High spatial reuse through densification of wireless networks and higher amount of bandwidth are necessary to meet the increase of wireless demand in the next decade. Small cells that operate at millimeter wave (MMW) band can fulfill these two criteria. Deploying fiber to many small cell MMW base stations can be expensive and wireless backhaul can reduce this cost. A fixed access backhaul network - where the available resource is fixed separately for access and backhaul links - cannot meet the dynamic traffic demand of the network. An integrated access backaul (IAB) network - where the summation of available resource is fixed for access and backhaul links - can dynamically change the partition between access and backhaul links and meet instantaneous demand of UEs acrosss the network. This work focuses on joint cost optimal fiber drop deployment, resource allocation and routing in an IAB network. Simulation results show that IAB can significantly reduce the fiber drop deployment cost of the network compared to fixed access backhaul network.

Given the cost and complexity associated with the deployment of a large number of radio frequency (RF) chains in millimeter wave (mmW) multi-input multi-output (MIMO) systems, this paper addresses the question of network efficiency considerations for hybrid precoding. We first establish the relevance of directional precoding structures as a low-complexity and robust solution to meet the data rate demands of single-user MIMO systems relative to the more complex and less robust eigen-mode-based precoding structures. Key to the relevance of the directional precoding structures is the sparsity of the mmW channel coupled with higher antenna dimensionality affordable due to smaller wavelengths. We then leverage the directional structure of the channel to propose a simple class of directional schedulers that offers a low-complexity and yet approximately fair separation plane in the user space. We then compare the performance of single-user precoding schemes with multi-user precoding schemes and show that from network efficiency considerations, it would be more worthwhile to expend the RF chain resource on multi-user transmissions.

The focus of this work is on cost and complexity- induced rank-constrained precoding for millimeter- wave (mmW) multi-input multi-output (MIMO) systems. The higher antenna dimensionality and sparse channel structure of mmW systems motivate directional precoding schemes over more complex and less robust eigen-mode-based precoding schemes. Building on a prior work that illustrated the superiority in design, robustness and performance of a directional rank-1 beamformer, this work considers higher-rank directional precoding structures for mmW systems. It is shown that directional precoders continue to enjoy similar properties in single-user MIMO (SU-MIMO) systems. But from a network efficiency consideration, it would be more worthwhile to expend the radio frequency (RF) chain resource on multi-user transmissions. Towards this goal, a directional scheduler and different directional beamforming structures are proposed here. These structures are shown to result in significant performance improvement over higher-rank SU-MIMO schemes.

High user density in stadiums and auditoriums is a challenging scenario for deploying future generation wireless networks. Due to the almost line-of-sight propagation model, the LTE base stations and WiFi access points would interfere with each other and result in reduced throughput in this scenario. Millimeter and microwave bands can play an important role in this regard due to their spatial reuse capability. This paper focuses on the coverage and capacity of the 28 GHz band in indoor stadiums and auditoriums. We model an indoor basketball stadium and human blockage loss. We consider two different types of human blockage models: (i) knife edge diffraction model and (ii) experimental blockage model. We develop the experimental blockage model while making measurements in a small-scale stadium-like seating area. Our experiments suggest that the boresight direction is irrelevant in the presence of multiple surrounding scattering objects. Scattering and reflections of signals from neighboring human bodies can provide strong signal paths at the millimeter wave band in indoor stadiums. Based on this model, we simulate the connectivity of the 28 GHz band with different sitting and standing patterns among audience. Simulation results suggest that ten base stations can sustain a sum rate of 76 Gbps and 227 Gbps with one and four RF chains respectively in 50 percentile case. We also show that the 28 GHz's sustainable data rate can be 11 and 33 times higher than the 5 GHz's sustainable data rate with one and four RF chains respectively.

The millimeter-wave (mm-Wave) frequency band ~30-300 GHz has received significant attention lately as a prospective band for 5G systems. Millimeter-wave frequencies have traditionally been used for backhaul, satellite and other fixed services. While these bands offer substantial amount of bandwidth and opportunity for spatial multiplexing, the propagation characteristics for terrestrial mobile usage need to be fully understood prior to system design. Towards this end, this paper presents preliminary indoor measurement results obtained using a channel sounder equipped with omni- and directional antennas at 2.9 GHz and 29 GHz as a comparative study of the two bands. The measurements are made within a Qualcomm building in Bridgewater, NJ, USA, for two separate floors, each representing a different yet representative type of office plan. We present measurements and estimated parameters for path loss, excess delay, RMS delay and analyze the power profile of received paths. In addition, we present several spherical scans of particular links to illustrate the 3-D angular spread of the received paths. This work represents initial results of an ongoing effort for comprehensive indoor and outdoor channel measurements. The measurements presented here, along with cited references, offer interesting insights into propagation conditions (e.g. loss, delay/angular spread etc.), coverage and robustness for mobile use of millimeter-wave bands. We believe additional extensive measurement campaigns in diverse settings by academia and industry would help facilitate the generation of usable channel models.

The focus of this paper is on beamforming in a millimeter-wave (mmW) multi-input multi-output (MIMO) set-up that has gained increasing traction in meeting the high data-rate requirements of next- generation wireless systems. For a given MIMO channel matrix, the optimality of beamforming with the dominant right-singular vector (RSV) at the transmit end and with the matched filter to the RSV at the receive end has been well-understood. When the channel matrix can be accurately captured by a physical (geometric) scattering model across multiple clusters/paths as is the case in mmW MIMO systems, we provide a physical interpretation for this optimal structure: beam steering across the different paths with appropriate power allocation and phase compensation. While such an explicit physical interpretation has not been provided hitherto, practical implementation of such a structure in a mmW system is fraught with considerable difficulties (complexity as well as cost) as it requires the use of per-antenna gain and phase control. This paper characterizes the loss in received SNR with an alternate low-complexity beamforming solution that needs only per-antenna phase control and corresponds to steering the beam to the dominant path at the transmit and receive ends. While the loss in received SNR can be arbitrarily large (theoretically), this loss is minimal in a large fraction of the channel realizations reinforcing the utility of directional beamforming as a good candidate solution for mmW MIMO systems.

In this paper, we conduct ray-tracing simulation studies using WinProp tool to understand the path loss, coverage area, diversity and delay spread of the 29GHz band in the indoor propagation environment, and compare the 29GHz indoor channel to the 2.9GHz indoor channel. In addition, we compare our simulation results with the measurement data from [4]. Based on our simulation results, we conclude that the 29GHz band has similar indoor channel characteristics as 2.9GHz and that any inherent impairment of the mmWave due to higher carrier frequency can easily compensated with beam forming techniques and other advanced algorithms. Specifically, our results show that the path loss exponents and the RMS delay spread values are similar at 29GHz and at 2.9GHz. In addition, our simulation results show that by having multiple access points on an office floor, we can significantly improve important diversity metrics such as the number of detectable beams and the angular separation of those beams, so as to make 29GHz system using beam forming technique more robust against beam blockage.

Utilization of millimeter wave (MMW) bands for supporting very high data rate cellular access has received much attention in recent times. The behavior of the wireless channel in these bands differ significantly from the sub-6 GHz counterparts. Ray tracing is a complementary effort to detailed measurements in providing quick insight into the coarse channel characteristics. In this paper, we model the 28 GHz outdoor channel through ray tracing using the WinProp tool. We simulate the single base station and multi base station scenarios in the downtown Manhattan area and study channel properties such as coverage and path loss, path diversity and delay spreads. We observe that a typical MMW base station provides coverage to a significant fraction of a 100m cell radius with path diversity, i.e. there exist secondary paths that may sustain the link on the failure of the first path. The improvements to coverage and path diversity with multiple base stations are also characterized. The above channel properties are compared with NYU's [4] results from the same region. We observe a reasonable match for many of the above parameters. The delay spreads with omni and beamformed antennas are also compared. The typical RMS delay spread in a beamformed scenario is seen to be in the order of 20-30 ns which is significantly smaller than the measured values of 100ns or more. This phenomenon is likely due to detailed clutter in the environment not being modeled.

The broadcast of periodic messages is a key functionality in vehicular ad hoc networks. In the emerging vehicular networks, IEEE 802.11p is the standard of choice to support the PHY and MAC layer functionalities. The broadcast process in IEEE 802.11p is based on the CSMA mechanism where a device transmitting a packet senses the channel for ongoing transmissions and performs a random back-off before accessing the channel. Without RTS/CTS mechanisms, carrier sensing is expected to provide a protection region around the transmitter where no other transmitters are allowed. The point process characterizing the concurrent transmitters is expected to enforce a minimum separation between concurrent transmitters. However, at increasing densities, the CSMA behavior breaks down to an ALOHA-like transmission pattern where concurrent transmitters are distributed as a Poisson point process, indicating the lack of protection around transmitters. In this paper, we model the CSMA mechanism as a slotted system and analytically characterize the critical node/packet arrival density where the CSMA mechanism approaches an ALOHAlike behavior. Further, we use tools from stochastic geometry to establish closed-form expressions for the performance metrics of the broadcast mechanism in the ALOHA regime. Finally, using ns2 (an unslotted asynchronous simulator), we compare the theoretical results with simulations.

Dedicated Short-Range Communications (DSRC) has been designed to support vehicular communications. In the U.S., DSRC operates in the 5.9 GHz licensed spectrum band. Its physical (PHY) and medium access control (MAC) layers, defined in the IEEE 802.11p standard, are based on the IEEE 802.11 family of Wi-Fi standards. Vehicular communication environments differ significantly from the sparse and low-velocity nomadic use cases of a typical Wi-Fi deployment. Thus, there are many challenges to adapt Wi-Fi technologies to support the unique requirements of vehicular communications such as achieving high and reliable performance in highly mobile, often densely populated, and frequently non-line-of-sight environments. The automotive and the communications industries, academia, and governments around the world have been devoting tremendous efforts to address these challenges, and significant achievements have been made. Remaining challenges can be addressed by the future versions of DSRC. In this paper, we investigate the current technologies used by DSRC to support vehicle safety communications, analyze existing and possible DSRC performance enhancements that can be realized in the near term, and provide a few initial thoughts on the DSRC evolution path.

Recent deployments of data-rich smart phones has provided a fresh impetus for designing, deploying and understanding the performance of wide area ad-hoc networks. The most popular medium access mechanism for such ad hoc networks is CSMA/CA with RTS/CTS. In this paper, using tools from stochastic geometry, we study and optimize the throughput performance of such networks. We show that in ad-hoc networks enabled with SIR based scheduling, a simple modification to the transmit power level - setting it to be inversely proportional to the square root of the link gain - leads to large improvements in network throughput. This simple power-level selection is optimal over the class of all ”local” transmit power selection strategies when channels are stationary, and further is at most a factor of two away from optimality in the fading case. Using stochastic geometric techniques, we also provide analytical expressions for the medium access probability in different scenarios.

Geographic routing with greedy relaying strategies are important routing schemes in sensor networks. These schemes assume that the nodes have perfect information about the location of the destination. When the destination is unit distance away, the asymptotic routing delays are Θ(1/M(n)) , where M(n) is the maximum distance traveled in one hop (transmission range). We consider three scenarios where: i) nodes have location errors (imprecise GPS); ii) only coarse geographic information about the destination is available, e.g., the quadrant in which the destination is located; and iii) only a small fraction of the nodes have routing information. In this paper, we show that even with such limited destination-location information, the routing delays are Θ(1/ M(n)) , and validate our analysis with simulations. Finally, we consider the throughput-capacity of networks with progressive routing strategies that take packets closer to the destination in every step, but not necessarily along a straight-line. While such a routing strategy could lead to spatial “hotspots” due to the suboptimal flows nonuniformly loading the network, we show that the effect of hot spots due to progressive routing does not reduce the network throughput-capacity in an order sense, i.e., is order-wise the same as the maximum achievable throughput-capacity.

Routing to mobile nodes in a wireless network is conventionally performed by associating a static IP address (or a geographic location) to each node, and routing to that address using routing tables at intermediate nodes that are updated periodically to reflect mobility-induced network topology changes. This mode of routing works when the mobiles' speeds as well as the number of mobiles are small. However, in the presence of large number of fast-moving mobiles, such approaches are infeasible and can lead to excessive overheads, routing failures and hence, throughput loss. In this paper, we consider a wireless network over a domain with a collection of static nodes (that form a connected cover of the domain) and mobile nodes, where the mobile nodes can move in an arbitrary (non-ergodic) manner over sub-domains of the network. For such a system, we develop new routing algorithms (based on a spatial multi-resolution search) that we show are efficient both in terms of routing overheads and throughput. In particular, we show that the achievable rate region of the proposed algorithm is within a poly-logarithmic constant of the optimal rate region with non-ergodic mobility.

We consider the problem of throughput-optimal routing over large-scale wireless ad-hoc networks. Gupta and Kumar (2000) showed that a throughput capacity (a uniform rate over all source-destination pairs) of thetas( 1/radicn log n ) is achievable in random planar networks, and the capacity is achieved by straight-line routes. In reality, both the network model and the traffic demands are likely to be highly non-uniform. In this paper, we first propose a randomized forwarding strategy based on geographic routing that achieves near-optimal throughput over random planar networks with an arbitrary number of routing holes (regions devoid of nodes) of varying sizes. Next, we study a random planar network with arbitrary source-destination pairs with arbitrary traffic demands. For such networks, we demonstrate a randomized local load-balancing algorithm that supports any traffic load that is within a poly-logarithmic factor of the throughput region. Our algorithms are based on geographic routing and hence inherit their advantageous properties of low- complexity, robustness and stability.

Geographic forwarding has been widely studied as a routing strategy for large wireless networks, mainly due to the low complexity of the routing algorithm, scalability of the routing information with network size and fast convergence times of routes. On a planar network with no holes, Gupta and Kumar (2000) have shown that a uniform traffic demand of ominus(1/radicn log n) is achievable. However, in a network with routing holes (regions on the plane which do not have active nodes), geographic routing schemes such as GPSR or GOAFR could cause the throughput capacity to significantly drop due to concentration of traffic on the face of the holes. Similarly, geographic schemes could fail to support non-uniform traffic patterns due to spatial congestion (traffic concentration) caused by greedy "straight-line" routing. In this paper, we first propose a randomized geographic routing scheme that can achieve a throughput capacity of ominus(1/radicn) (within a poly-logarithmic factor) even in networks with routing holes. Thus, we show that our scheme is throughput optimal (up to a poly-logarithmic factor) while preserving the inherent advantages of geographic routing. We also show that the routing delay incurred by our scheme is within a poly-logarithmic factor of the optimal throughput-delay trade-off curve. Next, we construct a geographic forwarding based routing scheme that can support wide variations in the traffic requirements (as much as ominus(1) rates for some nodes, while supporting ominus(1/radicn) for others). We finally show that the above two schemes can be combined to support non-uniform traffic demands in networks with holes.

We develop order bounds on the refresh rate of computing functions over large multi-hop sensor networks, with finite degree (finite neighbors for each node). The refresh rate quantifies how often the function can be re-computed with new data at sensor nodes. Giridhar and Kumar (2005) considered deterministic function computation for two important classes of functions (type-threshold and type-sensitive functions) and showed that for networks with high degree (random planar geometric graph with n nodes and average degree Theta(log n)), type- threshold functions (e.g. max) are easy to compute (refresh rate of Theta(1/log(n))), and type-sensitive functions (e.g. average) can be computed only at a rate Theta(1/log(n)). In this paper, we first show that type-threshold functions can be computed at an optimal refresh rate of Theta(1) over networks with finite degree. However, the computation of type-sensitive function do not become uniformly faster as the graph degree becomes smaller. We demonstrate that while some type-sensitive functions (such as computing parity) are not hard to compute in finite degree graphs (we can achieve a refresh rate proportional to (1/d), where d is the maximum graph degree), there exist type-sensitive functions (e.g., computing the average to one-bit precision) in this class that cannot be deterministically computed faster than Theta(1/log n) even for finite degree graphs. However, by relaxing the requirements to allow probabilistic guarantees, computing the average can be achieved at a refresh rate of Theta(1) over any graph with bounded degree and a refresh rate of Theta(1/ log log n) for random planar networks. Further, for random planar networks operating over an AWGN channel with signal power path-loss, we show that even refresh rates of Theta(1) can be achieved with vanishing distortion when the power path- loss exponent is strictly less than 4. Thus, relaxing deterministic computation guarantees to probabilistic requirements enables sizea

Flooding based querying and broadcasting schemes have low hop-delays of Theta(1/R(n)) to reach any node that is a unit distance away, where R(n) is the transmission range of any sensor node. However, in sensor networks with large radio ranges, flooding based broadcasting schemes cause many redundant transmissions leading to a storm problem. In this paper, we study the role of geographic information and state information (i.e., memory of previous messages or transmissions) in reducing the redundant transmissions in the network. We consider three broadcasting schemes with varying levels of local information where nodes have: (i) no geographic or state information, (ii) coarse geographic information about the origin of the broadcast, and (Hi) no geographic information, but remember previously received messages. For each of these network models, we demonstrate localized forwarding algorithms for (based on geography or state information) that achieve significant reductions in the transmission overheads while maintaining hop-delays comparable to flooding based schemes. We also consider the related problem of broadcasting to a set of spatially uniform points in the network (lattice points) in the regime where all nodes have only a local sense of direction and demonstrate an efficient sparse broadcast scheme based on a branching random walk that has a low number of packet transmissions. Thus, our results show that even with very little local information, it is possible to make schemes significantly more efficient.

Geographic routing with greedy relaying strategies have been widely studied as a routing scheme in sensor networks. These schemes assume that the nodes have perfect information about the location of the destination. We consider three scenarios: (i) where nodes have location errors (imprecise GPS), (ii) where only coarse geographic information about the destination is available, such as the quadrant or half-plane in which the destination is located, and (iii) where only a small fraction of the nodes have routing information. In this paper, we show that even with such imprecise or limited destination-location information, the routing delays are /spl Theta/(1/M(n)). We further show that routing delays of this magnitude can be obtained even if only a small fraction of the nodes have any location information, and other nodes simply forward the packet to a randomly chosen neighbor, and we validate our analysis with simulation. Finally, we consider the throughput-capacity of networks with progressive routing strategies that take packets closer to the destination in every step, but not necessarily along a straight-line. Such a routing strategy could potentially lead to spatial "hot spots" in the network where many data flows intersect at a spatial region (a node or group of nodes), due to "sub-optimal" routes with increased path-lengths. In this paper, we show that the effect of hot spots due to progressive routing does not reduce the network throughput-capacity in an order sense. In other words, the throughput-capacity with progressive routing is order-wise the same as the maximum achievable throughput-capacity.