Emerging new applications, such as Internet of Things (IoT), gigabit wireless connectivity, tactile internet, and many more are expected to impose new and diverse requirements on the design of the fifth generation (5G) of cellular communication systems. In recent years, many alternative multicarrier waveforms are being investigated with the aim to identify to what extent it would be possible to better address these diverse requirements compared to Cyclic Prefix (CP) OFDM, which is currently adopted by LTE/LTE-Advanced. However, most of these investigations considered in many research activities have focused on theoretical considerations or simulations and mainly with emphasis on only one alternative waveform. In this paper, we present ongoing work based on measurement results where real hardware experiments are conducted to investigate the performance of three waveform families: CP-OFDM, filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) and universal-filtered OFDM (UF-OFDM). FBMC/OQAM has the benefit of very low sidelobes leading to less inter-carrier interference in asynchronous and high mobility scenarios, whereas the typically long filter leads to low efficiency for short blocks and the use of OQAM makes it nontrivial to adopt certain schemes well-developed for CP-OFDM. UF-OFDM combines the benefits of CP-OFDM with the advantage of significantly lower out-of-band radiation while avoiding the drawbacks of FBMC/OQAM. In this paper we compare the performance of these alternative waveforms under several 5G targeted scenarios.

This article presents comprehensive comparisons of interleave division multiple access (IDMA) and direct sequence code division multiple access (DS-CDMA) in terms of performance and complexity assuming iterative multiuser detection. IDMA can be seen as a special case of DS-CDMA with spreading gain of one using very low rate code and user-specific interleavers for user separation. We focus on three suboptimum linear detectors: minimum mean square error (MMSE), rake (or matched filter), and soft-rake detectors from practical concerns. We analytically prove that the three detectors are equivalent for asynchronous users of IDMA on frequency flat channels for complex modulation alphabets. Such equivalence has been shown only for binary phase shift keying (BPSK) in the literature. The equivalence guarantees the MMSE solution for IDMA without computationally expensive matrix inversions or matrix-vector multiplications. This is generally not the case for DS-CDMA since DS-CDMA is sensitive to user asynchronism. We also discuss complexity aspects when the MMSE detector is used where we focus on essential differences in complexity between IDMA and DS-CDMA, instead of discussing particular complexity reduction techniques. Computer simulations are performed in various scenarios and the performance is analyzed by bit error rate simulations as well as by extrinsic information transfer (EXIT) charts. The analysis reveals the advantages of IDMA over DS-CDMA in terms of performance and complexity under practical considerations, particularly in highly user loaded scenarios.

This paper investigates multiple input multiple output (MIMO) transmission techniques based on realistic assumptions on feedback of channel state information. We consider three conventional techniques as the baseline: 3GPP long-term evolution (LTE) single user MIMO (SU-MIMO) based on implicit channel feedback, zero-forcing multiuser MIMO (ZF MU-MIMO) based on explicit channel feedback, and ZF MU-MIMO based on implicit channel feedback. SU-MIMO may not be able to exploit the full spatial dimension of the downlink MIMO channel. ZF MU-MIMO has the potential to improve the spectral efficiency, but the explicit channel feedback is not compatible with implicit feedback whereas implicit based ZF MU-MIMO is limited by performance and also the commonly assumed rank restriction makes it impossible to perform dynamic switching of SU/MU MIMO transmission. We propose a new hybrid scheme which enables such dynamic switching of SU/MU MIMO transmission by allowing UE to feed back the implicit channel information without any rank restriction. Computer simulation results show the benefits of the new hybrid scheme, which can properly switch to the better transmission mode in various correlation scenarios.

In this paper we propose a new efficient algorithm for finding good codebook targeting multiple input multiple output (MIMO) precoding transmission. The proposed algorithm is constrained optimization that satisfies a set of design constraints such as unitary, constant modulus, constrained alphabet, and nested. These constraints aim at practically important properties such as low complexity and low peak-to-average-power-ratio and have been carefully taken into account in the codebook design at 3GPP. Our algorithm is based on rotations of the discrete Fourier transform (DFT) matrix and attempts to maximize the minimum chordal distance between any two precoders in the codebook in each rank. The algorithm works iteratively: at each step it identifies a bottleneck precoder in terms of minimum chordal distance and then finds a better precoder replacing the bottleneck precoder. Simulation results illustrate that the codebook found by the proposed algorithm performs better than conventional codebooks in both single user MIMO as well as in dynamic switching of single user and multi user MIMO transmission.

Due to the limited feedback amount, the codebook and feedback design is very important. In order to achieve better tradeoff between overhead and performance, the double codebook structures are proposed and discussed, where a final precoder W is determined by two precoders W1and W2, which are selected from two different codebooks. In this paper, we give an overview of several double codebook structures and evaluate them based on system level simulations (SLS). We consider two frameworks to determine a final precoder: W1W2and W2W1where W1and W2track long-term/wideband and short term/subband channel properties, respectively. It is shown that under 4Tx assumption, double codebooks achieve gains compared with Rel.8 codebook when considering the multi-user (MU) MIMO and do not suffer in the case of single-user (SU) MIMO. Both W1W2and W2W1provide nearly the same gains over that for the Rel. 8 codebook with uniform linear array (ULA) deployment. Furthermore, W1W2framework performs better than W2W1framework with cross-polarized array (CPA) deployment.

This paper addresses the fairness of Medium Access Control (MAC) protocols that are capable of handling interference on the physical layer to a varying extent. The variation addressed is thereby not based on different physical layer techniques applied, but on differing attribute levels of the same physical layer technique. This can e.g., be a varying number of antennas at each node in case of Multiple Input Multiple Output (MIMO). Fair contention resolution in such protocols becomes a trade off between on the one hand avoiding a permanent overwhelming of low level devices with interference, and on the other hand blocking of all spatial reuse by these low level devices. The latter one would restrain the better equipped terminals from taking advantage of their excellent equipment. In order to balance both aspects, we propose the Fair and Adaptive Contention Resolution Algorithm (FACRA). The algorithm is fully distributed and requires only the exchange of a single parameter value between competing terminals. We show by means of simulations that the algorithm significantly increases the spatial reuse by avoiding frequent blocking by low level devices, without disadvantaging these devices significantly.

In wireless ad hoc networks, multiple access interference is the limiting factor for the overall system performance. The lack of any central control unit necessitates a careful joint design of both, the physical and the MAC layer. Two promising technologies that address this problem are power control based cross-layer design and multiuser detection based cross-layer design. While approaches in the first category suppress the interference by power control at the transmitter side, approaches in the second cancel interference at the receiver with a technique called multiuser detection. In this paper, we compare two cross-layer approaches that are representative of power control based cross-layer design and multiuser detection based cross-layer design, respectively. We separately investigate both, the solely PHY performance of the proposed solutions, and the gains achieved by the appropriate MAC design, in terms of overall throughput. Thus, we gain insight into the interaction between both layers that reflects the quality of the cross-layer designs. We test the performance in medium as well as high traffic load scenarios. Based on the obtained results, we show that applying power control in cross-layer designs can in dense traffic scenarios even lead to a worse performance than no cross-layer design at all. Thus, we conclude that in general MUD-based cross-layer design is the better alternative.

We investigate enhanced downlink multiuser multiple input multiple output (MU-MIMO) transmission schemes for 3GPP LTE-Advanced based on system level simulations. As compared to Rel. 8 LTE single user MIMO (SU-MIMO), MU-MIMO has the potential to better exploit the spatial dimension of MIMO channels in some scenarios. Although Rel. 8 LTE defines a simple MU-MIMO mode, more advanced MU-MIMO techniques are currently discussed towards Rel. 10 LTE-Advanced to further improve the spectral efficiency. In this paper, we focus on zero-forcing (ZF) MU-MIMO precoding with an explicit channel quantization mechanism called channel vector quantization (CVQ). Moreover, we consider two different criteria of CVQ at the receiver: either minimizing Euclidean distance or maximizing the signal to interference plus noise power ratio (SINR). A number of system level simulations reveal significant potential gains of ZF MU-MIMO as compared to those of Rel. 8 LTE SU/MU-MIMO in terms of both cell throughput and cell-edge user throughput when the spatial correlations at the base station (eNB) are relatively high. The potential gain of ZF MU-MIMO is shown to be further improved when the CVQ is based on the SINR maximization criterion instead of Euclidean distance minimization.

This paper presents comprehensive comparisons of interleave division multiple access (IDMA) and direct sequence code division multiple access (DS-CDMA) in terms of performance and complexity using iterative multiuser detection technique, where we restrict ourself to three suboptimum linear detectors: minimum mean square error (MMSE), rake (or matched filter), and soft-rake detectors from practical concerns. We first analytically compare these detectors, which are found to be equivalent for IDMA with asynchronous users on flat channels, whereas this does not hold for DS-CDMA, which is sensitive to user asynchronism. This implies that, on flat channels, simple detector suffices to get the MMSE output for IDMA while DS-CDMA requires more complexity such as computing matrix inversions. We also discuss several aspects of complexities for IDMA and DSCDMA when the MMSE detector is used. Computer simulations are performed in various scenarios and the performance is analyzed by bit error rate as well as by extrinsic information transfer chart. The analysis reveals some advantages of IDMA over DS-CDMA, particularly in highly user loaded scenarios.

We propose a new differential recursive update algorithm, which significantly reduces the complexity of iterative minimum mean square error (MMSE) detection and decoding techniques for spread spectrum communication systems. Although the MMSE detector is a less complex suboptimum solution comparing to the optimum one, it is still complex for iterative systems because a large number of matrix inversions need to be computed over different users, symbols, and iterations. The new algorithm enables a smooth transition from the MMSE detection with full complexity to the simple matched filter solution over iterations, thereby achieving complexity reduction. We show by means of computer simulations that the complexity is drastically reduced by the newly proposed algorithm at the expense of increased memory requirements.

Utilizing multiple interleavers recently attracted increasing attention in many research areas. Interleaver generation should be simple in order to avoid huge memory requirements for storing interleaving patterns. We propose to derive multiple interleavers by cyclically shifting and self-interleaving a common mother interleaver in a few steps. Our focus is on the good user separation in interleave division multiple access systems. The proposed method may also find other application areas such as multi-dimensional concatenated codes.

Conventional medium access control protocols are designed to avoid simultaneous transmissions, based on a simple collision model in the underlying physical layer. Recently, strong physical layer capabilities enabled by multiuser detection techniques have been studied in connection with simple medium access control protocols. We think that neither of these extreme approaches is optimum, in particular for general scenarios where network nodes with different signal processing capabilities coexist. Instead of dealing with interferences in either of the two layers alone, both medium access control and physical layer functionalities should cooperate and complement each other. We discuss several key aspects for designing such a new type of protocol, especially with an emphasis on iterative multiuser detection, which can provide a good tradeoff between performance and complexity. We propose a new protocol which satisfies these key aspects. We analyze its throughput bound and also perform numerical simulations. The results evidence excellent throughput improvements.

Interleave division multiple access (IDMA) recently attracted many research activities because of its excellent performance despite its reasonable low complexity. The low complexity is usually realized by the multiuser detector that applies an approximation similar to the rake receiver for CDMA systems. So far, this type of detector has been most frequently considered in IDMA literature. In this paper we investigate the performance of IDMA based on linearminimummeansquareerror(MMSE) detection. The MMSE detector is more complex than the rake-like approximation. At the price of the complexity, however, it is shown that the MMSE detector brings several advantages over the rake-like approach such as the superior performance on channels with spectrally poor characteristics, effective iterative processing for lower SNR values, faster convergence and therefore shorter decoding delays, and better performance for short block length. We also confirm that the complexity can be drastically reduced by the low rank approximation of the MMSE filter by its multistage representation without compromising on the performance.

Concatenated zigzag codes with iterative decoding show excellent performance close to the performance of turbo codes while having significantly lower decoding complexity. A strong code is built by concatenation of several weak high rate codes which encode differently interleaved versions of the data sequence. The need for multiple interleavers implies a new problem in interleaver design: Multiple interleavers have to be constructed which allow good performance in iterative decoding and are mutually random. Furthermore, for practical reasons, they should be constructed either from a simple equation or by simple permutations of a common mother interleaver in order to minimize the required memory for storage of the permutation pattern. We propose two simple methods for interleaver construction which meet those requirements. Our first proposal is to obtain multiple interleavers by cyclic shifts and self-interleaving from a common mother interleaver. The second proposal uses modified read out processes in intermediate steps during construction of the UMTS turbo code internal interleaver. Particularly the proposed cyclic shifted interleavers are shown to provide superior performance with zigzag codes compared to other interleaving schemes such as congruential interleavers.

We propose a simple strategy to generate multiple interleavers. As suggested in literature, using user-distinct interleavers is an effective means to separate multiple users, if applied in addition to user-specific spreading codes in code division multiple access (CDMA). In this paper we particularly focus on interleave division multiple access (IDMA), which has a close relation to CDMA, but users are separated only by user- distinct interleavers. Hence, multiple interleavers are essential system components for IDMA. Despite its importance, usually interleavers for such systems are randomly chosen and there are only few papers on the generation of multiple interleavers. We show that the conventional multiple interleavers proposed for CDMA are not sufficient for the user separation in IDMA. Moreover, in order to minimize memory requirements and signaling overheads to store and exchange interleavers, a simple interleaver construction rule is desirable in practical systems. Therefore, we propose to derive multiple interleavers from a single interleaver, common for all users, with only a few user- distinct cyclic shifts. Although this simple design is empirical and no optimality is claimed, simulation results show sufficiently good performance. We also proposed a simple procedure to exchange the information of interleavers.

Interleave Division Multiple Access (IDMA) is a multiple access scheme similar to CDMA and it relies on an iterative ("turbo") multiuser detection and decoding technique. IDMA applies a very low rate code which is typically realized by a convolutional code followed by a simple repetition code. We propose to limit the decoding of the convolutional code at the receiver aiming at complexity reduction. Although the transmitted bits cannot be recovered without decoding the convolutional code, decoding only the repetition code already gives some improvements of soft information to the next ("turbo") iteration. We show that such a simple scheme can be of advantage of IDMA against CDMA to achieve significant complexity reduction with graceful performance degradation. We also provide analyses to answer when it makes sense to apply the simple strategy.

Nonlinear minimum mean square error Tomlinson-Harashima precoding considered in this paper is an attractive solution for a scenario where a transmitter serves spatially separated receivers and no cooperation among them is possible. Unfortunately, the large performance gain against linear precoding comes along with significantly higher complexity than linear filters in the case of a large number of receivers. We show that superior performance of the nonlinear minimum mean square error Tomlinson-Harashima precoding can be obtained with complexity equivalent to linear precoding. Our proposed algorithm reduces the complexity by a factor of N/sub R/ which is the number of receivers.

This paper provides comprehensive comparison of the iterative multiuser detection techniques for code devision multiple access (CDMA) and interleave division multiple access (IDMA). We investigate the performance in various scenarios such as user-asynchronism, multipath channels, near-far problem, and overloaded scenarios. We develop our system model which is general enough to take into account the above mentioned scenarios as well as being capable of incorporating aspects relevant to these access schemes. We provide formal descriptions of both schemes using our system model that illuminates the similarities and differences of the two different schemes. Computer simulations are performed in a variety of scenarios. It is observed that IDMA performs better than or as good as CDMA despite its simplicity.

Enormous capacity advantage can be achieved on a flat MIMO channel compared to single antenna systems. V-BLAST was proposed to obtain such capacity advantage with low complexity. V-BLAST, however, requires multiple matrix (pseudo) inversions that are still computationally intensive for a large number of antennas. Much research has been attracted to reducing the complexity. Our contribution is to show that the MMSE block decision-feedback equalizer equivalent to the MMSE V-BLAST can be calculated via Cholesky factorization of the error covariance matrix with symmetric permutation. Forward and backward filters as well as detection order are jointly optimized with significantly reduced complexity. Simulation results show that MMSE V-BLAST performance can be achieved by the proposed scheme.

This paper addresses the severe performance degradation for OFDM systems in case of long channel delays exceeding a guard interval. Such long delays can be seen as an interference source on one hand and also as a valuable frequency diversity source on the other hand. Instead of simply suppressing such long delays, e.g. by beamforming at transmitter, the newly proposed preceding scheme attempts to find the optimum trade-off between the interference suppression and the diversity gain for the long delays. The optimization problem is formulated exploiting only long-term channel state information instead of the instantaneous one and it is solved based on the minimum mean square error criterion with a constraint on the transmit power. Computer simulation results show that the proposed scheme outperforms the conventional beamforming scheme.

We develop a new linear space-time precoding scheme for OFDM systems based on channel state information at the transmitter. The filter is derived for frequency-selective MlSO as well as for SISO channels using a minimum mean square error criterion with a constraint on transmit power. The guard interval of the resulting transmitted signal is no longer cyclic extension. Drawbacks of OFDM systems such as severe performance degradation for long channel delays exceeding the guard interval and vulnerability to possible nulls of the channel frequency response are addressed. Unlike existing equalizer variants at the receiver, our proposal combats these drawbacks by introducing computational complexity only at the transmitter while the receiver is kept simple as it is an advantage of OFDM systems against single carrier systems. Performance improvement has been confirmed by computer simulations.

We present space-time transmit filters for FDD DS-CDMA systems based on partial channel state information, i.e. we do not take into account the channel coefficients. Although the FDD transmit zero-forcing filter seems to be the most intuitive approach, we focus on the FDD transmit matched filter and the FDD transmit Wiener filter. Similarly to the respective receive filters and TDD transmit filters, the FDD matched filter maximizes the desired signal portion at the receiver and is optimum for low signal-to-noise-ratio scenarios, whereas the FDD transmit Wiener filter takes into account the noise power at the receiver and is therefore able to find an optimum trade-off between signal maximization and interference suppression. Additionally, we show that the FDD transmit matched filter is a type of eigenbeamforming. The simulation results reveal the excellent performance of the two FDD transmit filters. The FDD transmit Wiener filter even outperforms the TDD transmit matched filter for high signal-to-noise-ratio which is based on the instantaneous channel properties.