Random access networks with separable schemes

In this work, we study the finite-user stability region of a slotted ALOHA random access network with cooperative users. The network consists of multiple cooperating pairs where the users in each pair is allowed to cooperate by relaying each others' messages to the access point (AP). Spatial diversity gains are achieved since the packet of a cooperative user can be transmitted through independent fading channels, eg. the direct transmission path and the cooperative relaying path. Most works in the literature on cooperative communications focus on the physical layer aspects such as coding, modulation, transceiver signal processing etc. In this paper, we study the advantages of user cooperation from a MAC layer perspective and devise queueing strategies to exploit the cooperative diversity gains in a random access network. Extending upon our previous results for the two-user case, we study the stability of the finite-user cooperative system that consists of multiple cooperating pairs. By treating each cooperative pair as a transmission entity, we derive inner bounds for the finite-user stability region and propose a ranking system to characterize the transmission entities' relative tendency of being stable (or unstable).

The inherent nature of the physical setup and transmission mechanism in wireless ad hoc networks with random channel access, results in correlation between the link metrics of adjacent links, when considering path optimization problems. We identify a special structure inherent to the solution of Dynamic Programming (DP) problem arising in such an optimization over paths. According to this structure, the optimal policy tries to equalize the link metrics of adjacent links in a multi-hop route. We validate this structural property with simulations.

We consider a random access network in which the nodes need to optimize their channel access rates. The nodes are assumed to be rational and interested in their performance seen as a transmitter as well as a receiver. By casting this problem as a non-cooperative game, we derive conditions for the Nash equilibrium. We also show the existence of a Nash equilibrium when the nodes are constrained by their battery power (for this case, the constraints on the access rates of the nodes become coupled). For the special case where all nodes are each other's neighbors, we find that the equilibrium is given by the solution of a system of linear equations. An adaptive distributed scheme is then proposed for learning this equilibrium and its convergence is studied numerically.

Analysis and scheduling of practical network coding in OFDMA relay networks

As a subfield of network coding, PNC can effectively enhance the throughput of wireless networks by mapping superimposed signals at the receiver to other forms of user messages. Over the past 20 years, PNC has received significant research attention and has been widely studied in various communication scenarios, for example, TWRC, NOMA in 5G networks, random access networks, and so on. Later on, channel-coded PNC and related communication techniques were investigated to ensure network reliability, such as the design of channel code, low-complexity decoding, and cross-layer design. In this article, we briefly review the variants of channel-coded PNC-aided wireless communications with the aim of inspiring future research activities in this area. We also put forth open research problems along with a few selected research directions under PNC-aided frameworks.

In cooperative systems, users achieve spatial diversity and multihop gains by transmitting packets over multiple independent fading paths provided by their partners. Most previous works on cooperative communications focus on the physical layer aspects such as coding, modulation, and transceiver signal processing techniques. In this work, we study the advantages of user cooperation from a MAC layer perspective and devise queueing strategies to exploit cooperative gains in random access networks. Based on the conventional slotted ALOHA protocol, we propose a simple cooperative transmission mechanism for a two-user cooperative pair. We derive the two-user stability region of the proposed system and show the improvements compared to noncooperative systems. The benefits can be attributed to both physical layer cooperation, where users with good channels may relay for those with bad channels, and MAC layer cooperation, where system parameters can be chosen to enhance cooperation and reduce competition. Then, we extend the proposed strategy to a finite-user system that consists of multiple cooperative pairs. By treating each pair as a single transmission entity, we derive inner bounds for the finite-user stability region and propose a ranking system to characterize the transmission entities' relative tendency of being stable (or unstable).

Cooperative Communications in Random Access Networks

We study the age of information (AoI) in a random access network consisting of multiple source-destination pairs, where each source node is empowered by energy harvesting capability. Every source node transmits a sequence of data packets to its destination using only the harvested energy. Each data packet is encoded with finite-length codewords, characterizing the nature of short codeword transmissions in random access networks. By combining tools from bulk-service Markov chains with stochastic geometry, we derive an analytical expression for the network average AoI and obtain closed-form results in two special cases, i.e., the small and large energy buffer size scenarios. Our analysis reveals the trade-off between energy accumulation time and transmission success probability. We then optimize the network average AoI by jointly adjusting the update rate and the blocklength of the data packet. Our findings indicate that the optimal update rate should be set to one in the energy-constrained regime where the energy consumption rate exceeds the energy arrival rate. This also means if the optimal blocklength of the data packet is pre-configured, an energy buffer size supporting only one transmission is sufficient.

Age of information (AoI) is one of the key indicators to characterize the freshness of information in massive access networks. However, it is difficult to insure the freshness of information in such networks, since the random transmission of data packets from multiple sources will inevitably collide. In this paper, we introduce the asynchronous frame slotted ALOHA (FSA) random access to reduce channel conflicts and thus to improve the AoI performance in massive access networks. We first construct the asynchronous FSA model and derive the closed-form expression for average peak AoI (PAoI). Then, by tuning the frame size and transmission probability, the average PAoI for asynchronous FSA and synchronous FSA are investigated. Numerical results show that in comparison with synchronous FSA, asynchronous FSA achieves the better gain in average PAoI when the frame size is relatively large.

In this paper we analyze a multiple-input-multiple-output interference channel where nodes randomly distributed on a plane utilize interference alignment to reduce the point-to-point outage. We model the spatial distribution of the nodes as a spatial Poisson cluster point process with equal sized clusters. Each cluster uses intra-cluster interference alignment to suppress interference. We link the accuracy of channel state information to the distance between the nodes, i.e., for a fixed SNR, the quality of CSI degrades with increasing distance. Accounting for the inter-cluster unaligned interference, we compare intra-cluster interference alignment with open-loop spatial multiplexing. In our analysis we find common system setups where the benefits of using interference alignment over spatial multiplexing degrade the most due to the imperfect channel state information.

Compressed sensing in random access networks with applications to underwater monitoring

Wireless access networks are ever-changing their setup, topologies and provisions of coverage. Forthcoming deployment trends include densification of cells and convergence of the ‘native IP’ with cellular access networks with more than best-effort expectations. Greater meshing in networks due to practical deployment requirements is inevitable and needed. This paper proposes a routing protocol to address the “randomness” of topology interconnections, routing paths and sizes in emerging IP access networks. Termed as Minimum Set Cover (MSC) approach, it is a generalization of the NP and NP complete mathematical problem. The offline component of the MSC multiplies intra-domain routing installations, called Routing Planes (RPs) modelled as graphs, to suitably cover the whole routing topology prior to the traffic injections in the online component. We introduce novel offline optimization features using a dynamic cost function that plans availability of capacities and correlation of routing paths via the chosen set of RPs. Our simulations verify effective path diversity using MSC with its modest protocol overhead and a heuristic used for RPs selection. For randomly constructed access network topologies with 7, 18 and 33 routers and various meshing levels, our results show convincing suitability and key performances gains compared with rival routing solutions.

In this paper, we model the random multi-user multi-channel access network by using the well known occupancy problem from probability theory. Furthermore, we combine this with a network interference model in order to derive the achievable throughput capacity of such networks. The mathematical developments and results are illustrated through various simulations results. The proposed model is particularly relevant in analyzing the performance of networks where the users are not synchronized neither in time nor in frequency as it is often the case in various Internet of Things (IoT) applications.

A novel viewpoint to the collision resolution problem is introduced for wireless slotted random access networks. This viewpoint is based on signal separation principles borrowed from signal processing problems. The received collided packets are not discarded in this approach but are exploited to extract each individual user packet information. In particular, if k users collide in a given time slot, they repeat their transmission for a total of k times so that k copies of the collided packets are received. Then, the receiver has to resolve a k/spl times/k source mixing problem and separate each individual user. The proposed method does not introduce throughput penalties since it requires only k slots to transmit k colliding packets. Performance issues that are related to the implementation of the collision detection algorithm are studied. The protocol's parameters are optimized to maximize the system throughput.

We study the problem of multiple-packet bundling to improve spectral efficiency in cellular networks. The packet size of real-time data, such as voice over Internet Protocol (VoIP), is often very small. However, the common use of time-division multiplexing (TDM) limits the number of VoIP users supported because a packet has to wait until it receives a time slot, and if only one small VoIP packet is placed in a time slot, capacity is wasted. Packet bundling can alleviate such a problem by sharing a time slot among multiple users. A recent revision of cdma2000 1xEV-DO has introduced the concept of the multiuser packet (MUP) in the downlink to overcome limitations on the number of time slots. However, the efficacy of packet bundling is not well understood, particularly in the presence of time-varying channels. We propose a novel quality-of-service (QoS) and channel-aware packet bundling algorithm that takes advantage of adaptive modulation and coding. We show that optimal algorithms are nondeterministic polynomial time (NP)-complete, recommend heuristic approaches, and use analytical performance modeling to show the gains in capacity that can be achieved from our packet bundling algorithms. We show that channel utilization can be significantly increased by slightly delaying some real-time packets within their QoS requirements while bundling those packets with like channel conditions. We validate this paper through extensive OPNET simulations with a complete evolution-data optimized (EV-DO) implementation.