Xinchen Zhang

Joint Rate and SINR Coverage Analysis for Decoupled Uplink-Downlink Biased Cell Associations in HetNets

Load balancing by proactively offloading users onto small and otherwise lightly-loaded cells is critical for tapping the potential of dense heterogeneous cellular networks (HCNs). Offloading has mostly been studied for the downlink, where it is generally assumed that a user offloaded to a small cell will communicate with it on the uplink as well. The impact of coupled downlink-uplink offloading is not well understood. Uplink power control and spatial interference correlation further complicate the mathematical analysis as compared to the downlink. We propose an accurate and tractable model to characterize the uplink SINR and rate distribution in a multi-tier HCN as a function of the association rules and power control parameters. Joint uplink downlink rate coverage is also characterized. Using the developed analysis, it is shown that the optimal degree of channel inversion (for uplink power control) increases with load imbalance in the network. In sharp contrast to the downlink, minimum path loss association is shown to be optimal for uplink rate. Moreover, with minimum path loss association and full channel inversion, uplink SIR is shown to be invariant of infrastructure density. It is further shown that a decoupled association-employing differing association strategies for uplink and downlink-leads to significant improvement in joint uplink-downlink rate coverage over the standard coupled association in HCNs.

A Stochastic Geometry Analysis of Inter-Cell Interference Coordination and Intra-Cell Diversity

Inter-cell interference coordination (ICIC) and intra-cell diversity (ICD) play important roles in improving cellular downlink coverage. By modeling cellular base stations (BSs) as a homogeneous Poisson point process (PPP), this paper provides explicit finite-integral expressions for the coverage probability with ICIC and ICD, taking into account the temporal/spectral correlation of the signal and interference. In addition, we show that, in the high-reliability regime, where the user outage probability goes to zero, ICIC and ICD affect the network coverage in drastically different ways: ICD can provide order gain, whereas ICIC only offers linear gain. In the high-spectral efficiency regime where the SIR threshold goes to infinity, the order difference in the coverage probability does not exist; however, a linear difference makes ICIC a better scheme than ICD for realistic path loss exponents. Consequently, depending on the SIR requirements, different combinations of ICIC and ICD optimize the coverage probability.

Random Power Control in Poisson Networks

This paper studies power control strategies in interference-limited wireless networks with Poisson distributed nodes. We concentrate on two sets of strategies: single-node optimal power control (SNOPC) strategies and Nash equilibrium power control (NEPC) strategies. SNOPC strategies maximize the expected throughput of the power-controllable link given that all the other transmitters do not use power control. Under NEPC strategies, no individual node of the network can achieve a higher expected throughput by unilaterally deviating from these strategies. We show that under mean and peak power constraints at each transmitter, the SNOPC and NEPC strategies are ALOHA-type random on-off power control policies, whose transmit powers and transmit probabilities depend on the knowledge about the network at each transmitter. Moreover, the resulting NEPC strategies achieve a higher spatial average throughput of the network than constant power transmission. These results suggest that ALOHA can be viewed not only as a MAC scheme but also as a stable and efficient power control scheme.

Delay-optimal Power Control Policies

The delay till success (DTS) is the mean number of transmissions needed, averaged over the fading, until a single packet is successfully received (decoded) over a wireless link. This paper shows that under a mean and a peak power constraint, random power control can significantly reduce the DTS. We derive the optimal power control policies that minimize the DTS at one link of given length. For most commonly used fading distributions, these optimal power control policies are random on-off policies, whose parameters depend on the fading statistics and the link distance. We present two applications of this result in the context of noise-limited wireless networks: minimizing the local delay (mean delay for successful nearest-neighbor communication) and minimizing the local anycast delay (mean delay for a transmission to any node).

The aggregate throughput in random wireless networks with successive interference cancellation

The feasibility of successive interference cancellation (SIC) depends on the received power ordering from different users, which, in turn, depends on the fading distribution, path loss function and network geometry. Using a framework based on stochastic geometry, this paper studies the aggregate throughput in d-dimensional random wireless networks with SIC capability. We consider networks with arbitrary fading distribution, power-law path loss; the network geometry is governed by a non-uniform Poisson point process (PPP). Our results demonstrate how the performance of SIC changes as a function of the network geometry, fading distribution, and the path loss law. An important observation is that, in interference-limited networks, lower per-user information rate always results in higher aggregate throughput, while in noisy networks, there exists a positive optimal per-user rate at which the aggregate throughput is maximized.

Successive interference cancellation in downlink heterogeneous cellular networks

Using a multi-tier Poisson model, this paper studies the performance gain of successive interference cancellation (SIC) in the downlink of K-tier heterogeneous cellular networks (HCNs). For each tier, a fraction of base stations (BSs) is non-accessible. By using a framework based on the marked path loss process with fading and calculating the equivalent access probability, we analytically characterize the coverage probability, i.e., the probability of successfully connecting to at least one accessible BS, for a typical user equipment with finite or infinite SIC capability. The results show how the performance gain of SIC depends on many system parameters including path loss exponent, coding rate, fading distributions and BS accessibilities and densities. We show for contemporary OFDM-based HCNs, infinite SIC capability is often unnecessary. In fact, under typical system parameters, most of the gain of SIC comes from the ability of canceling only a single non-accessible BS.

The performance of successive interference cancellation in random wireless networks

This paper provides a unified framework to study the performance of successive interference cancellation (SIC) in wireless networks with arbitrary fading distribution and powerlaw path loss. An analytical characterization of the performance of SIC is given as a function of different system parameters. The results suggest that the marginal benefit of enabling the receiver to successively decode k users diminishes very fast with k, especially in networks of high dimensions and small path loss exponent. On the other hand, SIC is highly beneficial when the users are clustered around the receiver and/or very low-rate codes are used. In addition, with multiple packet reception, a lower per-user information rate always results in higher aggregate throughput in interference-limited networks. In contrast, there exists a positive optimal per-user rate that maximizes the aggregate throughput in noisy networks. The analytical results serve as useful tools to understand the potential gain of SIC in heterogeneous cellular networks (HCNs). Using these tools, this paper quantifies the gain of SIC on the coverage probability in HCNs with nonaccessible base stations. An interesting observation is that, for contemporary wireless systems (e.g., LTE and WiFi), most of the gain of SIC is achieved by canceling a single interferer.

On the spatial spectral efficiency of ITLinQ

Device-to-device (D2D) communication allows serving local wireless traffic bypassing the systems infrastructure. The interference in D2D can be controlled by carefully allocating users to orthogonal channels. This paper analytically characterizes the spectral efficiency of the ITLinQ channelization technique. The analysis relies on a stochastic geometry formulation, which enables obtaining compact expressions while opening the door to an optimization of ITLinQs parameters.

On decoding the kth strongest user in poisson networks with arbitrary fading distribution

Consider a d-dimensional network whose transmitters form a non-uniform Poisson point process and whose links are subject to arbitrary fading. Assuming interference from the k - 1 strongest users is canceled, we derive the probability of decoding the k-th strongest user in closed-form. As a special case, when k = 1, this probability is the standard coverage probability. This analytical result has immediate applications in networks with successive interference cancellation (SIC) capability. We use it to find closed-form upper and lower bounds on the probability of decoding at least k users and the mean number of successively decodable users. These bounds show that transmitter clustering is beneficial in exploiting SIC.

A 3-D Spatial Model for In-Building Wireless Networks With Correlated Shadowing

Consider orthogonal planes in the 3-D space representing floors and walls in a large building. These planes divide the space into rooms, where a wireless infrastructure is deployed. This paper is focused on the analysis of the correlated shadowing field created by this wireless infrastructure through the set of walls and floors. When the locations of the planes and wireless nodes are governed by Poisson processes, we obtain a simple stochastic model which captures the non-uniform nature of node deployment and room sizes. This model, which we propose to call the Poisson building, captures the complex in-building shadowing correlations, is scalable in the number of dimensions, and can be used for network performance analysis. It allows an exact mathematical characterization of the interference distribution in both infinite and finite buildings, which further leads to closed-form expressions for the coverage probabilities in in-building cellular networks and the success probability of in-building underlay D2D transmissions.