Spatial modulation (SM) added to traditional OFDM communications has been intensively studied as a candidate transmission method to convey high-speed, low-delay, power-efficient and high-mobility 5G communications in a reliable basis. This approach implies the use of multiple antennas at the transmitter. Then, the fundamental aspect revised in this work takes into account that in a single-carrier SM system, the selection of the active transmit antenna according to (part of) the information bits makes it possible to use a single power amplifier (PA) that is switched among the available antennas. On the other hand, in a conventional SM-OFDM system, every antenna needs to be continuously active as the index information is typically different for each subcarrier. Consequently, we propose a transmission scheme that precodes the information symbols in frequency domain, such that the global symbol period is split into partitions that enable a sequential operation of antennas which can be fed by a single PA. In addition, it is possible to establish that the proposed approach tends to be more robust against disturbances observed in high mobility environments.

As small satellites become more capable through miniaturized electronics and on-board processing, constellations of low-cost satellites lunched in Low-Earth Orbit (LEO) become feasible. The increase in the number of LEO satellites drives the need for frequency coexistence between the LEO constellation systems with the already existing geostationary (GSO) satellite networks. In this context, it is crucial to design the communication links paying special attention to interference analysis. This is particularly true when the LEO satellite constellation exploit inter-satellite communication links (ISL). In this paper, a radio frequency interference analysis based on simulation of the dynamic satellite constellation is presented and the design parameters of the inter-satellite links are analyzed. The results suggest that carefully choosing the design parameters of the intersatellite links, spectrum coexistence of LEO and GSO networks may be possible.

Machine-to-machine (M2M) communication represents a new paradigm for mobile cellular networks, where a massive number of low-cost devices request the transfer of small amounts of data without human intervention. One option to tackle this problem is obtained by combining random beamforming (RBF) with opportunistic scheduling. RBF can be used to induce larger channel fluctuations, and opportunistic scheduling can be used to select M2M devices when their overall channel quality is good. Traditional RBF does not fulfill M2M requirements, because overall channel quality needs to be tracked continuously. In order to tackle this limitation, a novel codebook-based RBF architecture that identifies in advance the time instants in which overall channel quality should be reported, within a coherence time window, is proposed. This opportunistic feedback mechanism reduces signaling overhead and enables energy saving at M2M devices. A simplified methodology is presented to evaluate the system mean data rate, using for this purpose closed form formulas derived from SNR distribution approximations. Results reveal that the performance loss that is experienced for introducing the proposed modifications to traditional RBF scheme is negligible. The concepts analyzed in this paper provide useful insights, and show that codebook-based RBF with simplified opportunistic scheduling algorithms is an excellent combination to provide wide-area M2M services with low-cost devices and limited signaling overhead.

This paper introduces a new parallel carrier recovery architecture suitable for ultra- high speed intradyne coherent optical receivers (e.g., ≥40Gb/s). The proposed scheme combines a novel low-latency parallel digital phase locked loop (DPLL) with a feedforward carrier phase recovery (CPR) algorithm. The new low-latency parallel DPLL is designed to compensate not only frequency offset, but also frequency fluctuations such as those induced by mechanical vibrations. It is well-known that nonzero frequency offset leads to higher phase error variance in a feedforward CPR. Furthermore, it has been recently shown that laser frequency instability caused by mechanical vibrations significantly degrades the performance of CPR. Other effects such as power supply noise may also introduce laser frequency fluctuations. To avoid receiver performance degradations, both frequency offset and frequency fluctuations should be compensated before the feedforward CPR block. We show that this task can be achieved by using a typical decision-directed serial DPLL. Then, a new approximation to the DPLL computation is introduced to enable a parallel-processing implementation in multigigabit per second receivers. The proposed technique reduces the latency within the feedback loop of the DPLL introduced by parallel processing, while providing a bandwidth and capture range close to those achieved by a serial DPLL. Simulation results demonstrate that the effects caused by frequency deviations can be eliminated with the proposed parallel carrier recovery architecture.

We propose a closed-loop scheme that combines opportunistic beamforming (OBF) and closed-loop (CL) transmit-diversity (TD) techniques. This quantized OBF scheme is compatible with CL TD algorithms, and allows to adaptively control the amount of feedback overhead according to the number of participating users. Closed-form expressions for the SNR gain of the quantized OBF proposal are derived. Important reductions in the amount of feedback overhead are observed without affecting the SNR gain considerably, when compared to CL TD techniques.

We study the performance of fair physical layer schedulers that select a pair of orthogonal users for simultaneous transmission. The schemes rely on partial channel state information compatible with 3GPP specifications and differ with respect to their usage of channel quality indicator in scheduling decisions. Closed-form expressions for the achievable sum-rate of three practical scheduling algorithms are derived.

This work analyzes the outage probability of fair physical layer schedulers that select a pair of orthogonal users for simultaneous transmission. These schemes rest on finite-rate channel state information compatible with 3GPP specifications and differ with respect to their usage of channel quality indicator in scheduling decisions. Closed-form approximations for the outage probability of these practical transmission schemes are derived. Important outage capacity gains are observed when these multiuser scheduling strategies are compared to the scheme that transmits sequentially to only one user at a time.

Reduced-complexity multiuser interference equalization and group decoding (ME-GD) is an attractive way to improve performance of multiuser transmissions for narrowband additive white Gaussian noise channels. ME-GD receivers have been investigated in the past for convolutionally coded synchronous multiple-access channels (MACs). In this paper, we propose a new ME-GD for trellis-coded asynchronous MACs. The coding information of a subset of users and the multiuser interference are jointly taken into account in the branch metric computation using per-survivor processing. The novel approach leads to a family of Viterbi decoders which offers a good tradeoff between performance and complexity. As a second contribution of this paper, we derive theoretical bounds for the performance of ME-GD considering the effect of error propagation, which is critical for asynchronous MACs. Numerical results show the excellent accuracy of the proposed bounds. Finally, based on the novel theoretical analysis, we derive a new criterion to achieve good tradeoffs between performance and complexity.

Previous works have shown that in case of multiple transmit antenna broadcast channels, the total system throughput can be greatly improved by combining spatial prefiltering (SP) with multiuser scheduling (MS) [G. Caire et al., 2003], [D. Samardzija et al., 2003]. However, most of the studies assume full channel state information at the transmitter (CSIT). In this paper, we study the achievable data rates within the current WCDMA HSDPA specification and present different schemes based on the WCDMA closed-loop transmit diversity (CLTD) modes. Although the achievable throughput is interference limited at high SNR due to partial CSIT, we show that at moderate power regimes a significant performance gain can be achieved by the proper combination of SP and MS concepts, compared to the systems that only transmit to the user with the best channel condition in each transmission time interval (TTI).

Joint reduced-state detection and group decoding (RSD-GD) is a promising approach to increase the spectral efficiency of multiple-access systems. Performance of RSD-GD receivers improves as coding information of more users is taken into account in the detection algorithm. For asynchronous channels, however, there is another approach to improve performance: use of more states of the multiple-access channel (MAC). Performance of RSD-GD receivers for asynchronous channels has been investigated using computer simulations; however, analytical studies of the performance of RSD-GD have not been reported so far. In this paper we derive upper bounds for the performance of RSD-GD over asynchronous MAC. The new results are based on an improved upper bound on the error event probability, which is significantly tighter than the traditional bound based on pairwise error events. Comparisons with values obtained from computer simulations show the excellent accuracy of the proposed bound.

Group decoding receivers (GDRs) are an attractive way to improve the performance of multiuser systems with highly correlated users. These reduced complexity receivers use jointly the coding information of a subset of users and multiuser information. Performance of GDR for synchronous multiple-access channels is upper bounded by E.A. Fain and M. Varanasi (see IEEE Trans. on Commun., vol.49, no.6, p.1021-32, 2001). However, upper bounds of GDR for asynchronous multiple-access channels (where the effect of error propagation is critical), have not been reported so far. We derive an upper bound for the performance of GDR for asynchronous multiple-access channels considering error propagation effects. We show that performance analysis based exclusively on minimum distance error events is poor, even at high signal-to-noise ratios (SNRs). To improve the accuracy of analytical estimates, new bounds accounting for the contribution of non-minimum distance error events and error propagation are derived. Comparisons with values obtained from computer simulations show the excellent accuracy of the proposed bounds. Our results also reveal that performance of GDR in channels with highly correlated users is severely affected by error propagation. To mitigate this problem, we propose a new scheme based on the combination of the generalized Viterbi algorithm and the GDR. We show that the proposed receiver is able nearly to eliminate error propagation with low additional complexity.

The combination of space-time coding and multiuser detection is an effective way to increase the spectral efficiency of multiple access systems. In this paper we propose a new multiuser decoder for space-time coded asynchronous DS-CDMA systems over Rayleigh fading channels. The proposed scheme is an extension of the one introduced in Corral-Briones et al. (2002). Complexity is reduced exploiting the fact that the trellis for the entire decoder can be decomposed into a set of trellises for subsets of users with a lower total state count. Different tradeoffs between performance and complexity can be achieved by adjusting the number of users decoded by each trellis. We investigate the behavior of the proposed receiver for a few users with moderate and high correlation. Simulation results show that the new scheme achieves an order of magnitude lower error rate than the conventional receiver (i.e., a single-user decoder that ignores multiuser interference) at almost the same complexity. Moreover, doubling the complexity, the proposed receiver performs near the single user bound. We also address the performance and complexity of the new scheme in high bandwidth efficiency situations, which are of high practical interest. It is found that performance close to the single user bound is achieved with our scheme at practical frame error rates with significantly lower complexity than the receiver proposed in Corral-Briones.

Space-time coding (STC) is an efficient technique to combat severe fading effects and improve system capacity in wireless communications. However, for CDMA communications, multi-antenna transmission increases the multiuser interference (MUI). In addition, if the data rate is high, the processing gain can be quite small, and thus the MUI is further enhanced. In this paper we propose a new non-iterative multiuser decoder for space-time coded asynchronous DS-CDMA. Non-iterative decoding schemes are important in situations where decoding delay must be minimized, for example in interactive applications such as videoconferencing. The proposed scheme introduces a novel reduced-state multiuser sequence detection algorithm suitable to decode few co-located users with high data rates (i.e. users within a beam with short spread sequences). We evaluate the performance of the proposed scheme in high and moderate multiuser interference channels. Simulation results show that a major performance improvement over a conventional receiver is achieved with the proposed scheme even in moderate interference channels. In high interference channels, where the conventional receiver is useless, the proposed scheme performs near the single user bound at practical frame error rates and with reasonable complexity.

We propose a new reduced-complexity multiuser receiver for asynchronous trellis-coded CDMA systems in AWGN channels. Based on the coding information of a subset of users and the set partitioning principles inherent in TCM, reduced-state trellises are constructed. The Viterbi algorithm is thus used to jointly decode the trellis codes of this subset of users and equalize the multiuser interference of the channel. As a special case of the proposed receiver, we study a structure that decodes only one user and equalizes the multiuser interference. Simulation results show that the performance of the new reduced complexity receiver approaches the trellis-coded single-user bound for both low and high-interference channels. Moreover, we compare the performance of the proposed receiver with that of previously proposed suboptimal structures. Our scheme is found to significantly outperform previously known suboptimal structures.

In channels where the delay spread is smaller than the chip interval (e.g., an IS-95 system operating in indoor environments), spread-spectrum signals do not give rise to path diversity. In this situation, maximal-ratio combiner (MRC) RAKE receivers with resolution reduction (RR) of the diversity receiver branches may be used by the mobile stations to provide diversity gain, significantly improving system performance. A new resolution reduction technique based on the use of a minimum mean-square-error diversity combiner (MMSEC) is proposed in this work. We show that, under very general assumptions, this new method of RR is optimal. A detailed study of the performance of a dual-branch MMSEC-RAKE receiver with RR in a typical indoor office environment is presented. In order to allow a simple practical implementation, a suboptimal structure of the MMSEC is also proposed. Numerical results show that this new receiver scheme provides a 1.2-dB improvement over the previously proposed RR technique based on MRC, and a 4.9-dB improvement over conventional MRC-RAKE receiver without RR, at a frame-error rate of 0.01 for the downlink of the IS-95 system in a typical indoor office environment.

In channels where the delay spread is smaller than the chip interval, spread spectrum signals do not give rise to path diversity, thus a classical maximal ratio combiner (MRC)-RAKE receiver sets to unity the number of branches. In this situation, MRC-RAKE receivers with resolution reduction (RR) of the diversity branches may be used to significantly improve performance in the forward link of DS-CDMA systems (Yang 1995). As a result of the use of RR techniques, the noise components at the finger outputs are correlated. It has been shown that the MRC is suboptimal in the presence of correlated noise components. In this paper, a minimum mean squared error combiner (MMSEC) RAKE receiver with RR is proposed. The performance of the RAKE receiver with two branches is studied by means of theoretical analysis and computer simulations. To allow a simple practical implementation, a suboptimal structure of the MMSEC is also proposed. Frame error rate (FER) is obtained by using a simulation system based on the forward link of the digital cellular standard IS-95 in a typical indoor radio channel. Our results show that this new receiver scheme can achieve a gain of 3.6 dB over a conventional MRC-RAKE receiver at an FER of 0.01, with small increase in complexity.