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Our motivation is to characterize the hitherto unexplored tradeoff between energy dissipation and delay in a buffer-aided two-hop link. First, the optimality of 2-D Transmission Activation Probability Space (TAPS) partitioning is formally shown both in the presence and absence of an outage region. Then, the relationship between packet energy dissipation and block delay is found, which is established with the aid of a buffer-state transition matrix. Moreover, the concept of “artificial outage” is introduced and applied for striking a flexible tradeoff between energy dissipation and block delay. Then, an algorithm is provided for finding the matching block delay and energy dissipation pairs. Our analysis and performance results demonstrate that the proposed methods provide a significantly wider and more flexible tradeoff range than all other methods, while reducing the average packet delay and/or the energy dissipation. [ABSTRACT FROM PUBLISHER]

In multihop diversity (MHD) aided multihop links, the nodes are assumed to have buffers for temporarily storing their received packets for further transmission at instances of good channel quality. Since adaptive modulation is employed, the number of packets in each time slot (TS) is affected both by the channel quality and the buffer fullness. During each time-slot (TS), the criterion used for activating a specific hop is that of transmitting the highest number of packets. When more than one hop is capable of transmitting the same number of packets, the particular hop having the highest channel quality (reliability) is activated. Hence, we refer to this regime as the maximum throughput adaptive rate transmission (MTART) scheme. The bit error ratio (BER), the outage probability, the throughput as well as the bandwidth-efficiency of the MTART scheme is analyzed. Our results demonstrate that our MTART regime has the potential of significantly outperforming conventional adaptive modulation. Naturally, the BER is improved by the buffering scheme advocated at the cost of an increased delay. Hence, the distribution of the end-to-end packet delay will also be characterized. [ABSTRACT FROM PUBLISHER]

In this contribution, we propose and study a bufferaided opportunistic routing (BOR) scheme, which combines the benefits of both opportunistic routing and multihop diversity (MHD) aided transmissions. It was conceived for a buffer-aided three-node network (B3NN) composed of a source node (SN), a buffer-aided relay node (RN) and a destination node (DN). In this network, there are three channels namely the SN-RN, RN-DN and SN-DN channels. The key motivation is that we are aiming for activating the specific channels requiring a recuded energy dissipation. In order to study this problem, a three-dimensional (3D) transmission activation probability space (TAPS) is proposed, which is divided into four regions representing each of the three channels plus an outage region. In a specific time slot (TS), the instantaneous channel fading values may be directly mapped to a specific point in this 3D channel space. The BOR scheme then relies on the position of this point to select the most appropriate channel for its transmission. Both the energy dissipation and the outage probability (OP) are investigated for transmission in this network. The results show that when the system is operated at a normalized throughput of 0.4 packet/TS, the energy dissipation was reduced by 24.8% to 77.6% compared to three different benchmark schemes. Alternatively, our technique is capable of reducing the OP by 89.6% when compared to conventional opportunistic routing. As in all buffer-aided system, the performance improved with the cost of higher packet delay, which is also studied. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a novel low-complexity single-user noncoherent combining scheme, named as the erasure-supported equal-gain combining assisted detector (ES-EGCD), for signal detection in fast frequency-hopping M-ary frequency-shift keying (FFH/MFSK) systems. In the proposed ES-EGCD, the decision variables are formed based on the principles of equal-gain combining (EGC), after erasing a number of entries having the largest values from each of the rows of a so-called detection matrix formed during detection. We derive the single-user bit-error rate (BER) of the FFH/MFSK systems employing the ES-EGCD and investigate by simulation the BER performance of the multiuser FFH/MFSK systems employing either the conventional EGC-assisted detector (EGCD) or the proposed ES-EGCD, when assuming communications over Rayleigh fading channels. Both random and optimum FFH addresses are considered in our studies. Our studies and performance results illustrate that the ES-EGCD is capable of significantly outperforming the conventional EGCD in multiuser scenarios. Although the ES-EGCD is a low-complexity single-user detector, it employs the capability to effectively mitigate multiuser interference (MUI). [ABSTRACT FROM PUBLISHER]

As a promising technique to improve achievable bandwidth efficiency, cognitive radio (CR) has attracted substantial research attention from both the academic and industrial communities. To improve the performance attained by the secondary user (SU), a novel hybrid CR system is proposed, which combines the conventional interweave and underlay paradigms to enhance the chance of the SU to access the spectrum. Queuing theory is invoked in this paper to analyze the impact of the primary user's maximum tolerable delay on the performance of the SU. Multiple queues are assumed for the SU, which is engaged in video communication. Apart from the Poisson traffic generation, we also model the classic Nakagami-m fading channel as a Poisson service process by utilizing the outage probability in the presence of cochannel interference. We optimize the hybrid interweave/underlay procedure to maximize the average service rate \muS, \max of the SU, as well as the queue's scheduling scheme, for the sake of minimizing the overall average delay (OAD). As a result, the OAD of the SU is reduced by up to 27% and 20%, compared with the proportion and round-robin schemes, respectively. [ABSTRACT FROM PUBLISHER]

In this paper, we study the asymptotic spectral efficiency (ASE) of the uplink multiple-input–multiple-output direct-sequence code-division multiple-access (MIMO-CDMA) systems supporting multiple users, when assuming communications over flat-fading channels with arbitrary correlation in the spatial domain. In the MIMO-CDMA systems considered, the system-channel-related matrix expressed by \mmb H is a block random matrix, whose entries may be correlated due to the spatial correlation. In this paper, the theory of operator-valued free probability is employed to derive the approximate asymptotic eigenvalue distribution (AED) of the correlation matrix \mmb R\mmb H = {\mmb H}^{H}{\mmb H}, where the size of the block matrix \mmb H tends to infinity. Based on the approximate AED, the spectral efficiency (SE) performance of multiuser MIMO-CDMA systems is studied by both analysis and simulations. Our simulation and numerical results show that the corresponding ASE is, in general, capable of providing very good prediction of the SE achieved by the practical MIMO-CDMA systems. [ABSTRACT FROM PUBLISHER]

In this paper, we exploit the benefits of the diversity gains that arise from a cluster of opportunistic relays (ORs) and from the independently fading subcarriers of multiple users. Our goal is to improve the energy efficiency of the decode-and-forward-based OR-assisted single-carrier frequency-division multiple-access (SC-FDMA) uplink, where the direct transmission (DT) link is unavailable. We proposed two joint dynamic resource allocation (JDRA) schemes by assuming that the pilot-aided channel-quality information (CQI) of all the users may be exchanged. Furthermore, we take the following two main aspects into account: 1) “first-hop quality awareness” (FHQA) for JDRA-aided OR and 2) “buffering delay awareness” in the context of interleaver-aided channel-coded systems. In addition, frequency-domain turbo equalizers are employed in both the relay and BS's receivers. Our results demonstrate that, compared to the DT benchmarker, the proposed FHQA JDRA schemes can achieve an energy reduction gain of 91% for a single-antenna base station (BS) receiver and up to 7.4% for a multiantenna BS when considering the energy consumption due to CQI exchange among relays. [ABSTRACT FROM PUBLISHER]

The rationale of our design is that although much of the literature of cooperative systems assumes perfect coherent detection, the assumption of having any channel estimates at the relays imposes an unreasonable burden on the relay station. Hence, non-coherently detected Reed-Solomon (ReS) coded Slow Frequency Hopping (SFH) assisted M-ary Frequency Shift Keying (FSK) is proposed for cooperative wireless networks, subjected to both partial-band interference and Rayleigh fading. Erasure insertion (EI) assisted ReS decoding based on the joint maximum output-ratio threshold test (MO-RTT) is investigated in order to evaluate the attainable system performance. Compared to the conventional error-correction-only decoder, the EI scheme may achieve an E_b/N_0 gain of approximately 3 dB at the Codeword Error Probability, P_w, of 10^-4, when employing the ReS(31,20) code combined with 32-FSK modulation. Additionally, we evaluated the system's performance, when either equal gain combining (EGC) or selection combining (SC) techniques are employed at the destination's receiver. The results demonstrated that in the presence of one and two assisting relays, the EGC scheme achieves gains of 1.5 dB and 1.0 dB at the P_w of 10^-6, respectively, compared to the SC arrangement. Furthermore, we demonstrated that for the same coding rate and packet size, the ReS(31,20) code using EI decoding is capable of outperforming convolutional coding, when 32-FSK modulation is considered, whilst LDPC coding had an edge over the above two schemes. [ABSTRACT FROM AUTHOR]

A multihop transmission scheme is proposed and studied, where all the relay nodes (RNs) of a multihop link are assumed to have buffers for temporarily storing their received packets. Hence, the RNs are operated under the so-called store-and-forward relaying scheme. As a benefit of storing packets at the RNs, during each time slot, the best hop typically has the highest signal-to-noise ratio (SNR), which can be selected from the set of hops that have packets awaiting transmission in the buffer. A packet is then transmitted over the best hop. This hop selection procedure is reminiscent of selection diversity, which is referred to here as multihop diversity (MHD). In this paper, we investigate both the bit error and outage probability and the delay performance of the MHD scheme when assuming that each hop experiences both propagation path loss and independent and identically distributed (i.i.d.) flat Rayleigh fading. The medium access control (MAC) layer implementation and several closed-form formulas are derived. Both numerical and simulation results are provided to characterize the achievable performance of the MHD scheme. Our performance results show that relying on multiple hops has the potential to provide a significant diversity gain, which may be exploited for enhancing the reliability of wireless multihop communications. [ABSTRACT FROM PUBLISHER]

The performance of multihop links is studied in this contribution by both analysis and simulations when communicating over generalized-K (KG) fading channels. The performance metrics considered include symbol error rate (SER), outage probability, level crossing rate (LCR), and average outage duration (AOD). First, the expressions for both the SER and outage probability are derived by approximating the probability density function (pdf) of the end-to-end signal-to-noise ratio (SNR) using an equivalent end-to-end pdf. We show that this equivalent end-to-end pdf is accurate for analyzing the outage probability. Then, the second-order statistics of LCR and AOD of multihop links are analyzed. Finally, the performance of multihop links is investigated by either simulations or evaluation of the expressions derived. Our performance results show that the analytical expressions obtained can be well justified by the simulation results. The studies show that the KG channel model and the expressions derived in this paper are highly efficient for the prediction of the performance metrics and statistics for the design of multihop communication links. [ABSTRACT FROM PUBLISHER]

A redundant residue number system (RRNS)-assisted multicarrier direct-sequence code-division dynamic multiple-access (MC/DS-CDDMA) scheme is proposed for application in cognitive radios (CRs). Taking the advantages of both multicarrier direct-sequence code-division multiple access and RRNS, the RRNS MC/DS-CDDMA has a low complexity for implementation and a high flexibility for reconfiguration and is robust to dynamic spectrums. Associated with the RRNS MC/DS-CDDMA, in this paper, the so-called receiver multiuser diversity aided multistage minimum mean-square error multiuser detector (RMD/MS-MMSE MUD) is considered for signal detection. Specifically, three types of RMD/MS-MMSE MUDs are proposed: Type-I.1, Type-II.1, and Type-I.2 RMD/MS-MMSE MUDs. In these MUD schemes, the Type-I.1 MUD carries out the joint detection of all the subcarrier signals using both the observations and the channel state information (CSI) of all the subcarriers. In the Type-I.2 and Type-II.1 RMD/MS-MMSE MUDs, the embedded MMSE-MUDs are implemented subcarrier by subcarrier independently. Furthermore, in the Type-I.2 RMD/MS-MMSE MUD, the autocorrelation matrices used by the MMSE-MUDs are free from CSI. Explicitly, both Type-II.1 and Type-I.2 RMD/MS-MMSE MUDs are suitable for operation in dynamic spectrum environments. Furthermore, our simulation results show that the preceding three types of RMD/MS-MMSE MUDs are capable of making the RRNS MC/DS-CDDMA systems achieve similar error and throughput performance. Owing to the foregoing merits, the RRNS MC/DS-CDDMA employing either Type-I.2 or Type-II.1 RMD/MS-MMSE MUD may be considered one of the highly promising dynamic multiple-access schemes for application in CR systems. [ABSTRACT FROM PUBLISHER]