In this paper, we propose a Reference Signal (RS) framework potentially for next generation wireless systems, which targets to systematical RS construction with nonorthogonal Space-Time Code (STC) designs. In particular, we embed parametrized non-orthogonal STCs into the RS design, by exploiting the properties of so-called ABBA STCs or its diagonal variants, where the diagonality is imposed by use of appropriate precoding and postcoding. The transformations of the ABBA coding vectors can be resolved at the receiver. The transformed matrices are used to enhance channel estimation quality with the aid of inherent power gain, self-interference mitigation, and diversity gain. The proposed framework can be flexibly integrated to existing RS sequences, defined in 5G New Radio (NR). In effect, this leads to parameterizable code space and controllable self-interference. The proposed RS construction can be easily extended to support$2^{M} \times 2^{M}$STC systems for$M \geq 2$, and can be used to track the channel response with up to$2^{M}$spatial links in parallel.

The performance of medium access control (MAC) depends on both spatial locations and traffic patterns of wireless agents. In contrast to conventional MAC policies, we propose a MAC solution that adapts to the prevailing spatial and temporal opportunities. The proposed solution is based on a decentralized partially observable Markov decision process (DEC-POMDP), which is able to handle wireless network dynamics described by a Markov model. A DEC-POMDP takes both sensor noise and partial observations into account, and yields MAC policies that are optimal for the network dynamics model. The DEC-POMDP MAC policies can be optimized for a freely chosen goal, such as maximal throughput or minimal latency, with the same algorithm. We make approximate optimization efficient by exploiting problem structure: the policies are optimized by a factored DEC-POMDP method, yielding highly compact state machine representations for MAC policies. Experiments show that our approach yields higher throughput and lower latency than CSMA/CA based comparison methods adapted to the current wireless network configuration.

This paper focuses on perturbation-based distributed beamforming with 1-bit feedback for wireless amplify-and-forward relay networks. We propose to use multiplicative perturbations based on Givens rotations to adapt the beamforming weights while guaranteeing a sum power constraint for the relays. This perturbation scheme is shown to be computationally efficient and easy to design, thus allowing for low-complexity relay nodes. An adaptation of the Givens rotation angle allows to approach optimum performance arbitrarily close. Numerical simulations demonstrate noticeable performance gains over additive perturbation schemes that have been exclusively considered up to now.

Opportunistic spectrum access, where cognitive radio devices detect available unused radio channels and exploit them for communication, avoiding collisions with existing users of the channels, is a central topic of research for future wireless communication. When each device has limited resources to sense which channels are available, the task becomes a reinforcement learning problem that has been studied with partially observable Markov decision processes (POMDPs). However, current POMDP solutions are based on simplistic representations where channels are simply on/off (transmitting or idle). We show that more complicated Markov models where on/off states are part of complicated behavior of the channel owner (primary user) yield better POMDPs achieving more successful transmissions and less collisions.

Single-user, multiuser, and network MIMO performance is evaluated for downlink cellular networks with 12 antennas per site, sectorization, universal frequency reuse, scheduled packet-data, and a dense population of stationary users. Compared to a single-user MIMO baseline system with 3 sectors per site, network MIMO coordination is found to increase throughput by a factor of 1.8 with intra-site coordination among antennas belonging to the same cell site. Intra-site coordination performs almost as well as a highly sectorized system with 12 sectors per site. Increasing the coordination cluster size from 1 to 7 sites increases the throughput gain factor to 2.5.

This paper deals with distributed beamforming techniques for wireless networks with half-duplex amplify-and- forward relays. Existing schemes optimize the beamforming weights based on the assumption that channel state information (CSI) is available at the relays. We propose to use adaptive beamforming based on deterministic perturbations and limited feedback (1-bit) from the destination to the relays in order to avoid CSI at the relays. Two scalable perturbation schemes are considered and practical implementation aspects are addressed. Simulation results confirm that the proposed techniques closely approach optimum performance and have satisfactory tracking properties in time-varying environments.

This paper addresses distributed scheduling in a self-organizing wireless network, assuming partial orthogonality of multiple transmitters. Previous work suggests that the optimal scheduling scheme depends on the degree of orthogonality of the interfering users. A fair efficient distributed scheduling scheme can be defined using a synchronous game for highly orthogonal transmitters (with orthogonality factor less than 0.5) and using an asynchronous game based on one-at-a-time transmission for orthogonality factors above 0.5. The distributed game heuristic only requires local node level information but still achieves a sum of rates at least 90 % of that obtained using the better of proportional fair scheduling and simultaneous transmission for a wide range of orthogonality factors. Assuming a slowly varying Rayleigh channel, an asynchronous scheduling game based on one-at-a-time transmission achieves the same sum of rates as proportional fair scheduling but only requires local node-level information.

The 17 papers in this special issue focus on MIMO systems and applications: field experience, practical aspects, limitations and challenges. The papers are summarized here.

This paper studies scheduling in a time-varying wireless channel where different users may have different delay-requirements. The scheduling problem is formulated as a dynamic programming problem, taking into account both user-specific delay penalties and the time-varying channel state. A scheduling rule similar to the exponential rule is derived, applying dynamic delay-penalized optimization. Threshold-based scheduling optimizes the throughput-delay trade-off in a single-user memoryless channel. Modifications of the optimal single-user scheduling rule to a multiuser channel are studied both in a time-correlated and in a memoryless environment. Numerical experiments suggest that a simplified threshold derived for a memoryless setting performs well in a time-correlated channel. The applicability of the scheduling model to a virtual fading channel with relay nodes is discussed.

In this paper, we consider adaptive transmission for quasi-orthogonal space-time block codes employing 4 transmit antennas. Orthogonal space-time block codes provide full transmit diversity and are an efficient means to reduce the negative effect of fading channels on the wireless link performance. However, these codes cannot provide full symbol rate with more than two transmit antennas and orthogonality has thus to be sacrificed in order to achieve the high data-rates promised by MIMO systems. We propose a closed-loop mode for two quasi-orthogonal space-time block codes with symbol rate 2 that aims at reducing the performance degradation caused by the loss of orthogonality. The feedback scheme is based on partial channel state information available at the emitter. The performance of the adaptive modes with various detection techniques over a Rayleigh flat-fading channel is assessed by means of numerical simulations.

We consider non-orthogonal space-time codes in multipath fading channels. For this, both minimizing the effect of the self-interference induced by the non-orthogonal code itself, and mitigating the inter-symbol-interference induced by the channel need to be addressed. We approach the problem by analyzing the example of rate one linear space-time transmissions from four transmit antennas. The starting point is the non-orthogonal ABBA-code minimizing the self-interference, which is one of the rate one quasi-orthogonal codes for four antennas. In addition to applying time reversal on the part of a frame consisting of complex conjugated symbols, the non-orthogonal structure allows for multiple non-equivalent permutations of the transmission symbols. We observe that in addition to separating symbols and complex conjugated symbols, it is essential to interleave the symbols so that time epochs during which copies of the same symbols are transmitted, are far from each other

This paper studies the applicability of selected high rate open-loop space-time codes and closed-loop beamforming concepts for future wireless systems, such as WCDMA/HSDPA. Several spectrally efficient space-time codes are evaluated and their performance is compared to related closed-loop concepts, where a feedback link from the receiver is used to aid (downlink) beamforming. The comparison is based on schemes that attain 4bps/Hz spectral efficiency, where either two parallel QPSK streams or a single 16-QAM stream is transmitted using up to four transmit antennas, and one or two receive antennas.

Space-time block codes for providing transmit diversity in wireless communication systems are considered. Based on the principles of linearity and unitarity, a complete classification of linear codes is given in the case when the symbol constellations are complex, and the code is based on a square matrix or restriction of such by deleting columns (antennas). Maximal rate delay optimal codes are constructed within this category. The maximal rates allowed by linearity and unitarity fall off exponentially with the number of transmit antennas.

Transmission over multiple input multiple output (MIMO) radio channels is considered. When multiple Rx antennas are present, one may simultaneously increase the rate and improve performance by optimizing transmit diversity, using nonorthogonal space-time block codes. This improves the performance of MIMO systems considerably, especially when the number of Tx and Rx antennas is small. As an example, we consider a 2 Tx, 2 Rx system, and construct a 2×2 nonorthogonal space-time block code with symbol rate 2, which performs 2.5 dB better than the corresponding BLAST system at BER 10-3.

We investigate methods to improve the rate of complex modulation space-time block codes for more than two Tx antennas. For concreteness we concentrate on 4 Tx antennas, for which the maximal rate for an orthogonal space-time block code is 3/4. We devise multi-modulation schemes, which increase the rate to 7/8 or 1. In these codes, the orthogonality of QPSK modulation is sacrificed for some symbols, but the orthogonality of the underlying space-time block code is kept intact. An increase in rate leads to a decrease in performance. The same is true if the rate increase is effected by sacrificing the orthogonality of the space-time block code itself. To assess the capacity enhancement due to the rate increase, space-time block codes with different rates are concatenated with turbo codes, with a puncturing scheme guaranteeing the same effective code rates for the concatenated systems. It is shown that with a constant rate for the concatenated scheme, increasing the space-time block code rate indeed improves the performance of the concatenated scheme. Also, it is shown that sacrificing the orthogonality of the space-time block code itself is more effective than sacrificing the orthogonality of the modulation.

Space-time block codes providing transmit diversity for communication over multiple transmitter channels are considered. A scheme giving ad possible 4 antenna, 4 epoch space-time block codes is presented. The scheme generalizes to an arbitrary number of Tx antennas, optimizing the transmission delay and modulator rate when full diversity is required. For four transmit antennas, the antenna power spectrum of the space-time block code is optimized.