Spectral Efficiency Loss Research Articles

Parallel Quadrature Spatial Modulation for Massive MIMO Systems With ICI Avoidance

Despite its efficiency in conventional multiple-input multiple-output (MIMO) wireless systems, quadrature spatial modulation (QSM) becomes less efficient in massive MIMO systems since it does not adapt to the number of antennas but always uses one or two out of them. To adopt QSM in massive MIMO systems, a parallel quadrature spatial modulation (PQSM) scheme is proposed in this paper. In PQSM, the transmit (Tx) antennas are divided equally into P > 1 groups, and the bit sequence to be transmitted during a time slot is divided into P+1 parts. Then, the first part is applied to map an M-QAM complex constellation symbol while the remaining P parts of the bitstream are used to perform P QSMs in parallel. By allowing a tradeoff between the spatial modulation order and signal constellation order, PQSM enables lower bit error rate (BER) with no loss of spectral efficiency compared with QSM. For a fixed signal constellation, PQSM yields higher spectral efficiency than QSM since more selected antenna indices can carry more data bits. The algorithm pertaining to the proposed scheme is designed, and an upper bound on the average bit error rate (ABER) is derived. Moreover, to minimize the ABER, an algorithm is developed to optimize the number of Tx antenna groups and the signal constellation order. Monte-Carlo simulation results demonstrate the superiority of PQSM over generalized SM and QSM, as well as the effectiveness of the developed performance analysis.

Performance Analysis of Two-Way Relaying in Cooperative Power Line Communications

This paper studies the performance of a half-duplex (HD) two-way relay (TWR) in power line communication (PLC) over a log-normal fading channel with impulsive noise. We consider the two common relaying protocols: amplify-and-forward (AF) and decode-and-forward (DF). For the DF relaying, we apply physical-layer network coding (PNC) and analog-network coding (ANC) to the PLC TWR. We derive analytic expressions for the average sum capacity and the outage probability of the system. The Monte Carlo simulations are provided throughout to validate our analysis. The analytical results show a tight approximation to the simulation results. We compare the one-way relay (OWR) to the TWR and show that the HD spectral efficiency loss incurred by the OWR can be sufficiently mitigated in PLC. However, the outage probability of the TWR is inferior to that of the OWR. To enhance the outage performance of the PLC TWR, we implement a hybrid PLC/wireless (HPW) system, where all nodes are equipped with the PLC and wireless capabilities. Data transmission occurs over the two parallel links. The diversity in the transmission allows the TWR to improve its outage performance in the AF and DF protocols. The impact of the impulsive noise, inherent to the PLC channels, is also highlighted in the simulation results. It is shown that the impulsive noise severely impairs system performance.

12 b/s/Hz Spectral Efficiency Over the C-band Based on Comb-Based Superchannels

We present transmission of three 50 × 24 Gbaud PM-256QAM superchannels with an overall spectral efficiency of 12 b/s/Hz over the entire C-band. Each comb-based superchannel combines a single shared optical pilot tone with a low-overhead pilot-based DSP. The pilot tone is used to lock the transmitter and the receiver comb seed lasers, suppressing carrier offsets. Combined with optimized insertion ratio of the digital pilot symbols, the total overhead is minimized, avoiding any excess loss in spectral efficiency. Our results demonstrate the feasibility to exploit the unique properties of frequency combs in the design of high spectral efficiency multi-Tb/s superchannels.

The spectral efficiency augmented HACO-OFDM based visible light communications

The conventional hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) has an obvious spectral efficiency loss due to the fact that the real part of the even subcarriers are not utilized. This paper proposes a scheme to augment the spectral efficiency of HACO-OFDM. In this scheme, more spectrum resources are exploited because the real part of selected even subcarrier group is utilized for data modulation at different depths. When the same modulation orders are used, the proposed scheme is able to achieve higher spectral efficiency than HACO-OFDM at the cost of acceptable increased computational complexity. When this scheme takes 5 depths, the joint modulation scheme uses 16QAM-4PAM and 64QAM-4PAM, the spectral efficiency of the proposed scheme can be increased by 31.33% and 23.48%, respectively. Moreover, compared with HACO-OFDM, the proposed scheme has advantages of lower PAPR and higher immunity to LED nonlinearity.

Energy- and spectral-efficiency of zero-forcing beamforming in massive MIMO systems with imperfect reciprocity calibration: bound and optimization

In a time-division duplex (TDD) system with massive multiple input multiple output (MIMO), channel reciprocity calibration (RC) is generally required in order to cope with the reciprocity mismatch between the uplink and downlink channel state information. Currently, evaluating the achievable spectral efficiency (SE) and energy efficiency (EE) of TDD massive MIMO systems with imperfect RC (IRC) mainly relies on exhausting Monte Carlo simulations and it is infeasible to precisely and concisely quantify the achievable SE and EE with IRC. In this study, a novel method is presented for tightly bounding the achievable SE of massive MIMO systems with zero-forcing beamforming under IRC. On the basis of the analytical results, we demonstrate key insights for practical system design with IRC in three aspects: the scaling rule for interference power, saturation region of the SE, and the bound on the SE loss. Finally, the trade-off between spectral and energy efficiencies in the presence of IRC is determined with algorithms developed to optimize SE (EE) under a constrained EE (SE) value. The loss of optimal total SE and EE due to IRC is also quantified, which shows that the loss of optimal EE is more sensitive to IRC in a typical range of transmit power values.

Performance optimization of cell-free massive MIMO system with power control approach

The cell-free massive MIMO (multiple-input multiple-output) system involves a large number of access points serving a smaller number of mobile users (MUs) over identical time/ frequency resource. By providing large number of service antennas closer to the MUs, the cell-free massive MIMO can offer great spectral efficiency, better macro-diversity and minimal path loss. Despite several advantages, the cell-free massive MIMO suffers from energy overloading caused by uncontrolled backhaul power consumption for large number of distributed access points (APs) and pilot contamination during channel estimation. In this paper, we have taken into consideration a cell-free massive MIMO system with APs equipped with multiple antennas performing time-division-duplex (TDD) operation. Here, all the APs coordinate through a constrained backhaul network for joint transmission of signals to all the users simultaneously by multiplying the received signal with the normalized conjugate of the estimated channel state information (CSI) and send back a rounded off version of the weighted pattern to the central processing unit (CPU). Finally, an effective user defined algorithm is presented involving selection and grouping of various APs based on their individual contributions for a particular MU to improve the overall performance of the system.

Multiple-Noise-Tolerant CO-OFDMA-PON Uplink Multiple Access Using AM-DAPSK-OFDM With Reflective ONUs

A reflective coherent orthogonal frequency-division multiple-access (OFDMA) passive optical network (PON) based on differential amplitude and phase-shift keying (DAPSK) OFDM is proposed herein. In the conventional OFDMA–PON uplink transmission, each optical network unit (ONU) signal has different channel and phase-noise characteristics. In addition, each ONU has an independent laser source; therefore, the carrier frequency offset (CFO) differs among the ONUs. These multiple noises worsen as the number of ONUs increases and adversely affect the spectral bandwidth and system complexity. The proposed system using reflective-type ONUs avoids multiple CFOs because every ONU shares the same laser source, which is transmitted from an optical line terminator. Moreover, by applying a differential modulation scheme to OFDM, we resolve the multiple channel and phase noises without spectral efficiency loss and system complexity increase. The spectral efficiency is maximized using an adaptive modulation technique. The proposed system is experimentally demonstrated in a 20-km OFDMA–PON uplink transmission link with two reflective ONUs. The results confirm the possibility of uplink multiple access without multiple-noise compensation processes. The optimal spectral efficiency in terms of bandwidth size and number of subcarriers is analyzed.

Diversity Enhancing Multiple-Mode OFDM With Index Modulation

As an emerging multi-carrier transmission scheme, multiple-mode orthogonal frequency division multiplexing with index modulation (MM-OFDM-IM) conveys information through multiple distinguishable constellations (or modes, alternatively) and their full permutations, increasing the spectral efficiency of OFDM-IM and classical OFDM. However, both MM-OFDM-IM and its extension, that is, MM-OFDM with in-phase/quadrature index modulation (MM-OFDM-IM-IQ), cannot provide any transmit diversity gain, which may be critical for ultra-reliable communications. In this paper, aiming at enhancing the diversity gain of MM-OFDM-IM(-IQ) schemes, coordinate interleaved (CI-)MM-OFDM-IM and linear constellation precoded (LCP-) MM-OFDM-IM-IQ are proposed, both of which achieve a diversity order of two without loss of spectral efficiency. The optimal rotation angle for CI-MM-OFDM-IM and the optimal precoding matrix for LCP-MM-OFDM-IM-IQ, in the sense of maximizing their coding gains, are derived in closed form. Computer simulations corroborate the advantages of the proposed schemes in terms of diversity and bit error rate performance toward next-generation wireless networks.

Pseudo-Pilot Coding Based Phase Noise Estimation for Coherent Optical FBMC-OQAM Transmissions

We propose a cost-efficient scheme called pseudo-pilot coding based phase noise estimation (PC-PNE) to compensate laser phase noises for filter-bank multicarrier (FBMC) transmission systems. The performance of the proposed PC-PNE with rectangular quadratic-amplitude modulation (QAM) is evaluated and compared with the state-of-the-art modified blind phase search (M-BPS) approach via extensive Monte Carlo simulations. Under 200 kHz combined laser linewidth, 32 Gbaud 256-subcarriers FBMC optical transmission equipped with PC-PNE provides a comparable performance with the complicated M-BPS method for QPSK, 16QAM and 64QAM. Moreover, a significant reduction in computational complexity (up to 50 times) is obtained with PC-PNE in comparison with M-BPS method at the cost of 3.1% spectral efficiency loss.

Spectral Efficiency of Mixed-ADC Massive MIMO

We study the spectral efficiency (SE) of a mixed-ADC massive MIMO system in which K single-antenna users communicate with a base station (BS) equipped with M antennas connected to N high-resolution ADCs and M-N one-bit ADCs. This architecture has been proposed as an approach for realizing massive MIMO systems with reasonable power consumption. First, we investigate the effectiveness of mixed-ADC architectures in overcoming the channel estimation error caused by coarse quantization. For the channel estimation phase, we study to what extent one can combat the SE loss by exploiting just N << M pairs of high-resolution ADCs. We extend the round-robin training scheme for mixed-ADC systems to include both high-resolution and one-bit quantized observations. Then, we analyze the impact of the resulting channel estimation error in the data detection phase. We consider random high-resolution ADC assignment and also analyze a simple antenna selection scheme to increase the SE. Analytical expressions are derived for the SE for maximum ratio combining (MRC) and numerical results are presented for zero-forcing (ZF) detection. Performance comparisons are made against systems with uniform ADC resolution and against mixed-ADC systems without round-robin training to illustrate under what conditions each approach provides the greatest benefit.

Energy-Efficient Hybrid Duplexing Strategy for Bidirectional Distributed Antenna Systems

We propose a bidirectional distributed antenna (DA) system where DAs are capable of working dynamically in hybrid duplex modes: full duplex (FD), half duplex, and sleep, which enables higher degree of freedom and hence much higher energy efficiency (EE) than DA sole-FD systems, with just marginal loss in spectral efficiency (SE). The proposed system also demonstrates significant EE and SE enhancement over FD colocated antenna systems. Compared to the above two benchmark systems, the proposed system also requires much simpler self-interference cancelation design for FD mode due to less cross talks between antennas. A low-complexity EE maximization scheme is proposed for the bidirectional DA system. A channel gain based DA clustering algorithm is first performed to activate/deactivate transmit/receive chains, which highlights the characteristics of DA deployment, and then a distributed hybrid duplexing algorithm is performed to optimize the downlink beamformer and the uplink transmission power. Various practical aspects are taken into account for system design, such as self-interference at DAs in FD mode, cochannel interference from uplink users to downlink users, and multiuser interference in both uplink and downlink. The effectiveness of the proposed system is verified by simulation results.

Enhancing Contention Resolution ALOHA Using Combining Techniques

Recently, random access protocols have acquired a new wave of interest, not only from the satellite communication community, but also from researchers active in fields like Internet of Things and machine-to-machine. Asynchronous (slot- and frame-wise) ALOHA-like random access protocols, are very attractive for such applications, enabling low complexity transmitters and avoiding time synchronization requirements. Evolutions of ALOHA employ time diversity through proactive replication of packets, but the time diversity is not fully exploited at the receiver. Combining techniques, as selection combining and maximal-ratio combining, are beneficial and are adopted in the enhanced contention resolution ALOHA (ECRA) scheme, presented here. A tight approximation of the packet loss rate for asynchronous random access, including ECRA, well suited for the low channel load region is derived. Finally, ECRA is evaluated in terms of spectral efficiency, throughput and packet loss rate in comparison with recent protocols, showing that it is able to largely outperform both slotted synchronous and asynchronous schemes.

Amplify-and-Forward Virtual Full-Duplex Relaying-Based Cooperative NOMA

This letter studies the performance of a virtual full-duplex relaying technique in cooperative non-orthogonal multiple access (NOMA)-based systems. In particular, two half-duplex amplify-and-forward relays imitate the operation of full-duplex relaying in cooperative NOMA systems using the successive relaying (SR) technique, in order to recover the loss of spectral efficiency due to the half-duplex constraint at the relays. The closed-form outage probability of the proposed scheme is derived. The simulation results show that the proposed SR-based cooperative NOMA system with an opportunistic power split factor achieves a lower outage probability and a higher ergodic rate than the existing SR-based orthogonal multiple access system and ideal full duplex relaying-based NOMA systems.

Relay Selection for Broadcast Underlay Cognitive Radio Networks Using AF and DF Relaying

A degree of cooperative diversity has been achieved with the introduction of relay nodes in wireless networks. However, such approach suffers from spectral efficiency loss. Therefore, different relay selection techniques have been proposed in the literature to mitigate this loss. In this paper, the problem of relay selection for broadcast cognitive radio networks (CRNs) is investigated. The CRN is composed of a primary transmitter and receiver, a secondary source and several destination nodes. It is assumed that the secondary source broadcasts a signal to N secondary destinations. We implicitly split the destination nodes into two sets: reliable nodes having an signal-to-noise ratio (SNR) larger than a predefined threshold (T) and unreliable nodes with SNR lower than T. The paper proposes several relay selection techniques based on the average SNR, instantaneous SNR, and best direct link. Relay selection is performed by a central node taking into account the interference level at primary receiver and channel state information between nodes of the secondary CRN. In addition, the paper provides the mathematical derivation and analysis of the bit error probability (BEP) of relay selection in broadcast CRNs for both amplify and forward and decode and forward relaying. We propose to select a single relay among reliable nodes that verify interference constraints (interference to primary receiver lower than the interference threshold \(\beta \)) to help the remaining unreliable nodes. The threshold T is optimized numerically to yield the lowest BEP. Our theoretical results are confirmed by simulation.

Mutual information of massive MIMO systems on block Rayleigh-faded channels

This paper presents mutual information achieved by a Massive multi-input multi-output (Ma-MIMO) systems on a block Rayleigh-fading Channels. Mutual information represents the highest achievable spectral efficiency (SE) in Ma-MIMO systems. Fast fading causes the loss in SE which is amplified by using multiple transmit antennas. The channel model considered here is block Rayleigh-fading Channels which resembles the practical scenario of cellular communication. Kronecker model is considered for modeling the spatially correlated channels. Minimum mean square error with successive interference cancellation (MMSE-SIC) is implemented to detect the channel and estimate the SE in the Uplink and Downlink channels. The simulation result shows linear MMSE and MMSE-SIC detector’s achievable SE for the Ma-MIMO systems.

Time reversal based frequency-domain equalization for underwater acoustic single-carrier transmissions

Time reversal processing generates a time-focused signal, after combining dispersive reception from a hydrophone array in the underwater acoustic channel. Because of this property, the time reversal method has been studied as a preprocessing scheme to support decision feedback equalization for underwater acoustic communications. Orthogonal frequency division multiplexing has also been combined with time reversal processing to improve spectral efficiency, as the equivalent compact channel impulse response allows short prefixes in the acoustic channel. Here, we show that time reversal processing can be used to facilitate prefix-free frequency domain equalization. Traditional frequency domain equalization schemes rely on the use of periodic prefixes, which bring a loss of spectral efficiency in the already-bandlimited underwater channel. The core idea is to use time reversal pre-processing to generate a delta-like equivalent impulse response to facilitate robust reconstruction of prefixes at the receiver for prefix-free single-carrier transmissions. The resultant receiver not only improves the spectral efficiency, but also supports communications over channels of variable fluctuation rates. The effectiveness of the receiver has been tested using the field measurements from the Gulf of Mexico in August 2016.

A resource allocation algorithm for throughput maximization with fairness increase based on virtual PRB in MIMO-OFDMA systems

This paper deals with a new resource allocation algorithm in downlink MIMO-OFDMA systems. The objective is to maximize the system throughput with respect to fairness criteria since some users may experience bad channel conditions for a long time. Known to be NP-hard, the original optimization problem is divided into two sub-problems where radio resource allocation and power allocation are performed separately. Firstly, a recursive PRB allocation algorithm is performed aiming at maximizing the system throughput. In LTE systems, 41% of sub-carriers are considered unused which introduces spectral efficiency loss. As solution, the eNodeB aggregates the unused sub-carriers by each user to construct a “virtual” PRB to be allocated to seldom served user for fairness and throughput increase. Secondly, power allocation is performed to select a more appropriate MCS.

BER Performance of Stratified ACO-OFDM for Optical Wireless Communications over Multipath Channel

In intensity modulation/direct detection- (IM/DD-) based optical OFDM systems, the requirement of the input signal to be real and positive unipolar imposes a reduction of system performances. Among previously proposed unipolar optical OFDM schemes for optical wireless communications (OWC), asymmetrically clipped optical OFDM (ACO-OFDM) and direct current biased optical OFDM (DCO-OFDM) are the most accepted ones. But those proposed schemes experience either spectral efficiency loss or energy efficiency loss which is a big challenge to realize high speed OWC. To improve the spectral and energy efficiencies, we previously proposed a multistratum-based stratified asymmetrically clipped optical OFDM (STACO-OFDM), and its performance was analyzed for AWGN channel. STACO-OFDM utilizes even subcarriers on the first stratum and odd subcarriers on the rest of strata to transmit multiple ACO-OFDM frames simultaneously. STACO-OFDM provides equal spectral efficiency as DCO-OFDM and better spectral efficiency compared to ACO-OFDM. In this paper, we analyze the BER performance of STACO-OFDM under the effect of multipath fading. The theoretical bit error rate (BER) bound is derived and compared with the simulation results, and good agreement is achieved. Moreover, STACO-OFDM shows better BER performance compared to ACO-OFDM and DCO-OFDM.

Cooperative NOMA With Incremental Relaying: Performance Analysis and Optimization

In conventional cooperative non-orthogonal multiple access (NOMA) networks, spectral efficiency loss occurs due to a half-duplex constraint. To address this issue, we propose an incremental cooperative NOMA (ICN) protocol for a two-user downlink network. In particular, this protocol allows the source to adaptively switch between a direct NOMA transmission mode and a cooperative NOMA transmission mode according to a 1-bit feedback from the far user. We analytically prove that the proposed ICN protocol outperforms the conventional cooperative NOMA protocol. In addition, an optimal power allocation strategy at the source is studied to minimize the asymptotic system outage probability. Finally, numerical results validate our theoretical analysis, present insights, and quantify the enhancement achieved over the benchmark scheme.

Compressive sensing approach in multicarrier sparsely indexing modulation systems

Recently the indexed modulation (IM) technique in conjunction with the multi-carrier modulation gains an increasing attention. It conveys additional information on the subcarrier indices by activating specific subcarriers in the frequency domain besides the conventional amplitude-phase modulation of the activated subcarriers. Orthogonal frequency division multiplexing (OFDM) with IM (OFDM-IM) is deeply compared with the classical OFDM. It leads to an attractive trade-off between the spectral efficiency (SE) and the energy efficiency (EE). In this paper, the concept of the combinatorial modulation is introduced from a new point of view. The sparsity mapping is suggested intentionally to enable the compressive sensing (CS) concept in the data recovery process to provide further performance and EE enhancement without SE loss. Generating artificial data sparsity in the frequency domain along with naturally embedded channel sparsity in the time domain allows joint data recovery and channel estimation in a double sparsity framework. Based on simulation results, the performance of the proposed approach agrees with the predicted CS superiority even under low signal-to-noise ratio without channel coding. Moreover, the proposed sparsely indexed modulation system outperforms the conventional OFDM system and the OFDM-IM system in terms of error performance, peak-to-average power ratio (PAPR) and energy efficiency under the same spectral efficiency.