A wireless massive multiple-input multiple-output (MIMO) system entails a large number of base station antennas serving a much smaller number of users, with large gains in spectral efficiency and energy efficiency compared with the conventional MIMO technology. Until recently, it was believed that as the number of base station antennas tends to infinity, the performance of such systems is limited by directed inter-cellular interference caused by unavoidable re-use of training sequences (pilot contamination) by users in different cells. We devise a new concept of large-scale fading precoding (LSFP) that leads to the effective elimination of inter-cell interference. The main idea of LSFP is that base stations linearly combine messages aimed at users from different cells that re-use the same training sequence. Crucially, the combining coefficients depend only on the large-scale fading coefficients between the users and the base stations. These coefficients change slowly and their number does not depend on the number of base station antennas. Thus, the traffic between base stations stays constant even if the number of antennas tends to infinity. Furthermore, we derive a capacity lower bound for massive MIMO systems with LSFP and a finite number of base station antennas. In this regime, mitigation of all types of interference, not only the pilot contamination, is required. We consider optimal and suboptimal LSFP precodings that take into account all sources of interference. Our simulations results show that LSFP provides significant gain even for the case of moderate number of base station antennas.

To enable simultaneous transmit and receive operations on the same frequency band, also known as full duplex, on adjacent panels of a millimeter wave (mmWave) base station (BS), we benchmark the amount of leakage interference from the transmitter to the receiver. We construct a self-interference channel model consisting of line-of-sight (LOS) leakage and non-LOS (NLOS) reflections from nearby clusters. From simulations, we further show that due to antenna beamforming in mmWave systems, the LOS leakage interference is subdued, whereas the NLOS leakage is amplified. For some BS designs, the NLOS leakage is likely to be the dominant interference. It impacts system designs in two ways. First, the worst case interference can still saturate the receiver chain and would require analog and digital cancellation in addition to the isolation provided by antenna beamforming. Second, due to the delay associated with NLOS reflective components, the cancelers would need to accommodate a large dynamic delay.

This paper explores the use of selection diversity for interference alignment systems. Multiple distinct alignment modes exist for certain types of interference network. By selecting the best alignment mode to support the signal-to-noise rate (SNR) of the worst user, a diversity gain improvement can be observed. This paper shows that in a 3-user double-antenna interference channel, a system switching between two alignment modes achieves a diversity gain of 2 in the fixed-rate regime and the maximum degree-of-freedom (DoF) gain of 3 in the variable-rate regime. Consequently, we disprove a previous feasibility condition for diversity in alignment systems, which requires a trade-off between the DoF gain and the diversity gain.

Interference alignment (IA) was shown to be effective for interference management in improving transmission rate in terms of the degree of freedom (DoF) gain. On the other hand, orthogonal space-time block codes were widely used in point-to-point multiantenna channels to enhance transmission reliability in terms of the diversity gain. In this paper, we connect these two ideas, i.e., IA and space-time block coding, to improve the designs of alignment precoders for multiuser networks. Specifically, we consider the use of Alamouti codes for IA because of their rate-one transmission and achievability of full diversity in point-to-point systems. The Alamouti codes protect the desired link by introducing orthogonality between the two symbols in one Alamouti codeword, and create alignment at the interfering receiver. We show that the proposed alignment methods can maintain the maximum DoF gain and improve the ergodic mutual information in the long-term regime, while increasing the diversity gain to 2 in the short-term regime. The presented examples of interference networks have two antennas at each node and include the two-user X channel, the interfering multiaccess channel, and the interfering broadcast channel.

We explore the feasibility of linear interference alignment using finite signaling dimensions and symbol extensions. In the non-zero total intersection regime, we show that the number of sources is upperbounded by a function of channel diversity. The available diversity places a fundamental constraint on the number of signal spaces that can overlap at one destination (where they are undesired) while maintaining the resolvability of a subset of those signals (desired signals) at another destination. Specifically, the number of such signal spaces cannot be larger than the channel diversity. This is the diversity constraint that we identify in this paper. Not only is the proposed method applicable for X channels, interference channels, and their rank-deficient counterparts, it can also be used for both circular symmetric signaling (CSS) and asymmetric complex signaling (ACS) over a combination of frequency and MIMO channels with arbitrary symbol extensions.

This paper considers a scenario for multi-user multi-input multi-output (MIMO) communication systems when perfect channel state information at the transmitter (CSIT) is given while the equivalent channel state information at the receiver (CSIR) is not available. Such an assumption is valid for the downlink multi-user MIMO systems with linear precoders that depend on channels to all receivers. We propose a concept called dual systems with zero-forcing design based on the duality principle, originally proposed to relate the Gaussian multi-access channel (MAC) and the Gaussian broadcast channel (BC). For the K-user N×2 MIMO BC with N antennas at the transmitter and two antennas at each of the K receivers, we design a downlink interference cancellation (IC) transmission scheme by obtaining the dual of uplink MAC systems employing IC methods. The transmitter simultaneously sends K precoded Alamouti codes, one for each user. Each receiver zero-forces the unintended user's Alamouti codes and decouples its own data streams using two simple linear operations independent of CSIR. Analysis shows that the proposed scheme achieves a diversity gain of 2(N - K + 1) for equal energy constellations with short-term power and rate constraints. Power allocation among K users can also be performed to improve the array gain but not the diversity gain. Numerical results demonstrate that the bit error rate of the downlink IC scheme has a diversity gain improvement compared to the block diagonalization (BD) method, which requires global channel information at each node.

Alamouti codes have been widely recognized for their full-diversity performance, while they provide a symbol rate of one with symbol-by-symbol decoding complexity. To decode an Alamouti code, the receiver requires the channel state information (CSI), while the transmitter operates blindly without CSI. This paper investigates the dual scenario when CSI is available only at the transmitter and not at the receiver. Such an assumption is valid for time-division duplex (TDD) systems where the transmitter obtains the CSI from the reciprocal channel and the receiver does not want to estimate the channel because of complexity. We propose dual Alamouti codes that do not require CSI at the receiver and exploit transmit CSI to achieve the performance of full-diversity with rate-one and symbol-by-symbol decoding complexity. Moreover, the dual codes provide the same bit error rate (BER) performance as the original Alamouti codes for equal-energy constellations. The dual Alamouti codes have potential applications in TDD systems to save the cost of training and simplify receiver design.

Interference alignment increases transmission rate in terms of multiplexing gain for X channel. In this paper, we propose a fixed-rate transmission scheme over a two-user X channel where each of the two double-antenna transmitters has independent messages for each of the two double-antenna receivers. Each transmitter encodes symbols using Alamouti codes followed by beamformers that align interference at unintended receivers. The receiver removes the aligned interference and decouples symbols using interference cancellation followed by symbol-by-symbol decoding. Our analysis shows that the proposed scheme achieves a diversity gain of 2 at the maximum node-to-node symbol-rate of 2 over 3.

We study multi-user transmission and detection schemes for a multi-access relay network (MARN) with linear constraints at all nodes. In a (J, Ja, Ra, M) MARN, J sources, each equipped with Jaantennas, communicate to one M-antenna destination through one Ra-antenna relay. A new protocol called IC-Relay-TDMA is proposed which takes two phases. During the first phase, symbols of different sources are transmitted concurrently to the relay. At the relay, interference cancellation (IC) techniques, previously proposed for systems with direct transmission, are applied to decouple the information of different sources without decoding. During the second phase, symbols of different sources are forwarded to the destination in a time division multi-access (TDMA) fashion. At the destination, the maximum-likelihood (ML) decoding is performed source-by-source. The protocol of IC-Relay-TDMA requires the number of relay antennas no less than the number of sources, i.e., Ra≥ J. Through outage analysis, the achievable diversity gain of the proposed scheme is shown to be min {Ja(Ra- J + 1), RaM}. When M ≤ Ja(1- J-1/Ra), the proposed scheme achieves the maximum interference-free (int-free) diversity gain RaM. Since concurrent transmission is allowed during the first phase, compared to full TDMA transmission, the proposed scheme achieves the same diversity, but with a higher symbol rate.

This paper considers multi-access relay networks (MARNs) where two single-antenna users communicate to one N-antenna receiver via two hops of transmissions through one R-antenna relay. Nodes in the network are under two linear constraints. The relay linearly maps its received signal to generate its forwarding signal without decoding; the receiver has linear decoding complexity in the number of users. Since the relay-receiver link has more transmission paths than the link from each user to the relay, a two-step protocol, called TDMA-ICRec, is proposed. In the first step, both users timeshare the user-relay link. Linear combining is then performed at the relay to maximize the signal-to-noise-ratio (SNR) for each user. In the second step, the relay forwards both users' symbols concurrently to the receiver to enhance the transmission rate. At the receiver, an interference cancellation (IC) technique is used to decouple the users and ML decoding is conducted to recover each user's symbols. Two network scenarios are studied when the relay has two antennas and four antennas. Through analysis and simulation, when the receiver has more than two antennas, TDMA-ICRec achieves the same diversity as the full-TDMA-DSTC interference-free (int-free) scheme, yet with higher symbol rate.

This paper is on interference cancellation (IC) schemes for a two-user relay network where users are allowed to communicate simultaneously. The considered networks have one double-antenna half-duplex relay, single-antenna receiver but three scenarios on both users: single-antenna, double-antenna, and four-antenna. We apply the IC scheme, which was originally proposed for multi-antenna multi-user direct communication systems, to multi-user relay networks and propose a protocol called IC-Relay-TDMA, in which the relay cancels user interference and then amplifies and forwards the interference-free (int-free) user information to the receiver in TDMA. The maximum likelihood (ML) decoding at the receiver can be conducted symbol by symbol for both networks with single-antenna users and double-antenna users. Compared to the full TDMA scheme in which each user is allocated different time slots from end to end to avoid interference, IC-Relay-TDMA achieves the same diversity with a higher symbol rate when both users have two or four antennas.

Recent research has shown that differential transmission (DT) can be applied to STBC systems so that the receiver does not need channel state information (CSI) when compared to coherent detector. In this paper, we propose a generalized DT (GDT) scheme, which has a frame structure consisting of two parts, reference-block part and normal-block part. In contrast to the conventional DT (CDT) scheme, where each transmitted block is differentially encoded based on previous transmitted block, each GDT block in both of the two parts is differentially encoded based on previous reference block. It is shown that allocating more transmit power to the reference-block part leads to a more reliable reference for the differential detector. We discuss about the optimal power allocation between the reference-block part and the normal-block part. Simulation results show that the proposed GDT scheme achieves significant performance improvement over CDT and such performance can be very close to that of the coherent detection.

Distributed space time coding can achieve full spatial diversity in wireless relay networks. However, it requires accurate symbol-level synchronization and priori coordination between cooperative relay terminals, which is difficult to implement in distributed networks. In this paper, we propose a novel scheme based on interleaving to achieve cooperative diversity in both synchronous and asynchronous networks with little protocol overhead. A low complexity iterative detection algorithm is also proposed to combine signals from different relays at the receiver. The simulation results demonstrate the comparable performance to space-time codes based cooperative schemes which require perfect synchronization and coordination.

A novel algorithm is proposed in this literature to estimate the transmit and receive antenna correlation matrix , RTand RR, in MIMO system. Theoretical analysis indicates that the estimation performance is improved when more training number is used. Interestingly, the performance of the estimation of antenna correlation matrix at one side is highly correlated to the correlation degree of the antennas at the other side, but not the correlation degree of itself. Moreover, the relationship between estimation performance and antenna numbers is also presented. Simulation results show that mean error of this algorithm is generally below 0.05 when 200 training symbols are used.