Mahbas, A., Zhu, H. and Wang, J. (2019). Impact of Small Cells Overlapping on Mobility Management. IEEE Transactions on Wireless Communications [Online]. Available at: http://dx.doi.org/10.1109/TWC.2018.2889465.
The mobility management will be more complex and will have a great impact on the quality of service (QoS) in the future cellular networks, as these networks will have to handle a huge number of user equipment (UEs) and their frequent handoffs due to very dense short-footage small cells. This paper presents a framework to model and derive the coverage of small cells, the cell sojourn time and the handoff rate in multi-tier small cell networks. The distribution of the small cells around a reference UE’s path is studied by taking into consideration
the overlaps among the small cells. Two types of handoff rates are introduced to estimate the load managed by different cells, where inter-frequency handoff (IRH) rate and intra-frequency handoff (IAH) rate represent the fraction of handoffs managed by the first tier and the other tiers, respectively. Our analysis shows that ignoring the overlaps among the small cells affects the accuracy of the results ignificantly. The simulation results validate the accuracy of the analytical results and also show the impact of different parameters such as the small cell density, the number of tiers and the size of the small cells on the small cell sojourn time, the macro cell sojourn time and the handoff rate.
Nair, M., Wang, J., Leiba, Y., Zhu, H., Gomes, N. and Wang, J. (2018). Exploiting Low Complexity Beam Allocation in Multi-User Switched Beam Millimeter Wave Systems. IEEE Access [Online]:1-11. Available at: https://doi.org/10.1109/ACCESS.2018.2887003.
Switched-beam systems offer a promising solution for realizing multi-user communications at
millimeter wave (mmWave) frequencies. A low-complexity beam allocation (LBA) algorithm has been
proposed to solve the challenging problem of maximizing sum data-rates. However, there are practical
limitations in mmWave systems, such as restrictions in the number of available radio frequency (RF)
transceiver chains at the base station (BS), sensitivity to sidelobe interference and the beam generation
techniques. In this paper, using generalized beam-patterns, we present the maximum sum data-rates
achievable in switched-beam mmWave systems compared to fixed-beam systems by applying LBA. Then,
the impact on maximum sum data rates of actual beam-patterns, obtained from a practical mmWave lens
antenna, which have higher and non-uniform sidelobes compared to the theoretical beams, is assessed.
Finally, as a guide for practical wireless system design, benchmarks are established for relative sidelobe levels
that provide acceptable sum data-rate performance when considering generalized beam patterns.
Kai, Y., Wang, J., Zhu, H. and Wang, J. (2018). Resource Allocation and Performance Analysis of Cellular-assisted OFDMA Device-to-Device Communications. IEEE Transactions on Wireless Communications [Online]. Available at: http://dx.doi.org/10.1109/TWC.2018.2880956.
Resource allocation of cellular-assisted device-todevice (D2D) communication is very challenging when frequency reuse is considered among multiple D2D pairs within a cell, as intense inter D2D interference is difficult to tackle and generally causes extremely large signaling overhead for channel state information (CSI) acquisition. In this paper, a novel resource allocation framework for cellular-assisted D2D communication is developed with low signaling overhead while maintaining high system capacity. By utilizing the spatial dispersion property
of D2D pairs, a geography-based sub-cell division strategy is proposed to divide the cell into multiple sub-cells and D2D pairs within one sub-cell are formed into one group. Then, sub-cell resource allocation is performed independently among sub-cells without the need of any prior knowledge of inter D2D interference. Under the proposed resource allocation framework, a tractable approximation for the inter D2D interference modelling is obtained and a computationally efficient expression for the average ergodic sum capacity of the cell is derived. The expression further allows us to obtain the optimal number of sub-cells, which is an important parameter for maximizing the average ergodic sum capacity of the cell. It is shown that with small CSI feedback, system capacity can be improved significantly by adopting the
proposed resource allocation framework, especially in dense D2D deployed systems.
Albasry, H., Zhu, H. and Wang, J. (2018). In-band Emission Interference in D2D-enabled Cellular Networks: Modelling, Analysis, and Mitigation. IEEE Transactions on Wireless Communications [Online]. Available at: https://doi.org/10.1109/TWC.2018.2866833?.
Next generation network is considered as a device to device (D2D)-enabled system. The overlay in-band scheme can be used by the cellular user equipments (CUEs) and D2D user equipments (DUEs) to send data. The cellular and D2D links experience the in-band emission interference (IEI) from the DUEs that use the adjacent frequencies. This paper models the IEI impact by using the stochastic geometry and analytically investigates this impact on cellular and D2D links. The IEI intercell and IEI intra-cell are separately assessed, and the expected D2D resource block (DRB) reuse factor is evaluated. Further, distance-density based (DDB) strategy is proposed to mitigate the IEI by controlling the number and location of served DUEs for each DRB. Also, optimal power allocation (OPA) algorithm is proposed by calculating the optimal DUEs transmission power profile that mitigates IEI and maximizes the DUEs sum rate. The performance is improved significantly for the proposed methods. The application scenario is identified for each mitigation method.
Ye, Z., Pan, C., Zhu, H. and Wang, J. (2018). Tradeoff Caching Strategy of Outage Probability and Fronthaul Usage in Cloud-RAN. IEEE Transactions on Vehicular Technology [Online] 67. Available at: http://dx.doi.org/10.1109/TVT.2018.2797957.
In this paper, tradeoff content caching strategy is proposed to jointly minimize the cell average outage probability and fronthaul usage in cloud radio access network (Cloud-RAN). At first, an accurate closed form expression of the outage probability conditioned on the user’s location is presented, and the cell average outage probability is obtained through the composite Simpson’s integration. The caching strategy for jointly optimizing the cell average outage probability and fronthaul usage is then formulated as a weighted sum minimization problem, which is a nonlinear 0-1 integer problem. Two heuristic algorithms are proposed to solve the problem. Firstly, a genetic algorithm (GA) based approach is proposed. Numerical results show that the performance of the proposed GA-based approach with significantly reduced computational complexity is close to the optimal performance achieved by exhaustive search based caching strategy, and the GA-based approach can improve the performance by up to 47.5% on average than the typical probabilistic caching strategy. Secondly, in order to further reduce the computational complexity, a mode selection approach is proposed. Numerical results show that this approach can achieve near-optimal performance over a wide range of the weighting factors through a single computation.
Wang, J., Zhu, H., Gomes, N. and Wang, J. (2018). Frequency Reuse of Beam Allocation for Multiuser Massive MIMO Systems. IEEE Transactions on Wireless Communications [Online]. Available at: https://doi.org/10.1109/TWC.2018.2793227.
Abstract—Massive multiple-input-multiple-output (MIMO)
has become a promising technique to provide high-data-rate
communication in fifth-generation (5G) mobile systems, thanks to
its ability to form narrow and high-gain beams. Among various
massive MIMO beamforming techniques, the fixed-beam scheme
has attracted considerable attention due to its simplicity. In this
paper, we focus on a fixed-beam based multiuser massive MIMO
system where each user is served by a beam allocated to it. To
maximize the sum data rate, a greedy beam allocation algorithm
is proposed under the practical condition that the number of
radio frequency (RF) chains is smaller than the number of users.
Simulation results show that our proposed greedy algorithm
achieves nearly optimal sum data rate. As only the sum data rate
is optimized, there are some “worst-case” users who could suffer
from strong inter-beam interference and thus experience low
data rate. To improve the individual data rates of the worst-case
users while maintaining the sum data rate, an adaptive frequency
reuse scheme is proposed. Simulation results corroborate that our
proposed adaptive frequency reuse strategy can greatly improve
the worst-case users’ data rates and the max-min fairness among
served users without sacrificing the sum data rate.
Daghal, A., Zhu, H. and Wang, J. (2018). Content Delivery Analysis in Multiple Devices to Single Device Communications. IEEE Transactions on Vehicular Technology [Online]. Available at: https://doi.org/10.1109/TVT.2018.2865012.
Content caching at mobile user devices (UDs) utilizing device to device (D2D) communications is a promising technology to enhance the performance of mobile networks, in terms of latency, throughput, energy consumption, and so on. In this paper, a novel method of content delivery using multiple devices to single device (MDSD) communications through D2D links is presented. In this method, the Zipf distribution with exponent shape parameter is adopted to model the content caching popularity for the analysis of the achievable signal to interference plus noise ratio (SINR). In order to investigate the advantage of the proposed MDSD method, firstly, a closedform expression of the outage probability is theoretically derived for a single D2D communication to evaluate the success of content delivery to a reference UD. Secondly, the expression of the outage probability for MDSD communication is derived, where the outage probability is modeled as a function of content caching popularity, the density of UDs, and the size of cooperative area. The research work is further extended to address the frequency reuse among different UDs in one cell, where a frequency band factor is introduced, and the optimal radius of the cooperative area is introduced and analysed. The analytical results, validated by the simulation results, show that the outage probability decreases drastically when the popularity of the content increases, or the radius of the cooperative area increases. Using the given closed-form expression of the outage probability, the area spectral efficiency (ASE) of the system is presented. Furthermore, the results show that as the frequency band factor increases, the outage probability decreases, as well as the ASE decreases. Finally, it is shown that the MDSD outperforms the single D2D-based method.
Pan, C., Zhu, H., Gomes, N. and Wang, J. (2017). Joint User Selection and Energy Minimization for Ultra-Dense Multi-channel C-RAN with Incomplete CSI. IEEE Journal on Selected Areas in Communications [Online] 35:1809-1824. Available at: http://dx.doi.org/10.1109/JSAC.2017.2710858.
This paper provides a unified framework to deal with the challenges arising in dense cloud radio access networks (C-RAN), which include huge power consumption, limited fronthaul capacity, heavy computational complexity, unavailability of full channel state information (CSI), etc. Specifically, we aim to jointly optimize the remote radio head (RRH) selection, user equipment (UE)-RRH associations and beam-vectors to minimize the total network power consumption (NPC) for dense multi-channel downlink C-RAN with incomplete CSI subject to per-RRH power constraints, each UE’s total rate requirement, and fronthaul link capacity constraints. This optimization problem is NP-hard. In addition, due to the incomplete CSI, the exact expression of UEs’ rate expression is intractable. We first conservatively replace UEs’ rate expression with its lower-bound. Then, based on the successive convex approximation (SCA) technique and the relationship between the data rate and the mean square error (MSE), we propose a single-layer iterative algorithm to solve the NPC minimization problem with convergence guarantee. In each iteration of the algorithm, the Lagrange dual decomposition method is used to derive the structure of the optimal beam-vectors, which facilitates the parallel computations at the Baseband unit (BBU) pool. Furthermore, a bisection UE selection algorithm is proposed to guarantee the feasibility of the problem. Simulation results show the benefits of the proposed algorithms and the fact that a limited amount of CSI is sufficient to achieve performance close to that obtained when perfect CSI is possessed.
Pan, C., Zhu, H., Gomes, N. and Wang, J. (2017). Joint Precoding and RRH selection for User-centric Green MIMO C-RAN. IEEE Transactions on Wireless Communications [Online] 16:2891-2906. Available at: http://dx.doi.org/10.1109/TWC.2017.2671358.
This paper jointly optimizes the precoding matrices and the set of active remote radio heads (RRHs) to minimize the network power consumption (NPC) for a user-centric cloud radio access network (C-RAN), where both the RRHs and users have multiple antennas and each user is served by its nearby RRHs. Both users’ rate requirements and per-RRH power constraints are considered. Due to these conflicting constraints, this optimization problem may be infeasible. In this paper, we propose to solve this problem in two stages. In Stage I, a low-complexity user selection algorithm is proposed to find the largest subset of feasible users. In Stage II, a low-complexity algorithm is proposed to solve the optimization problem with the users selected from Stage I. Specifically, the re-weighted l1-norm minimization method is used to transform the original problem with non-smooth objective function into a series of weighted power minimization (WPM) problems, each of which can be solved by the weighted minimum mean square error (WMMSE) method. The solution obtained by the WMMSE method is proved to satisfy the Karush-Kuhn-Tucker (KKT) conditions of the WPM problem. Moreover, a low-complexity algorithm based on Newton’s method and the gradient descent method is developed to update the precoder matrices in each iteration of the WMMSE method. Simulation results demonstrate the rapid convergence of the proposed algorithms and the benefits of equipping multiple antennas at the user side. Moreover, the proposed algorithm is shown to achieve near-optimal performance in terms of NPC.
Pan, Y., Pan, C., Zhu, H., Ahmed, Q., Chen, M. and Wang, J. (2017). On Consideration of Content Preference and Sharing Willingness in D2D Assisted Offloading. IEEE Journal on Selected Areas in Communications [Online] 35:978-993. Available at: http://dx.doi.org/10.1109/JSAC.2017.2680938.
Device-to-device (D2D) assisted offloading heavily depends on the participation of human users. The content preference and sharing willingness of human users are two crucial factors in the D2D assisted offloading. In this paper, with consideration of these two factors, the optimal content pushing strategy is investigated by formulating an optimization problem to maximize the offloading gain measured by the offloaded traffic. Users are placed into groups according to their content preferences, and share content with intergroup and intragroup users at different sharing probabilities. Although the optimization problem is nonconvex, the closed-form optimal solution for a special case is obtained, when the sharing probabilities for intergroup and intragroup users are the same. Furthermore, an alternative group optimization (AGO) algorithm is proposed to solve the general case of the optimization problem. Finally, simulation results are provided to demonstrate the offloading performance achieved by the optimal pushing strategy for the special case and AGO algorithm. An interesting conclusion drawn is that the group with the largest number of interested users is not necessarily given the highest pushing probability. It is more important to give high pushing probability to users with high sharing willingness.
Huseyin, H., Zhu, H. and Wang, J. (2017). Performance of Non-orthogonal Multiple Access (NOMA) with a Novel Asynchronous Interference Cancellation Technique. IEEE Transactions on Communications [Online]. Available at: https://doi.org/10.1109/TCOMM.2016.2640307.
The non-orthogonal multiple access (NOMA) allows one subcarrier to be allocated to more than one user at the same time in an orthogonal frequency division multiplexing (OFDM) system. NOMA is a promising technique to provide high throughput due to frequency reuse within a cell. In this paper, a novel interference cancellation (IC) technique is proposed for asynchronous NOMA systems. The proposed IC technique exploits a triangular pattern to perform the IC from all interfering users for the desired user. The bit error rate (BER) and capacity performance analysis of an uplink NOMA system with the proposed IC technique is presented, along with the comparison to orthogonal frequency division multiple access (OFDMA) systems. The numerical and simulation results show that the NOMA with the proposed asynchronous IC technique outperforms the OFDMA. It is also shown that employing iterative IC provides significant performance gain for NOMA and the number of required iterations depends on the modulation level and the detection method.With hard-decision, two iterations are sufficient, however with soft-decision, two iterations are enough only for low modulation level, and more iterations are desirable for high modulation level.
Wang, J., Zhu, H., Dai, L., Gomes, N. and Wang, J. (2016). Low-Complexity Beam Allocation for Switched-Beam Based Multiuser Massive MIMO Systems. IEEE Transactions on Wireless Communications [Online] 15:8236-8248. Available at: http://dx.doi.org/10.1109/TWC.2016.2613517.
This paper addresses the beam allocation problem in a switched-beam based massive multiple-input-multiple-output (MIMO) system working at the millimeter wave (mmWave) frequency band, with the target of maximizing the sum data rate. This beam allocation problem can be formulated as a combinatorial optimization problem under two constraints that each user uses at most one beam for its data transmission and each beam serves at most one user. The brute-force search is a straightforward method to solve this optimization problem. However, for a massive MIMO system with a large number of beams N, the brute-force search results in intractable complexity O(NK), where K is the number of users. In this paper, in order to solve the beam allocation problem with affordable complexity, a suboptimal low-complexity beam allocation (LBA) algorithm is developed based on submodular optimization theory, which has been shown to be a powerful tool for solving combinatorial optimization problems. Simulation results show that our proposed LBA algorithm achieves nearly optimal sum data rate with complexity O(K logN). Furthermore, the average service ratio, i.e., the ratio of the number of users being served to the total number of users, is theoretically analyzed and derived as an explicit function of the ratio N=K.
Wei, H., Wang, D., Zhu, H., Wang, J., Sun, S. and You, X. (2016). Mutual Coupling Calibration for Multiuser Massive MIMO Systems. IEEE Transactions on Wireless Communications [Online] 15:606-619. Available at: http://doi.org/10.1109/TWC.2015.2476467.
Massive multiple-input multiple-output (MIMO) is a promising technique to greatly increase the spectral efficiency and may be adopted by the next generation mobile communication systems. Base stations (BSs) equipped with large-scale antennas can serve multiple users simultaneously by exploiting the downlink precoding in time division duplex (TDD) mode. However, channel state information (CSI) of uplink transmissions cannot be simply used for downlink precoding, because the gain mismatches of the transceiver radio frequency (RF) circuits disable the channel reciprocity. In this paper, we focus on antenna calibration for massive MIMO systems with maximal ratio transmit (MRT) precoding to solve the channel nonreciprocity problem. A new calibration method, called mutual coupling calibration, is proposed by using the effect of mutual coupling between adjacent antennas. By exploiting this method, the BS can perform the calibration without extra hardware circuit and users' involvement. We also build up the model of calibration error and derive the closed-form expressions of the ergodic sum-rates for evaluating the impact of calibration error on system performance. Simulation results verify the high calibration accuracy of the proposed method and show the significant improvement of system performance by performing antenna calibration.
Lopes, L., Sofia, R., Haci, H. and Zhu, H. (2015). A Proposal for Dynamic Frequency Sharing in Wireless Networks. IEEE/ACM Transactions on Networking [Online] 24. Available at: http://doi.org/10.1109/TNET.2015.2477560.
Wireless networks are today employed as complementary access technology, implemented on the last hop towards the Internet end-user. The shared media that wireless deployments provide and which is relevant to interconnect multiple users has a limited technical design, as only one device can be served per unit of time, design aspect that limits the potential applicability of wireless in dense environments. This paper proposes and evaluates a novel MAC-layer mechanism that extends current wireless networks with the possibility to perform downstream transmission to multiple devices within a single transmission time-frame, resulting in improved fairness for all devices. The mechanism, which is software-defined, is backward-compatible with current wireless standards and does not require any hardware changes. The solution has been validated in a realistic testbed, and the paper provides details concerning the computational aspects of our solution; a description of the implementation; and results extracted under different realistic scenarios in terms of throughput, packet loss, as well as jitter.
Alade, T., Zhu, H. and Wang, J. (2015). Uplink Spectral Efficiency Analysis of In-Building Distributed Antenna Systems. IEEE Transactions on Wireless Communications [Online] 14:4063-4074. Available at: http://doi.org/10.1109/TWC.2015.2416235.
Providing high data rate wireless transmissions has been difficult in indoor environments, particularly in multi-floor buildings. One way to achieve high data rate wireless transmissions is to reduce the radio transmission distance between the transmitter and the receiver by using distributed antenna systems (DASs) and employing frequency reuse. However, due to the reuse of the limited available spectrum, co-channel interference can severely degrade system capacity. In this paper, the uplink spectral efficiency of an in-building DAS with frequency reuse is studied, where remote antenna units (RAUs) deployed on each floor throughout the building are connected to a central unit (CU) where received signals are processed. The impact of co-channel interference on system performance is investigated by using a propagation channel model derived from multi-floor, in-building measurement results. The proposed scheme exploits the penetration loss of the signal through the floors, resulting in frequency reuse in spatially separated floors, which increases system spectral efficiency and also reduces co-channel interference. A comparative analysis with conventional co-located antenna deployment at the floor center is provided. Location based RAU selection and deployment options are investigated. System performance is evaluated in terms of location-specific spectral efficiency for a range of potential mobile terminal (MT) locations and various in-building propagation characteristics.
Xin, Y., Wang, D., Li, J., Zhu, H., Wang, J. and You, X. (2015). Area Spectral Efficiency and Area Energy Efficiency of Massive MIMO Cellular Systems. IEEE Transactions on Vehicular Technology [Online] 65:3243-3254. Available at: http://doi.org/10.1109/TVT.2015.2436896.
This paper investigates the performance of massive
multiple input multiple output (MIMO) cellular systems with
pilot contamination. We derive a closed-form approximation of
the area spectral efficiency (ASE) for uplink multi-cell multiuser
massive MIMO systems by using an uniformly distributed
user location model. Considering a practical power consumption
model, we obtain the area energy efficiency (AEE) in a cell. The
theoretical results of ASE and AEE can be used to investigate
some system parameters of massive MIMO cellular system, such
as the number of antennas at the base station, the number of
users, and the number of symbols over which the channel remains
constant. We also study the optimal pilot-to-data power ratio
(PDPR) for both ASE maximization and AEE maximization.
Sofia, R., Bogliolo, A., Sivrikaya, F., Zhu, H., Marce, O. and Valerdi, D. (2014). User centric networking and services: Part I [Guest Editorial]. IEEE Communications Magazine [Online] 52:18-18. Available at: http://dx.doi.org/10.1109/MCOM.2014.6894447.
User-centric networks (UCNs) are a recent architectural trend of self-organizing autonomic networks where the Internet end user cooperates by sharing network services and resources. UCNs are spontaneous and grassroots deployments of wireless architectures, ad hoc or infrastructured, often involving low-cost equipment.
Sofia, R., Bogliolo, A., Sivrikaya, F., Zhu, H., Marce, O. and Valerdi, D. (2014). User-centric networking and services: part 2 [Guest Editorial]. IEEE Communications Magazine [Online] 52:16-16. Available at: http://dx.doi.org/10.1109/MCOM.2014.6979946.
Iser-centric networks (UCNs) can be seen as a recent architectural trend of self-organizing autonomic networks where the Internet end user cooperates by sharing network services and resources. UCNs are characterized by spontaneous and grassroots deployments of wireless architectures, where users on such environments roam frequently and are also owners of networking equipment. Common to UCNs is a social behavior that heavily impacts network operation from an end-to-end perspective.
Zhu, H. and Wang, J. (2014). Performance Analysis of Chunk-Based Resource Allocation in Multi-Cell OFDMA Systems. IEEE Journal on Selected Areas in Communications [Online] 32:367-375. Available at: http://dx.doi.org/10.1109/JSAC.2014.141216.
In the orthogonal frequency division multiple access (OFDMA) system, one of the efficient and low complex methods to allocate radio resources among multiple users is chunk-based resource allocation, which groups a number of adjacent subcarriers into a chunk and allocates resources chunk by chunk. In this paper, performance analysis of chunk-based resource allocation is studied in the multi-cell OFDMA environment. Fractional frequency reuse (FFR) is considered in the cellular OFDMA. Basically, FFR divides each cell into central and edge areas where two different values of the frequency reuse factor are assumed. This paper analytically evaluates how spectral efficiency performance is affected by system parameters, including radius ratio of the central area to the whole cell, transmit signal to noise ratio (SNR), number of users, number of subcarriers per chunk, and coherence bandwidth. The numerical results show that there exists an optimal radius ratio to achieve the highest spectral efficiency in the proposed research. The optimal radius ratio is about 0.7, which is almost irrespective of the SNR, number of users, and number of subcarriers per chunk. In other words, the sizes of the central area and the edge area of the whole cell are almost equal when achieving the optimal performance.
Zhu, H. and Wang, J. (2013). Radio Resource Allocation in Multiuser Distributed Antenna Systems. IEEE Journal on Selected Areas in Communications [Online] 31:2058-2066. Available at: http://dx.doi.org/10.1109/JSAC.2013.131008.
The distributed antenna system (DAS) is a promising system to provide high data rate transmissions in wireless communications. So far, most researches in the performance analysis for the DAS have been focused on single-user cases. This paper aims to investigate and present an optimal resource allocation scheme in downlink multiuser DASs. Different from the resource allocation in a conventional cellular system, the resource allocation in the multiuser DAS must consider multiple channel conditions of the transmission links between multiple remote antenna unites (RAUs) and any user, which increases the complexity of the resource allocation. In order to provide insights in the multiuser DAS, the effect of the number of RAUs used to communicate with each user is investigated extensively. In addition to power allocation, subcarrier allocation and bit allocation are studied in the orthogonal frequency division multiple access (OFDMA) DAS. To reduce the complexity of the resource allocation in the downlink multiuser DAS, the chunk allocation technique is adopted by grouping a set of contiguous subcarriers into one chunk and allocating resources chunk by chunk to users and an equivalent channel fading factor per subcarrier for a user is proposed to represent the combined channel gain from multiple RAUs to the user. The numerical results show that by using the proposed resource allocation method, each user only needs to be connected to a few RAUs, depending on the location of the user.
Osman, H., Zhu, H., Toumpakaris, D. and Wang, J. (2013). Achievable Rate Evaluation of In-Building Distributed Antenna Systems. IEEE Transactions on Wireless Communications [Online] 12:3510-3521. Available at: http://dx.doi.org/10.1109/TWC.2013.060513.121755.
Over the last few years high data rate wireless transmission has gained considerable attention in hot spot areas, including high buildings. It has been demonstrated that distributed antenna systems (DASs) improve the performance of the indoor environment by covering dead spots. In this paper, the achievable rate is investigated for indoor high data rate wireless transmission in high buildings when DASs are employed. In the system, radio transmission/reception within one or several neighbouring floors is controlled by one central unit (CU) exploiting the entire system bandwidth. In order to efficiently use the spectrum, the same frequency channels are reused among floors controlled by different CUs. The radio signals in high buildings can propagate vertically and reach the neighbouring floors. Therefore, the performance of the system is limited by cochannel interference. Direct propagation inside the building and reflection from nearby buildings are considered in the channel model. The achievable rate of the system is evaluated through analysis and simulation. The impact of representative system parameters on the achievable rate, including the number of remote antenna units (RAUs), the frequency reuse factor, the floor penetration loss, the path loss exponent, and the Nakagami fading parameter, is discussed.
Raoof, R., Zhu, H. and Wang, J. (2018). Pilot Allocation and Sum-rate Analysis in Distributed Massive MIMO Systems. In: 2017 IEEE 86th Vehicular Technology Conference (VTC-Fall). IEEE. Available at: http://dx.doi.org/10.1109/VTCFall.2017.8288013.
In distributed massive multi-input multi-output (DM-MIMO) systems, orthogonal pilot sequences are generally utilized to acquire the channel state information (CSI). However, this highly restricts the number of users simultaneously served. In this paper, a pilot reuse within a single cell DM-MIMO system is proposed to serve more users than the available pilot sequences. The reuse in this strategy is applied so that maximum achievable sum-rate is satisfied with the constraint of predefined pilot resource. On this basis, two users in different subcells separated by a large distance and satisfying a specific signal to interference plus noise ratio (SINR) level can share the same pilot sequence. An expression for SINR is derived for any pair of users who use the same pilot. Based on this expression, an algorithm is proposed to choose which pairs of users are able to use the same pilot with the constraint of satisfying the minimum SINR required for these users. The simulation results demonstrate that the uplink achievable sum-rate for the proposed strategy is higher than both cases when no pilot reuse or random pilot reuse are considered.
Albasry, H., Zhu, H. and Wang, J. (2018). The Impact of In-Band Emission Interference in D2D-Enabled Cellular Networks. In: GLOBECOM 2017 - 2017 IEEE Global Communications Conference. IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2017.8254841.
The overlay in-band device to device (D2D) scheme can be used by cellular user equipments (CUEs) and D2D user equipments (DUEs) to transmit the uplink and D2D data in the channel. The CUEs experience in-band emission interference (IEI) from the DUEs that transmit D2D signals in the adjacent channels. This paper investigates the IEI impact in D2D-enabled cellular networks and evaluates the cellular system performance by deriving the coverage probability and average data rate of typical CUE under different DUEs densities. Further, the intra-cell IEI and inter-cell IEI are separately examined to calculate the dominating interference in the system. Moreover, we propose density-based strategy (DBS) to mitigate IEI in D2D-enabled networks and evaluate the data rate gain. The reduction percentage of DUEs that can be served for each time slot is derived as a trade-off metric of the strategy. The results show the IEI impact is significant and should be considered to evaluate the system performance accurately. We evaluate the IEI impact by changing the DUEs density and DUEs transmission power. Additionally, a remarkable result finds that the intra-cell and inter-cell IEI dominate the typical CUE coverage probability similarly at low DUEs density, whilst at high DUEs density the intra-cell IEI does. The DBS results depict the remarkable improvement of the typical CUE data rate and define the upper bound of the data rate that can be achieved by DBS.
Daghal, A., Zhu, H. and Wang, J. (2018). Performance analysis of Mobile Content Delivery in Multiple Devices to Single Device Communication. In: GLOBECOM 2017 - 2017 IEEE Global Communications Conference. IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2017.8254839.
In wireless communication networks, caching and delivering popular content via the device to device (D2D) communication has recently been proposed as an exciting and innovative technology in order to offload network data traffic. In this paper, a novel method of content delivery using multiple devices to the single device (MDSD) communication via D2D links is presented. An expression of the outage probability (Pout) is analytically derived and validated by simulation to determine the success of the content delivery to the user equipment (UE). Zipf distribution with exponent shape parameter p is adopted to model the UE requests and content caching popularity which affects the achievable link data rate (Ra). The results show that Pout decreases as the popularity of the content increases. Meanwhile, MDSD improves the UE experience in terms of Pout substantially compared to the single D2D link based method.
Ali, W., Wang, J., Zhu, H. and Wang, J. (2018). An Expedited Predictive Distributed Antenna System Based Handover Scheme for High-Speed Railway. In: GLOBECOM 2017 - 2017 IEEE Global Communications Conference. IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2017.8254579.
High-speed train has drawn considerable attention and become one of the most preferable conveyance mechanism. Each year the manufacture corporations reach a higher speed record which is expected to attain 1000 km/h by 2021 using hyperloop one technology. Moving at such a high speed results in a high handover (HO) rate which makes it challenging for high speed railway (HSR) mobile wireless communication to preserve steady link performance. Employing distributed antenna systems (DASs) along with the two-hop architecture, this paper proposes a fast predictive HO algorithm. In this strategy, the serving cell starts the HO preparation phase in advance by inferring the train current location. Issuing the HO preparation phase in advance reduces the HO latency and reduces the HO command failure probability as well. Lower HO command failure probability means lower HO failure probability which could greatly improve the end-users quality of services (QoS). The analytical results show that the proposed scheme performs better compared with the conventional HO scheme.
Mahbas, A., Zhu, H. and Wang, J. (2018). Mobility Management in Small Cell Networks. In: GLOBECOM 2017 - 2017 IEEE Global Communications Conference. IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2017.8254570.
The cell sojourn time and the handoff rate are considered as the main parameters in the mobility management of the cellular systems. In this paper, we address the mobility management in a two-tier heterogeneous network (HetNet) and propose a framework to study the impact of different system parameters on the handoff rate and the small cell sojourn time. In the proposed framework, the overlapping coverage among the small cells and the number of overlaps on the path of a reference user equipment (UE) are derived to obtain the actual time that the reference UE spends in each small cell during its movement from the starting point to the destination point. The results show the accuracy of the analysis in this paper in comparison to the analysis when ignoring the impact of the overlaps. The results also show the importance of considering the overlaps among the small cells in dense HetNets.
Khadka, A., Adachi, K., Sun, S., Wang, J., Zhu, H. and Wang, J. (2018). Cooperative Transmission Strategy Over Users’ Mobility for Downlink Distributed Antenna Systems. In: GLOBECOM 2017 - 2017 IEEE Global Communications Conference. IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2017.8254565.
Previously, a scheme in  is proposed for the outdated channel state information (CSI) problem, for data transmission in time division duplex (TDD) systems. In user movement environment, the actual channel of data transmission at downlink time slot is different from the estimated channel due to channel variation. In this paper the effect of different user mobility on TDD downlink multiuser distributed antenna system is investigated. An efficient autocorrelation based feedback interval technique is proposed and updates CSI at less cost of the downlink time slots. In the proposed technique, the frequency of CSI feedback for different users is proportional to their speed. Cooperative clusters are formed to maximize sum rate where channel gain based antenna selection and user clustering based on SINR threshold is applied to reduce computational complexity. Numerical results show that sum rate superiority of the proposed scheme over the user mobility.
Alluhaibi, O., Ahmed, Q., Pan, C. and Zhu, H. (2017). Capacity Maximisation for Hybrid Digital-to-Analog Beamforming mm-Wave Systems. In: 2016 IEEE Global Communications Conference (GLOBECOM). IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2016.7841649.
Millimetre waves (mm-Waves) with massive multiple input and multiple output (MIMO) have the potential to fulfill fifth generation (5G) traffic demands. In this paper, a hybrid digital-to-analog (D-A) precoding system is investigated and a particle swarm optimisation (PSO) based joint D-A precoding optimisation algorithm is proposed. This algorithm maximises the capacity of the hybrid D-A mm-Wave massive MIMO system. The proposed algorithm is compared with three known hybrid D-A precoding algorithms. The analytical and simulation results show that the proposed algorithm achieves higher capacity than the existing hybrid D-A precoding algorithms.
Nair, M., Ahmed, Q. and Zhu, H. (2017). Hybrid Digital-to-Analog Beamforming for Millimeter-Wave Systems with High User Density. In: 2016 IEEE Global Communications Conference (GLOBECOM). IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2016.7841879.
Millimeter-wave (mm-Wave) systems with hybrid digital-to-analog beamforming (D-A BF) have the potential to fulfill 5G traffic demands. The capacity of mmWave systems is severely limited as each radio frequency (RF) transceiver chain in current base station (BS) architectures support only a particular user. In order to overcome this problem when high density of users are present, a new algorithm is proposed in this paper. This algorithm operates on the principle of selection combining (SC). This algorithm is compared with the state of the art hybrid D-A BF. The simulation results show that our proposed hybrid D-A BF using SC supports higher density of users per RF chain. Furthermore, our proposed algorithm achieves higher capacity than what is achieved by the current hybrid D-A BF systems.
Pan, C., Zhu, H., Gomes, N. and Wang, J. (2017). Joint Precoding and RRH selection for Green MIMO C-RAN. In: 2016 IEEE Global Communications Conference (GLOBECOM). IEEE. Available at: http://dx.doi.org/10.1109/GLOCOM.2016.7841930.
This paper jointly optimizes the precoding matrices and the set of active remote radio heads (RRHs) to minimize the network power consumption for a cloud radio access network (C-RAN) where both the RRHs and users all have multiple antennas. Both users’ rate requirements and per-RRH power constraints are considered. Due to these conflicting constraints, this optimization problem may be infeasible. We propose to solve this problem with two phases. In Phase I, a new approach is proposed to check the feasibility of the original problem. If the feasibility is guaranteed, in Phase II, a low-complexity algorithm is proposed to solve the original optimization problem. Simulation results demonstrate the rapid convergence of the proposed algorithms and the benefits of equipping multiple antennas at the user side.
Xin, Y., Wang, D., Li, J., Zhu, H., Wang, J. and You, X. (2016). Area Spectral Efficiency and Energy Efficiency Analysis in Downlink Massive MIMO Systems. In: 2015 IEEE 82nd Vehicular Technology Conference (VTC2015-Fall). IEEE, pp. 1-5. Available at: https://dx.doi.org/10.1109/VTCFall.2015.7390846.
We consider the downlink multi-user multi-cell massive MIMO systems, assuming that the number of antennas at base station (BS) and the number of users are large. Our system model accounts for channel estimation, pilot contamination, and uniformly random user location distribution. We derive the approximation of area spectral efficiency (ASE) with regularized zero-forcing (RZF) precoding technique which are proven to be accurate via simulation results. With a realistic power consumption model considering not only transmit power but also the fundamental power for operating the circuit at transmitter and receiver, we analyze the performance of area energy efficiency (AEE). Finally, based on the proposed power consumption model, we determine the optimal number of antennas at BS aimed at maximizing AEE when transmit power is given.
Haci, H. and Zhu, H. (2013). Novel scheduling characteristics for mixture of real-time and non-real-time traffic. In: 2013 IEEE Wireless Communications and Networking Conference (WCNC). IEEE, pp. 1733-1738. Available at: http://dx.doi.org/10.1109/WCNC.2013.6554825.
This paper presents comprehensive performance analyses of a novel scheduling policy for wireless communications where users have mixed real-time (RT) and non-real-time (nRT) traffic. Also, it introduces novel characteristics to scheduling where the QoS perceived by RT and nRT streams within a mixed traffic system can be differentiated and managed. Performance comparisons with popular proportional fair scheduling policy as well as the convergence analysis are also included.
Alade, T., Zhu, H. and Osman, H. (2013). The impact of antenna selection and location on the performance of DAS in a multi-storey building. In: 2013 IEEE Wireless Communications and Networking Conference (WCNC). IEEE, pp. 3213-3218. Available at: http://dx.doi.org/10.1109/WCNC.2013.6555077.
It is well known that providing high data rate wireless mobile services is a major challenge in indoor environments, particularly in multi-storey buildings. One way to achieve high data rate wireless transmissions is to reduce the radio transmission distance between the transmitter and the receiver by using distributed antenna systems (DASs) employing frequency reuse. However, due to the reuse of the limited available frequency spectrum, co-channel interference can severely degrade system performance. In this paper, the uplink performance of an in-building DAS with frequency reuse is studied, where remote antenna units (RAUs) deployed on each floor throughout the building are connected to a central unit (CU) where received signals are processed. The impact of RAU selection and location strategies on the performance of the interference-limited system is analysed by using a propagation channel model derived from multi-floor, in-building measurement results. The proposed scheme exploits the penetration loss of the signal through the floors, resulting in frequency reuse in spatially separated floors, which increases system spectral efficiency and also reduces co-channel interference. RAU location is shown to be a dominant factor influencing the levels of co-channel interference, and consequently, have a major implications on system performance.
Osman, H., Zhu, H., Alade, T. and Toumpakaris, D. (2013). Wireless downlink high data rate transmission in multi-floor buildings. In: 2013 IEEE Wireless Communications and Networking Conference (WCNC). IEEE, pp. 3449-3454. Available at: http://dx.doi.org/10.1109/WCNC.2013.6555118.
In this paper, the achievable rate of distributed antenna systems (DASs) in high buildings is investigated for indoor downlink high data rate wireless transmission. A DAS, where radio transmission/reception within one or several neighbouring floors is controlled by a central unit (CU) exploiting the entire system bandwidth is considered. The same frequency channels are reused by all the CUs, co-channel interference is caused. A mathematical channel model for transmission to one user per floor including direct signal propagation between floors within the building and reflection from a nearby building is built based on practical measurements. Using the model, it is shown that the indoor path loss exponent and the Nakagami channel fading parameter have a significant effect on the achievable rate performance.