SEFDM detection as an algorithm that achieves ML bit error rate (BER) performance. However, SD complexity is variable depending on the noise as well as the conditioning of the system. In this paper, the use of Fixed complexity Sphere Decoder (FSD) for the detection of SEFDM signal is proposed. The FSD is more suitable for hardware implementation as it eradicates the variable complexity characteristic of the Sphere Decoder algorithm whilst providing competitive bit error rate (BER) performance. The paper shows how the FSD can be applied to detect SEFDM signals and investigate the performance of the FSD in terms of the bit error rate (BER). Simulations results show that the FSD results in minor error penalties that can be traded-off with complexity.

Spectrally Efficient Frequency Division Multiplexing (SEFDM) systems aim to reduce the utilized spectrum by multiplexing non-orthogonal overlapped carriers. Since the per carrier transmission rate is maintained, SEFDM yields higher spectral efficiency relative to an equivalent Orthogonal Frequency Division Multiplexing (OFDM) system. Yet, due to the loss of the orthogonality, detection of the SEFDM system requires overly complex detectors. In this work, new SEFDM receivers that offer substantial complexity reduction with a competitive Bit Error Rate (BER) performance are presented. The Truncated Singular Value Decomposition (TSVD) is proposed as an efficient tool to overcome the ill conditioning of the system caused by the orthogonality collapse. The performance of the system with respect to the system size and spectrum saving is examined by extensive numerical simulations. It is shown that the TSVD detector outperforms linear detectors such as Zero Forcing (ZF) and Minimum Mean Squared Error (MMSE) detectors in terms of BER. Furthermore, a combination of TSVD with the Fixed Sphere Decoder (FSD) algorithm is proposed and tested for the first time. This novel FSD-TSVD receiver achieves near -optimum performance in terms of BER with a fixed and reduced complexity for systems with bandwidth savings of up to 40%.

Wireless multi-hop architectures are increasingly used in many wireless networks. However, it is very difficult to analyse and guarantee end-to-end delay performance over multi-hop wireless paths. In this paper, we develop a cross-layer analytical model to characterise the multi-hop delay performance and derive new mathematical formulae of (DBVP), delay mean and jitter of end-to-end communications. The analytical model is verified by computer simulations, and the results show that the mathematical formulae and simulations are in good agreement.

We propose a novel FIR filter architecture that mitigates sub-critical timing violations as they occur in the pipeline structure by momentarily bypassing affected coefficients. Timing violations are detected using known in situ circuit-level techniques based on late transition detection at timing end points. The approach enables operation with a small but non-zero logical error rate, such that process, voltage and temperature margins can be eliminated without compromising stop-band attenuation. The proposed architecture is implemented in a 90nm CMOS process technology using a typical commercial standard cell implementation flow and verified using full model SPICE simulations. The filter operates at a maximum clock frequency of 420 MHz at 1 V, with an estimated area and power overhead of 26% and 24% respectively compared to a conventional implementation. At the typical process and temperature corner, the proposed architecture can be scaled in voltage down to the point of first failure at 730 mV, thereby achieving a 53% power saving, with no detectable degradation in stop-band attenuation characteristics.

This paper investigates the transmission in time dispersive channels of Spectrally Efficient Frequency Division Multiplexed (SEFDM) signals, where carrier orthogonality is intentionally violated in order to increase bandwidth efficiency. Sufficient statistics of the transmitted SEFDM signal can be obtained by projecting the received signal onto an orthonormal base generated at the receiver using an Iterative Modified Gram Schmidt (IMGS) procedure. In order to reduce the computational complexity resulting from Inter-Carrier Interference (ICI), detection has been implemented based on a Regularized Sphere Decoding (RSD) algorithm. The proposed scheme was previously tested in Additive White Gaussian Noise (AWGN) for various SEFDM signal parameters. In the present work, these results are extended to account for the effect of time dispersive channels. Randomly generated SEFDM symbols are used as pilots to provide estimates of the channel impulse response in systems with or without cyclic prefixes. A joint equalization-detection is subsequently performed in a RSD stage. We show that it is possible to detect optimally SEFDM signals of small dimensionality (e.g. N = 32), with up to 20% bandwidth gain with respect to OFDM systems of the same symbol-rate. This indicates that the wireless transmission of non orthogonal SEFDM signals is tangible.

Spectrally Efficient Frequency Division Multiplexing (SEFDM) system proposes enhanced spectrum utilization in contrast to Orthogonal Frequency Division Multiplexing system (OFDM). Spectral efficiency is increased by relaxing the orthogonality condition while maintaining the same transmission rate per individual channel, hence, for the same bandwidth allocation SEFDM offers higher throughput than OFDM. However, the loss of orthogonality necessitates complex algorithms for the recovery of the signal. In this work, we propose a precoding strategy that greatly simplifies the detection of the signal. The strategy facilitates simpler detection for the same bandwidth savings as an equivalent uncoded SEFDM system. The strategy is based on localizing the effects of the lost orthogonality in a portion of the transmitted symbols. Detection of the preserved channels is a simple zero forcing (ZF) estimator and the rest of the symbols can be detected using complex detectors such as the maximum likelihood (ML) detector. Simple architecture of the precoded SEFDM system based on IDFT/DFT blocks for transmission and reception is proposed. Extensive numerical investigations in AWGN channel confirmed favorable bit error rate (BER) performance of the new system with a much reduced complexity.

This paper investigates the transmission of frequency division multiplexed (FDM) signals, where carrier orthogonality is intentionally violated in order to increase bandwidth efficiency. In analogy to conventional OFDM, signal generation relies on an inverse fractional Fourier transform (IFRFT) that can be implemented with O(N log2N) algorithmic complexity. Optimal maximum likelihood (ML) detection is overly complex due to the presence of substantial intercarrier interference (ICI). Consequently, we investigate an alternative detection mechanism based on the generalized sphere decoding (GSD) algorithm. We examine the bandwidth efficiency and the error performance in additive white gaussian noise (AWGN), for various FDM signal parameters. In particular, we show that it is possible to detect optimally and efficiently FDM signals, with 25% bandwidth gain with respect to analogous OFDM signals. This indicates that the transmission of spectrally efficient non orthogonal FDM signals is tangible.

Recent years have witnessed some interest in Spectrally Efficient Frequency Division Multiplexing (SEFDM) communications systems, where subcarrier orthogonality is intentionally violated to improve the spectral efficiency at the expense of system complexity. This paper investigates reliable polynomial-time hard detection techniques for SEFDM systems, by relaxing the optimal combinatorial Maximum Likelihood (ML) detection to a Semidefinite Program (SDP). SDP can be solved in almost cubic complexity over the number of the SEFDM subcarriers, N. However, the relaxation results into a degradation of the system error performance. In particular, we study the effect of the number of SEFDM subcarriers, N, and the subcarrier separation, ¿f, on the SDP relaxation gap in the presence of Additive White Gaussian Noise (AWGN). We find that as N increases and/or ¿f decreases, the SDP estimate gradually diverges from the optimal solution. To overcome this problem, we propose the use of a boxed ML procedure around the SDP estimate. We show by simulation that the SDP-ML combination approximates the optimum detection for N ¿ 32 subcarriers and up to 20% of bandwidth reduction with respect to an equivalent Orthogonal FDM (OFDM). Our SDP results show a small error penalty when compared to optimal Sphere Decoders (SD), whose computational effort is random and noise dependant, and thereby indicate that our proposed technique is useable in practical SEFDM systems with a moderate number of subcarriers.

In this paper, we investigate the possibility of reliable and computationally efficient detection for spectrally efficient non-orthogonal Multiplexing (FDM) system, exhibiting varying levels of intercarrier interference. Optimum detection is based on the Maximum Likelihood (ML) principle. However, ML is impractical due to its computational complexity. On the other hand, linear detection techniques such as Zero Forcing (ZF) and Minimum Mean Square Error (MMSE) exhibit poor performance. Consequently, we explore the combination of MMSE estimation with ML estimation around a neighborhood of the MMSE estimate. We evaluate the performance of the different schemes in Additive White Gaussian Noise (AWGN), with reference to the number of FDM carriers and their frequency separation. The combined MMSE-ML scheme achieves a near optimum error performance with polynomial complexity for a small number of BPSK FDM carriers. For QPSK modulation the performance of the proposed system improves for a large number of ML comparisons. In all cases, the detectability of the FDM signal is bounded by the signal dimension and the carriers frequency distance.

We examine the effect of the optical fiber group velocity dispersion (GVD) on wavelength-hopping time-spreading (WHTS) optical code division multiple access (OCDMA) networks. We introduce a new and generic modeling methodology for multi access interference (MAI) and bit error rate (BER) estimation. This methodology is applicable to realistic networks with fully asynchronous operation and is valid for any type of code. The efficacy of our model is tested using two different codes; prime-hop and Bin's one coincidence codes. Modeling results and studies of BER performance are reported and indicate the inappropriateness of standard single mode fibers for use in WHTS-OCDMA networks when contrasted to dispersion shifted fibers (DSFs).

We investigate the influence of different Walsh-Hadamard (WH) code allocation techniques on the performance of multi-carrier code division multiple access (CDMA) systems in the presence of high power amplifier (HPA) non-linearities. We consider two different multi-carrier CDMA schemes: multi-carrier CDMA (MC-CDMA) and multi-carrier direct sequence CDMA (MC-DS-CDMA) and analyse their performance in terms of total degradation and spectral spreading for different numbers of active users.

The combination of code division multiple access (CDMA) and multi-carrier modulation (MCM) (referred to as multi-carrier CDMA) has been proposed as a possible candidate for future generations of wireless/mobile systems. Much work has been done on the performance of such schemes in various environments, with different channel models, predominantly assuming linear channels. Accordingly, in this paper we explore the influence of the effects of non-linearities (introduced by the transmitter high power amplifier (HPA)) on multi-carrier CDMA systems using higher order modulation schemes. We consider two techniques for combining CDMA with MCM: multi-carrier CDMA (MC-CDMA) and multi-carrier direct sequence CDMA (MC-DS-CDMA) and analyse their performance in the presence of an HPA and additive white Gaussian noise (AWGN), paying particular attention to the total degradation and the spectral spreading.

To maximize reconfigurability in software radio receivers, digitization should occur as close to the antenna as possible. Bandpass sampling allows the digitization of bandpass signals at RF or intermediate frequencies without significantly increasing the sampling rate. This enables a more flexible receiver to be realized allowing for many radio functions to be defined in software. Sampling of signals at high carrier frequencies has associated problems such as the effects of aperture jitter. Jitter can limit the frequency at which digitization occurs and also degrade receiver performance. This paper investigates the effects of jitter on signal quality in bandpass sampling systems.