Coexistence between heterogeneous wireless networks

Abstract

The proliferation of data-intensive applications has led to an exponential growth in wireless data traffic over the last decade. Solutions to accommodate this increase in traffic demand include (a) improvements in technology, for example, using high order modulation and coding schemes and multiple-input multiple-output (MIMO) systems, (b) network densification via deployment of small cell networks, and (c) efficient use of spectrum including spectrum refarming and spectrum sharing amongst different networks. While each solution has its own advantages and associated challenges, in this thesis we focus on spectrum sharing between networks. Sharing spectrum bands, which are underutilized temporally or spatially, is a promising strategy to address the demand for spectrum. However, it introduces novel challenges that result from the networks having to coexist with each other. Coexistence could be challenging for several reasons, including disparity in spectrum access rights assigned to the networks by regulatory bodies and differences in technologies and utilities of the networks sharing the spectrum. For instance, in a paradigm shift in the US and Europe, the spectrum licensed to TV operators for exclusive use was opened for use by low power unlicensed devices, provided they did not impair the reception of TV broadcast at TV receivers. More recently, spectrum that was allocated for use by Intelligent Transportation Systems (ITS) was opened up by the Federal Communications Commission (FCC) for high throughput WiFi networks. This resulted in a coexistence scenario where while the networks have equal rights to the spectrum, they care for the different utilities of information timeliness and throughput, respectively. In this thesis, we address in detail the above two scenarios of coexistence for a CSMA/CA based access of the shared spectrum. Motivated by the distinct behavior of the network that cares for information timeliness and the growing interest in real-time monitoring applications, we conclude the thesis with novel insights on spectrum sharing amongst selfish nodes that care for timely delivery of information updates. TV Whitespaces (TVWS) refers to the spectrum licensed for TV broadcast that was opened up by regulators for use by secondary (unlicensed) devices. We investigate the deployment of White-Fi networks of secondary devices, which coexist with TV networks, and the resulting throughputs. White-Fi networks use WiFi-like physical layer and medium access control (MAC) mechanisms. Unlike WiFi networks that operate in the 2.4 and 5 GHz bands and typically have a coverage of up to 100 m, outdoor White-Fi cells have much larger coverage of up to 5 km. As a result, nodes in a White-Fi cell see significant spatial heterogeneity in channel availability and link quality. We model the MAC throughput of a multi-cell city-wide White-Fi network. We formulate a throughput maximization problem for the White-Fi network under the constraint that its nodes’ maximum aggregate interference at TV receivers is within acceptable limits. We propose a heuristic method and illustrate its efficacy over hypothetical deployments of White-Fi networks coexisting with real TV networks in the US cities of Columbus and Denver, which are good examples of heterogeneity in channel availability and link quality in TVWS. Our proposed framework provides useful insights. For instance, we show that while Columbus has higher channel availability than Denver, surprisingly, its network throughput is lower, indicating that more channels may not result in increased throughput. Next, we investigate the coexistence of two networks, one of which cares for information timeliness and the other for throughput. This is motivated by a recent ruling in which the FCC opened up the 5.85−5.925 GHz ITS band, used for vehicular networking, for the unlicensed 802.11ac/802.11ax devices. While both networks have similar spectrum access rights, the incumbents of the ITS band, i.e., vehicular nodes, value timely delivery of information updates, and the sharers, i.e., the WiFi devices, desire high throughput. This novel spectrum sharing scenario raises an interesting question of whether such networks would cooperate or compete for spectrum access. We address this question using a game theoretic approach. We capture the timeliness of information using the metric of age of information. We refer to the network that cares for timeliness as an age optimizing network (AON) and the other as a throughput optimizing network (TON). We study their coexistence under the assumption that both networks share the spectrum using a CSMA/CA based access mechanism and that the AON aims to minimize the age of updates while the TON seeks to maximize throughput. We employ a repeated game-theoretic approach that allows us to answer whether a simple coexistence etiquette that enables cooperation between networks is self-enforceable. Specifically, we introduce a coordination device, which is a randomized signaling device that allows the AON and the TON to access the spectrum in a non-interfering manner. The networks employ a grim trigger strategy when cooperating which ensures that networks would disobey the device only if competition were more beneficial than cooperation in the long run. We apply the proposed etiquette to two distinct practical medium access settings: (a) when collision slots (more than one node accesses the spectrum leading to all transmissions received in error) are at least as large as successful transmission (interference-free) slots, and (b) collision slots are smaller than successful transmission slots. To exemplify, the former holds when networks use the basic access mechanism defined for the 802.11 MAC and the latter is true for networks employing the RTS/CTS based access mechanism. We show that for both medium access settings, while cooperation is self-enforceable when networks have a small number of nodes, networks prefer competition when they grow in size. Our study of coexisting age and throughput optimizing networks shows that an age optimizing network behaves differently from a throughput optimizing one. This motivated us to consider the coexistence of nodes that care for timeliness of information and share the same spectrum. As before, we employ a game theoretic approach. We formulate a non-cooperative one-shot game with nodes as players and age of information as their utilities. We investigate nodes’ equilibrium strategies in a CSMA/CA slot for the aforementioned medium access settings, i.e., when collisions are longer than successful transmissions and when collisions are shorter. For each setting, we provide insights into how competing nodes that value timeliness share the spectrum. We find that access settings exert strong incentive effects. Specifically, we show that under decentralized decision making by nodes, when collisions are shorter, transmit is a weakly dominant strategy, and when collisions are longer, no dominant strategy exists. For the latter case, we analytically derive the mixed strategy Nash equilibrium for when the ages at the beginning of the slot satisfy certain conditions.