Downlink Channel Capacity Research Articles

Uplink and Downlink Channel Capacity of Massive MIMO Enabled UAV Communications Links

Enhanced communication support to UAV networks can be achieved by integrating UAVs into existing cellular networks as aerial users. But there are many challenges and inadequacies in integrating UAVs with cellular networks. By utilizing multiple antennas at ground base stations, these inadequacies of existing cellular networks can be mitigated. With the use of massive MIMO technology, wherein cellular base stations are mounted with hundreds of antennas, the performance of UAV communication links gets significantly improved. In this article, we present performance evaluation of massive MIMO enabled UAV communication links, by initially covering UAV cellular communications along with its potential benefits and challenges. We then carry out the performance evaluation of UAV communication links by using basic multiple antenna techniques, shortcomings of using point to point MIMO and Multi user MIMO (MU-MIMO). Lastly we derive uplink and downlink channel capacity expressions for evaluating massive MIMO enabled UAV communication links along with few numerical results.

Performance Analysis of Spatially Correlated Cylindrical Antenna Array for 3D Massive MIMO

The spectral efficiency of three-dimensional (3D) massive MIMO is impaired due to the spatial correlation (SC) between closed-spaced elements of antenna arrays (AAs). This paper investigates the impacts of arbitrary Q-power variations of the Power Cosine distribution for a precise characterization of the SC. This is done by presenting mathematical expressions characterizing the SC of cylindrical antenna array (CAA) regarding Student's -t, Von Mises and Cosine distributions, for the direction of arrival (DoA) of the electromagnetic waves impinging on the receiving antenna. Unlike the Student's -t and Von Mises distribution, our outcome recorded huge mismatch between theoretical and simulation correlation coefficients for arbitrary Q-power values above ten (10) which also reduced the downlink channel capacity. The practical relevance is that even though higher Q-power values are the key parameters for system performance, meticulous selection of arbitrary Q-power values above ten (10) is essential to minimize errors regarding channel modelling.

Massive MIMO with Transceiver Hardware Impairments: Performance Analysis and Phase Noise Error Minimization

In this paper, we investigate the impact of hardware impairments (HWIs) on the performance of a downlink massive MIMO system. We consider a single-cell system with maximum ratio transmission (MRT) as precoding scheme, and with all the HWIs characteristics such as phase noise, distortion noise, and amplified thermal noise. Based on the system model, we derive closed-form expressions for a typical user data rate under two scenarios: when a common local oscillator (CLO) is used at the base station and when separated oscillators (SLOs) are used. We also derive closed-form expressions for the downlink transmit power required for some desired per-user data rate under each scenario. Compared to the conventional system with ideal transceiver hardware, our results show that impairments of hardware make a finite upper limit on the user’s downlink channel capacity; and as the number of base station antennas grows large, it is only the hardware impairments at the users that mainly limit the capacity. Our results also show that SLOs configuration provides higher data rate than CLO at the price of higher power consumption. An approach to minimize the effect of the hardware impairments on the system performance is also proposed in the paper. In our approach, we show that by reducing the cell size, the effect of accumulated phase noise during channel estimation time is minimized and hence the user capacity is increased, and the downlink transmit power is decreased.

A Simple and Effective Approach for Transmit Antenna Selection in Multiuser Massive MIMO Leveraging Submodularity

Massive MIMO systems are expected to enable great improvements in spectral and energy efficiency. Realizing these benefits in practice, however, is hindered by the cost and complexity of implementing large-scale antenna systems. A potential solution is to use transmit antenna selection for reducing the number of radio-frequency (RF) chains at the base station. In this paper, we consider the NP-hard discrete optimization problem of performing transmit antenna selection in the downlink of a single cell, multiuser massive MIMO system by maximizing the downlink sum-rate capacity with fixed user power allocation subject to various RF switching constraints. Whereas prior work has focused on using convex relaxation based schemes, which lack theoretical performance guarantees and can be computationally demanding, we adopt a very different approach. We establish that the objective function of this antenna selection problem is monotone and satisfies an important property known as submodularity , while the RF switching constraints are expressible as the independent sets of a matroid. This implies that a simple greedy algorithm can be used to guarantee a constant-factor approximation for all problem instances. Simulations indicate that greedy selection yields a near-optimal solution in practice and captures a significant fraction of the total downlink channel capacity at substantially lower complexity relative to convex relaxation based approaches, even with very few RF chains. This paves the way for substantial reduction in hardware complexity of massive MIMO systems while using very simple algorithms.

Ergodic Channel Capacity Analysis of Downlink in the Hybrid Satellite-Terrestrial Cooperative System

In this paper, the ergodic channel capacity of the downlink is analyzed for a hybrid satellite-terrestrial cooperative system, which consists of a satellite (the source), a mobile terminal (the destination), and several gap fillers (the relays) located at the ground. The links between the satellite and the relays and the link between the satellite and the destination experience independent shadowed Rician fading, and the links between the relays and the destination experience Rayleigh fading. The maximal ratio combining technique is used at the destination to combine the direct signal received from the satellite and the relayed signals from relays with different cooperative protocols, namely amplify-and-forward (AF) and decode-and-forward (DF). The moment generating function (MGF)-based approach is adopted to derive the closed-form expressions of the ergodic downlink channel capacity of the hybrid satellite-terrestrial cooperative system. The numerical results are compared with Monte Carlo simulations and numerical results calculated with the existing analytical expressions. Comparison results show that the analytical expression derived with the MGF-based approach can achieve a higher accuracy in the low signal-to-noise ratio (SNR) regime for the single relay scenario, and significantly reduce the computational complexity with a little loss of accuracy for multiple relays scenario. On the other hand, the ergodic downlink channel capacity of the hybrid satellite-terrestrial DF cooperative system is generally higher than that of the AF cooperative system. Moreover, the ergodic channel capacity of the hybrid satellite-terrestrial cooperative system decreases as the number of participating relays increases, which can be overcome using the best relay selection strategy. In addition, the ergodic downlink channel capacity of the hybrid satellite-terrestrial cooperative system increases when the channel condition of the link between the satellite and the relay goes better, and is larger than that of the no relay land mobile satellite system when the transmitted SNR is below a certain value.

Full-Duplex Meets Multiuser MIMO: Comparisons and Analysis

Efficient utilization of wireless communication resources and increases in the capacity of the communication networks have become very crucial claims for next-generation cellular networks. Full-duplex (FD) is a very promising technique that allows for the efficient use of wireless communication resources, given that the self-interference level can be suppressed to an acceptable level. In this paper, we consider a simple FD communication system, consisting of one FD access point (AP) and two half-duplex (HD) mobile users, and investigate when it is more advantageous for the communication network to operate in FD or HD multiuser multiple-input multiple-output (MIMO) mode. Since FD transmission is degraded by self-interference while HD suffers from spatial correlation between MIMO antennas, which causes rate loss, we study the effect of both the self-interference cancelation parameter at the FD AP and the mutual distance between the HD users on FD performances and the effect of the spatial correlation coefficient on HD MIMO. Afterward, a switching criterion is proposed, which chooses the operation mode that maximizes the downlink channel capacity while maintaining the uplink channel capacity at a certain level. Subsequently, based on our study of the system's parameters that affect the performance of both FD and HD, theoretical thresholds for these parameters are derived. Finally, numerical analysis is presented, verifying the validity of the optimization problem solution and the derived thresholds.

Robust Non-linear Precoder for Multiuser MISO Systems Based on Delay and Channel Quantization

In multiuser multiple-input single-output (MISO) systems, non-linear precoder is able to achieve the theoretical sum capacity of downlink channel with perfect channel state information (CSI). However, the perfect CSI is not available at the transmitter in practical system, especially in frequency division duplex (FDD) system where the imperfect CSI is the delayed, quantized channel direction information relayed back from the receiver through a dedicated feedback channel. So the performance of conventional non-linear precoder degrades significantly. In this paper, a robust non-linear Tomlinson---Harashima precoding (THP) based on sum mean squared error (SMSE) minimization for the downlink of multiuser MISO FDD systems is proposed. The proposed precoder is robust to the channel uncertainties arising from channel delay and quantization error. Furthermore, an improved non-linear THP with channel magnitude information (CMI) consideration is introduced to compensate the instantaneous CMI shortage at the transmitter. Additionally, the computational complexity of both proposed precoders can be reduced remarkably by Cholesky factorization with symmetric permutation. Simulation results demonstrate the improvement in bit error ratio performance and illustrate the SMSE performance of the proposed algorithms compared with conventional THP with perfect CSI in the literature.

On Cellular MIMO Channel Capacity

To increase the transmission rate without bandwidth expansion, the multiple-input multiple-output (MIMO) technique has recently been attracting much attention. The MIMO channel capacity in a cellular system is affected by the interference from neighboring co-channel cells. In this paper, we introduce the cellular channel capacity and evaluate its outage capacity, taking into account the frequency-reuse factor, path loss exponent, standard deviation of shadowing loss, and transmission power of a base station (BS). Furthermore, we compare the cellular MIMO downlink channel capacity with those of other multi-antenna transmission techniques such as single-input multiple-output (SIMO) and space-time block coded multiple-input single-output (STBC-MISO). We show that the optimum frequency-reuse factor F that maximizes 10%-outage capacity is 3 and both 50%- and 90%-outage capacities is 1 irrespective of the type of multi-antenna transmission technique, where q%-outage capacity is defined as the channel capacity that gives an outage probability of q%. We also show that the cellular MIMO channel capacity is always higher than those of SIMO and STBC-MISO.

On antijamming in general CDMA systems-part I: multiuser capacity analysis

The capacity of a general multiuser CDMA jamming channel is analyzed when the receiver has or lacks jammer state information. Analyses are carried out for noncooperative and cooperative users in uplink and downlink static and Nakagami fading channels. The results are applied to a versatile multiaccess channel model which is based on the time-bandwidth dimensionality and is considered for the user capacity analysis of a variety of multiuser spread spectrum systems contaminated by jamming. In this model, different users are distinguished by signatures of either direct sequence or hopping type. It is found that the jammer should spread its energy evenly over all degrees of freedom in order to minimize the average capacity. Also, the capacity behavior appears to be dominated more by knowledge of jammer state information than by the effects of fading. The capacity of downlink channels in cooperative schemes is found to be more sensitive to changes in fading severity, compared with the capacity of other kinds of channels.

Interference mitigation using transmitter filters in CDMA systems

We consider the use of transmitter filters as pre-equalizers at the base station to mitigate the interference in code-division multiple-access (CDMA) systems over a multipath fading channel. It is shown that the interference at the receiver of the mobile station can be mitigated by the transmitter filters and, thereby, the capacity of downlink channel can be improved. The minimum variance distortionless response (MVDR) beamforming technique in antenna arrays has been utilized to find the coefficients of the interference mitigation (IM) transmitter filters. To design IM transmitter filters, we assume that the channel information is available. This can be true in the time-division duplexing (TDD) mode because of the channel reciprocity. In the frequency-division duplexing mode, the downlink channel impulse responses (CIRs) shall be be transmitted to the base station. To transmit the CIRs, we consider an encoding method. In addition, to compensate inherent feedback delay, the channel prediction is also utilized.

Multi-user downlink beamforming with multiple transmit and receive antennas

Downlink multiple-input, multiple-output (MIMO) systems assuming a large number of base station antennas, a small number of mobile station antennas, and rich-scattering, quasi-stationary, and flat-fading channel environments are considered. A MIMO space-division multiple access (SDMA) technique is proposed which enables concurrent transmission of multiple data streams to each multiple mobile station. The proposed method can achieve much higher downlink channel capacity than conventional MIMO techniques.