A potential use case for the sixth-generation wireless networks is the widespread adoption of eXtended Reality (XR) on mobile devices. These applications require high data rates, which may exhaust the limited available bandwidth ofmid-band(MB) frequencies, comprising frequency range 1 (FR1), i.e., sub-6 GHz, and the lower part of frequency range 3 (FR3), spanning 7-24 GHz, especially in dense urban clusters. In contrast, thehigh-band(HB) frequencies, including the frequency range 2 (FR2), i.e., millimeter-wave frequencies, and the upper part of FR3, offer more bandwidth, but are prone to reliability issues due to obstructions that block line-of-sight links between user equipment (UEs) and base stations (BSs). Therefore, we propose offloading high data rate UEs experiencing link outages in HB to MB. Using a cell-free massive multiple-input multiple-output architecture in MB, we ensure stable throughput for both low data rate UEs and offloaded high data rate UEs, while distributing the traffic load across more BSs. Our results demonstrate significant reduction in the mean outage duration and a hundred-fold reduction in the mean outage probability, with minimal impact on service quality for low data rate UEs, offering a cost-effective, practical solution for reliable HB deployments.

Next generation cellular networks will rely heavily on mmWave and THz spectrum for the abundantly available bandwidth. However, these frequencies suffer from high path and penetration losses. One way to improve the performance is network densification which comes with higher operational cost. To reduce the operational cost of Base Station (BS) deployment, the 3GPP has proposed the Integrated Access and Backhaul (IAB) architecture. Nonetheless, when a handover does occur in IAB networks, even with minimal handover time, the traffic that is already en route to the UE is delayed, as the new serving BS must retrieve the packets from either the previous serving BS or the core. To address these challenges, we propose Fast Wireless Backhaul (FWB), a new wireless backhaul solution. FWB takes advantage of the multi-connectivity of UEs and the high-capacity low-cost wireless backhaul promised by IAB to reduce latency and increase reliability in case of unexpected blockages on the wireless signal path. In FWB, the BSs serving the UE participate in a multicast tree, and receive all packets designated for the UE, but only one BS transmits packets to the UE. In the event of a link failure between the UE and the serving BS, another BS takes over to maintain the data plane connection, without first having to retrieve undelivered downlink packets. We believe our architecture can enable mission critical applications with stringent latency and reliability requirements.

Device-to-device (D2D) communication using 5G New Radio (NR) sidelink (SL) is envisioned to be a key enabler of high speed, low latency applications, with automated driving being the prime use case. To meet the high data rate requirements, it is essential for SL devices to be able to operate in mmWave/sub-THz frequencies where bandwidth is abundant. Consequently, several enhancements will be needed to the current version of NR SL, which is mainly designed for sub-6 GHz frequencies. Beamforming based highly directional transmission/reception used at high carrier frequencies result in directional deafness in other directions. For SL UE autonomous resource allocation, termed as Mode 2 of NR SL in 3GPP, this results in a UE’s inability to detect transmissions not aligned to its primary direction of reception, which leads to high packet errors. In this paper, we focus on system wide performance of 3GPP based NR SL Mode 2 resource allocation for directional systems at mmWave/sub-THz frequencies. We propose a composite strategy that comprises paired SL control transmission and sensing, whereby SL UEs transmit and receive the SL control information in an additional “paired” direction, directly opposite to their intended direction of transmission. This helps eliminate hidden node interference while avoiding too many exposed nodes. System level simulations in NR V2X highway scenarios show significant performance improvement of the proposed scheme over conventional solutions like omnidirectional or directional transmission/reception of SL control information.

Wireless communication over terahertz (THz) frequency bands is envisioned as the key enabler of many applications and services offered in 6G networks. The abundantly available bandwidth in THz frequencies can satisfy the ultra-high user throughput requirements and accommodate a massive number of connected devices. However, poor propagation characteristics, shadowing, and blockages may result in sudden outages and necessitate frequent handovers. Therefore, an inefficient handover procedure will impose severe challenges in meeting the ultra-high reliability and low latency requirements of emerging applications. In blockage driven mmWave and THz networks, a higher multi-connectivity degree and efficient handover procedures are needed to reduce the data plane interruptions and to achieve high reliability. We present an analytical model to study the impact of handover procedures and multi-connectivity degree on the latency and reliability of blockage driven wireless networks. From the network protocol design perspective, our study offers a quick and accurate way to envisage how network architecture and protocols should evolve in terms of multi-connectivity degrees and handover procedural efficiency. Our results suggest that, for THz systems, coverage range should be increased even if it comes at the cost of increased initial access and base station discovery times.

With the new emerging throughput-intensive ultralow latency applications, there is a need for a transport layer protocol that can achieve high throughput with low latency. One promising candidate is TCP BBR, a protocol developed by Google, with the aim of achieving high throughput and low latency by operating around the Bandwidth Delay Product (BDP) of the bottleneck link. Google reported significant throughput gains and much lower latency relative to TCP Cubic following the deployment of BBR in their high-speed wide area wired network. As most of these emerging applications will be supported by Millimeter Wave (mmWave) wireless networks, BBR should achieve both high throughput and ultra-low latency in these settings. However, in our preliminary experiments with BBR over a mmWave wireless link operating at 60 GHz, we observed a severe degradation in throughput that we were able to attribute to high delay variation on the link. In this paper, we show that “throughput collapse” occurs when BBR's estimate of minimum RTT is less than half of the average RTT of the uncongested link (as when delay jitter is large). We demonstrate this phenomenon and explain the underlying reasons for it using a series of controlled experiments on the CloudLab testbed. We also present a mathematical analysis of BBR, which matches our experimental results closely. Based on our analysis, we propose and experimentally evaluate potential solutions that can overcome the throughput collapse without addina sianificant latency.

With the increasing number of base stations (BSs) and network densification in 5G, interference management using link scheduling and power control are vital for better utilization of radio resources. However, the complexity of solving link scheduling and the power control problem grows exponentially with the number of BS. Due to high computation time, previous methods are useful for research purposes but impractical for real time usage. In this paper we propose to use deep neural networks (DNNs) to approximate optimal link scheduling and power control for the case with multiple small cells. A deep Q-network (DQN) estimates a suitable schedule, then a DNN allocates power for the corresponding schedule. Simulation results show that compared with Geometric Programming based power allocation and exhaustive search based scheduling, the proposed method achieves over five orders of magnitude speed-up with less than nine percent performance loss, making real time usage practical.

Fifth Generation (5G) Millimeter Wave (mmWave) cellular networks are expected to serve a large set of throughput-intensive, ultra-reliable, and ultra-low latency applications. To meet these stringent requirements, while minimizing the network cost, the 3rdGeneration Partnership Project has proposed a new transport architecture, where certain functional blocks can be placed closer to the network edge. In this architecture, however, blockages and shadowing in 5G mmWave cellular networks may lead to frequent handovers (HOs) causing significant performance degradation. To meet the ultra-reliable and low-latency requirements of applications and services in an environment with frequent HOs, we propose the Fast Inter-Base Station Ring (FIBR) architecture, where Base Stations (BSs) that are in close proximity are grouped together, interconnected by a bi-directional counter-rotating buffer insertion ring network. FIBR enables high-speed control signaling and fast-switching among BSs during HOs, while allowing the user equipment to maintain a high degree of connectivity. We demonstrate that the FIBR architecture efficiently handles frequent HO events in mmWave cellular systems, and thus more effectively satisfies the QoS requirements of 5G applications.

Millimeter wave (mmWave) communications provide great potential for next-generation cellular networks to meet the demands of fast-growing mobile data traffic with plentiful spectrum available. However, in a mmWave cellular system, the shadowing and blockage effects lead to the intermittent connectivity, and the handovers are more frequent. This paper investigates an "all- mmWave" cloud radio access network (cloud-RAN), in which both the fronthaul and the radio access links operate at mmWave. To address the intermittent transmissions, we allow the mobile users (MUs) to establish multiple connections to the central unit over the remote radio heads (RRHs). Specifically, we propose a multipath transmission framework by leveraging the "all- mmWave" cloud-RAN architecture, which makes decisions of the RRH association and the packet transmission scheduling according to the time- varying network statistics, such that a MU experiences the minimum queueing delay and packet drops. The joint RRH association and transmission scheduling problem is formulated as a Markov decision process (MDP). Due to the problem size, a low-complexity online learning scheme is put forward, which requires no a priori statistic information of network dynamics. Simulations show that our proposed scheme outperforms the state-of- art baselines, in terms of average queue length and average packet dropping rate.

Full duplex (FD) communications, which increases spectral efficiency through simultaneous transmission and reception on the same frequency band, is a promising technology to meet the demand of next generation wireless networks. In this paper, we consider the application of such FD communication to self-backhauled small cells. We consider a FD capable small cell base station (BS) being wirelessly backhauled by a FD capable macro-cell BS. FD communication enables simultaneous back-haul and access transmissions at small cell BSs, which reduces the need to orthogonalize allocated spectrum between access and backhaul. However, in such simultaneous operations, all the links experience higher interference, which significantly suppresses the gains of FD operations. We propose an interference-aware scheduling method to maximize the FD gain across multiple UEs in both uplink and downlink directions, while maintaining a level of fairness between all UEs. It jointly schedules the appropriate links and traffic based on the back-pressure algorithm, and allocates appropriate transmission powers to the scheduled links using Geometric Programming. Our simulation results show that the proposed scheduler nearly doubles the throughput of small cells compared to traditional half-duplex self-backhauling, even when the uplink/downlink traffic is asymmetric.

As latency sensitive applications, such as online video chatting and virtual reality become popular, end-to-end latency prediction is becoming an important problem. Traditional methods for latency prediction are static, and are unsuitable to predict time-varying round-trip times between the servers and edge devices. Though distance-feature decomposition is able to predict time-varying round-trip times with time sampled information, it fails to utilize network similarity for better prediction. When the time correlation is weak between different sample matrices, it performs no better than static prediction algorithms. But in most cases, as long as network structure remains the same, network similarity still exists even if round-trip times change greatly over time. In this paper we show that similar patterns of round-trip time sequences exist both across time and among different pairs of devices. By exploiting both time correlation and network similarity, we can achieve much lower prediction error even if time correlation of 3D sampled data is weak.

In a cellular network, connections between the Base Station (BS) and Mobile Stations (MS) may fail when the channel is in a deep fade. Shadow fading is large-scale fading which can cause significant received power loss for a wide area. This will lead to lost connections and/or packet loss which is harmful to mobile users, especially to those who are using real-time applications such as video conferencing. Cooperative communication is an efficient way to reduce outage and provide better Quality of Service (QoS) support for delay sensitive applications. A third station, which is often referred as a relay, can be used to forward signals between the BS and the MS. This paper focuses on a study of the performance of relay deployments under correlated shadow fading. We consider the downlink direction in a single cell deployment, for which the shadowing effect is modeled as an angle and distance based correlated shadowing. The received signal-to- noise ratio (SNR) is then calculated by assuming jointly Gaussian shadow fading at the MS. Simulation results show channel variations over time with fixed user speed under different relay deployments. These results demonstrate that a modest number of relays can improve the performance of real-time applications significantly.

The acute disparity between increasing bandwidth demand and available spectrum has brought millimeter wave (mmWave) bands to the forefront of candidate solutions for the next-generation cellular networks. Highly directional transmissions are essential for cellular communication in these frequencies to compensate for higher isotropic path loss. This reliance on directional beamforming, however, complicates initial cell search since mobiles and base stations must jointly search over a potentially large angular directional space to locate a suitable path to initiate communication. To address this problem, this paper proposes a directional cell discovery procedure where base stations periodically transmit synchronization signals, potentially in time-varying random directions, to scan the angular space. Detectors for these signals are derived based on a Generalized Likelihood Ratio Test (GLRT) under various signal and receiver assumptions. The detectors are then simulated under realistic design parameters and channels based on actual experimental measurements at 28 GHz in New York City. The study reveals two key findings: 1) digital beamforming can significantly outperform analog beamforming even when digital beamforming uses very low quantization to compensate for the additional power requirements and 2) omnidirectional transmissions of the synchronization signals from the base station generally outperform random directional scanning.

Recent progress in establishing the capability of radios to operate in full duplex mode on a single channel has been attracting growing attention from many researchers. We extend this work by considering the application to small cells, in particular resource-managed cellular systems similar to the TDD variant of LTE. We derive conditions where full duplex operation provides improved throughput compared to half duplex for a single cell scenario. We present a hybrid scheduler that defaults to half duplex operation but can assign full duplex timeslots when it is advantageous to do so. We compare the performance of such a scheduler with a traditional half duplex scheduler in terms of throughput and energy efficiency. Our simulation results show that we achieve as much as 81% of the capacity doubling promised by full duplex, with limitations deriving from interference effects specific to full duplex operation.

Recent progress on self-interference cancellation technology in a full-duplex system makes self-interference no longer an obstacle that restricts the system from capacity gain. In this paper, assuming both relays and access point (AP) are capable of working in full- duplex mode, we focus on to what extent a wireless relaying network can benefit from full-duplex technology. We first evaluate a conventional full-duplex relaying scheme for IEEE 802.11 networks, where each source picks its optimum relay. Then we propose a novel relaying strategy, where the downlink and uplink traffic can share a common relay/link simultaneously. Using this strategy, we design a hybrid scheduling algorithm that opportunistically switches between direct link, half-duplex and full-duplex relaying. Simulation results shows our hybrid scheme can provide up to 80% more throughput than a traditional half-duplex relaying scheme. These results can be extended to cellular networks.

As cellular networks are turning into a platform for ubiquitous data access, cellular operators are facing a severe data capacity crisis due to the exponential growth of traffic generated by mobile users. In this work, we investigate the benefits of sharing infrastructure and spectrum among two cellular operators. Specifically, we provide a multi-cell analytical model using stochastic geometry to identify the performance gain under different sharing strategies, which gives tractable and accurate results. To validate the performance using a realistic setting, we conduct extensive simulations for a multi-cell OFDMA system using real base station locations. Both analytical and simulation results show that even a simple cooperation strategy between two similar operators, where they share spectrum and base stations, roughly quadruples capacity as compared to the capacity of a single operator. This is equivalent to doubling the capacity per customer, providing a strong incentive for operators to cooperate, if not actually merge.

Relay-assisted cooperative wireless communication has been shown to have significant performance gains over the legacy direct transmission scheme. Compared with single relay based cooperation schemes, utilizing multiple relays further improves the reliability and rate of transmissions. Distributed space-time coding (DSTC), as one of the schemes to utilize multiple relays, requires tight coordination between relays and does not perform well in a distributed environment with mobility. In this paper, a cooperative medium access control (MAC) layer protocol, called STiCMAC, is designed to allow multiple relays to transmit at the same time in an IEEE 802.11 network. The transmission is based on a novel DSTC scheme called randomized distributed space-time coding (R-DSTC), which requires minimum coordination. Unlike conventional cooperation schemes that pick nodes with good links, STiCMAC picks a transmission mode that could most improve the end-to-end data rate. Any station that correctly receives from the source can act as a relay and participate in forwarding. The MAC protocol is implemented in a fully decentralized manner and is able to opportunistically recruit relays on the fly, thus making it robust to channel variations and user mobility. Simulation results show that the network capacity and delay performance are greatly improved, especially in a mobile environment.

In next generation broadband wireless networks, the orthogonal frequency division multiple access (OFDMA) technique provides a flexible physical interface for mobile users (MS) to share the radio resources. Since OFDMA allows MSs in different cells to reuse the same frequency subchannels to increase network spectrum utilization, MSs at the edge of a cell suffer from severe inter-cell interference from co-channel MSs in the neighboring cells. In WiMAX and LTE networks, fractional frequency reuse (FFR) is employed to mitigate the interference for edge users, but may incur a cost in terms of a lower overall spectrum utilization. In this paper, we propose a novel frequency planning scheme to improve the signal-to-interference (SIR) ratio of edge MSs in a multi-cell OFDMA system. A cell is partitioned in a way that minimizes the interference to the edge areas while sustaining a full frequency reuse factor (FRF) for each cell. Simulation results show that our scheme substantially outperforms FFR methods in terms of SIR under the condition of full frequency reuse and high traffic load.

With the increased popularity of mobile multimedia services, efficient and robust video multicast strategies are of critical importance. Cooperative communications has been shown to improve the robustness and the data rates for point-to-point transmission. In this paper, a two-hop cooperative transmission scheme for multicast in infrastructure-based networks is used, where multiple relays forward the data simultaneously using randomized distributed space time codes (RDSTC). This randomized cooperative transmission is further integrated with layered video coding and packet level forward error correction (FEC) to enable efficient and robust video multicast. Three different schemes are proposed to find the system operating parameters based on the availability of the channel information at the source station: RDSTC with full channel information, RDSTC with limited channel information, and RDSTC with node count. The performance of these three schemes are compared with rate adaptive direct transmission and conventional multicast that does not use rate adaptation. The results show that while rate-adaptive direct transmission provides better video quality than conventional multicast, all three proposed randomized cooperative schemes outperform both strategies significantly as long as the network has enough nodes. Furthermore, the performance gap between RDSTC with full channel information and RDSTC with limited channel information or node count is relatively small, indicating the robustness of the proposed cooperative multicast system using RDSTC.

In this paper we study a two-hop cooperative transmission scheme where multiple relays forward the data simultaneously using Randomized Distributed Space Time Codes (R-DSTC). We propose to integrate this randomized cooperative transmission with layered video coding and packet level Forward Error Correction (FEC) to enable error resilient video multicast. Data rates in both hops as well as the FEC rate are adopted to maximize the video quality. Our results show that while rate-adaptive direct transmission provides better video quality than conventional multicast, randomized cooperative scheme outperforms both strategies significantly.

Wireless video multicast efficiently streams video to multiple receivers. When designing a wireless video multicast system, the system must be able to handle (i) packet losses induced by the underlying wireless channel and (ii) receiver heterogeneity in channel condition in a multicast group. We propose Layer Bargaining, a wireless video multicast design in infrastructure-based wireless networks that simultaneously addresses both of the above problems. To combat packet losses and improve received video quality, we propose a layered hybrid ARQ scheme that provides unequal protection to layered video by exploring light-weight feedback. To deal with receiver heterogeneity, we propose a framework based on Nash bargaining game for operating point selection in a multicast group. Our simulation results show that the layered hybrid ARQ scheme significantly outperforms the conventional hybrid ARQ scheme with single layer video and the layered FEC scheme. The results also show that the game-based operating point selection provides a high overall system performance while facilitating fairness among receivers. We carefully examine the overhead and the computational complexity of our proposed schemes, through both theoretical analysis and OPNET simulation/experiment, and show that Layer Bargaining is practically feasible under representative settings.

A key technology in cooperative communications is distributed space-time coding (DSTC) which achieves spatial diversity gain from multiple relays. A novel DSTC, called randomized distributed space-time coding (R-DSTC), shows considerable advantages over a regular DSTC in terms of system complexity. In this paper, we exploit the benefits of R-DSTC physical (PHY) layer and develop a distributed and opportunistic medium access control (MAC) layer protocol for R-DSTC deployment in an IEEE 802.11 wireless local area network (WLAN). Unlike other cooperative MAC designs, in our proposed PHY-MAC cross-layer framework, there is no need to decide which stations will serve as relays before each packet transmission. Instead, the MAC layer opportunistically recruits relay stations on the fly; any station that receives a packet from the source correctly forwards it to the destination. Through extensive simulations, we validate the efficiency of our MAC layer protocol and demonstrate that network capacity and delay performance is considerably improved with respect to legacy IEEE 802.11g network.

Cooperative communication is a technique that can be employed to meet the increased throughput needs of next-generation WiMAX systems. In a cooperative scenario, multiple stations can jointly emulate the antenna elements of a multi-input multi-output (MIMO) system in a distributed fashion. Although distributed space-time coding (DSTC) is being considered by the IEEE 802.16j/16 m standards for spatial diversity gain, it has several inherent drawbacks. These are addressed in the recently invented randomized distributed space-time coding, called R-DSTC. In this paper, we present the framework for the R-DSTC technique in the emerging relay-assisted WiMAX network, and develop a cooperative medium access control (MAC) layer protocol, called CoopMAX, for R-DSTC deployment in an IEEE 802.16 system. Our scheme couples the MAC layer with the physical (PHY) layer for performance optimization. The PHY layer yields significant diversity gain, while the MAC layer achieves a substantial end-to-end throughput gain. Through extensive simulations, we evaluate the performance of CoopMAX and show that it can generate capacity gains of up to about 77% for an IEEE 802.16 network.

In this paper, we design a simple and robust cooperation scheme that allows multiple relay stations, with only one antenna each, multiplex transmissions to a BS with multiple antennas. The data transmission is over two hops, where the source first sends information to relaying nodes. In the second hop, the data packet is split into multiple parallel streams in a deterministic manner. Each participating relaying node transmits a random and independent linear combination of the signal streams that would have been transmitted by all the elements of a multi-antenna system. This random processing eliminates the need to index and allocate a code for each participating relay, and thus makes the system more efficient and robust. We use the information-theoretic capacity as an upper bound for what can be achieved using this scheme. Since this scheme can achieve higher spatial multiplexing gain for the relay-destination link, it greatly improves the effective data rate for stations at the edge of the cell. This scheme requires multiple antennas at the receiver and is therefore more suitable for uplink transmission in wireless LANs and cellular networks.

In the next-generation WiMAX system, cooperative communication is being considered as an advanced technique to increase the throughput and improve the signal quality. In a cooperative scenario, multiple stations can jointly emulate the antenna elements of a multi-input multi-output (MIMO) system in a distributed fashion. Unlike conventional space-time coding (STC) mechanisms used by a IEEE 802.16e antenna array, distributed space-time coding (DSTC) is employed across the cooperating stations to achieve a higher spatial diversity gain. In this paper, we present the framework for DSTC in the emerging relay-assisted WiMAX network, and develop a cooperative MAC layer protocol, called CoopMAX, for DSTC deployment in a WiMAX system. Through extensive simulations, we evaluate the performance of CoopMAX and show that DSTC can yield capacity gains of up to about 50% for the uplink of an IEEE 802.16 network.

In a distributed wireless network, it is possible to employ several relays and mimic a multiple antenna transmission system. In this paper we propose a MAC layer solution that allows multiple relays to send information to the receiver at unison, using a randomized distributed space time code. The randomized space-time coding can recruit relays on the fly, thus significantly reducing signaling overhead. The cross-layer design between physical layer and MAC layer involves relay discovery and rate adaptation, and results in improvements in throughput and delay performance. The design is dynamic and can be adapted to changing network conditions. The proposed MAC scheme can be integrated into various wireless technologies such as distributed contention based networks (e.g., IEEE 802.11 BSS and ad hoc mode) as well as centralized multiple access networks (e.g., IEEE 802.16).

Cooperation in wireless networks has shown significant performance gains in comparison to legacy wireless networks. Cooperative wireless protocols achieve such efficiency by enabling cooperation among nodes to exploit spatial diversity. CoopMAC is a medium access control (MAC) protocol that enables cooperation by using an intermediate node as a helper to a direct communication under poor channel conditions. The helper is typically located in a position where it experiences a good channel with both the source and destination. Therefore, it increases the efficiency of the communication by forwarding a packet from the source to the destination using high transmission rates. In an earlier attempt, we demonstrated the benefits of cooperation at the MAC layer by implementing the CoopMAC protocol using an open source wireless driver platform. However, due to some limitations posed by the hardware, the full potential of the protocol could not be explored. In this paper, we proceed with a complete implementation of the cooperative MAC protocol using an OFDM based software defined radio (SDR) platform. We investigate the benefits of the SDR approach, describe the details of the implementation, as well as the experiments we run in order to evaluate the protocol. Experimental results show that CoopMAC can easily be implemented and can lead to a significant improvement in the performance of wireless networks.

Due to the broadcast nature of wireless signals, a wireless transmission intended for a particular destination station can be overheard by other neighboring stations. A focus of recent research activities in cooperative communications is to achieve spatial diversity gains by requiring these neighboring stations to retransmit the overheard information to the final destination. In this paper we demonstrate that such cooperation among stations in a wireless LAN (WLAN) can achieve both higher throughput and lower interference. We present the design for a medium access control protocol called CoopMAC, in which high data rate stations assist low data rate stations in their transmission by forwarding their traffic. In our proposed protocol, using the overheard transmissions, each low data rate node maintains a table, called a CoopTable, of potential helper nodes that can assist in its transmissions. During transmission, each low data rate node selects either direct transmission or transmission through a helper node in order to minimize the total transmission time. Using analysis, simulation and testbed experimentation, we quantify the increase in the total network throughput, and the reduction in delay, if such cooperative transmissions are utilized. The CoopMAC protocol is simple and backward compatible with the legacy 802.11 system. In this paper, we also demonstrate a reduction in the signal-to-interference ratio in a dense deployment of 802.11 access points, which in some cases is a more important consequence of cooperation

In this paper, a novel idea of user cooperation in wireless networks has been exploited to improve the performance of the IEEE 802.11 medium access control (MAC) protocol. The new MAC protocol leverages the multi-rate capability of IEEE 802.11b and allows the mobile stations (STA) far away from the access point (AP) to transmit at a higher rate by using an intermediate station as a relay. Two specific variations of the new MAC protocol, namely CoopMAC I and CoopMAC II, are introduced in the paper. Both are able to increase the throughput of the whole network and reduce the average packet delay. Moreover, CoopMAC II also maintains backward compatibility with the legacy 802.11 protocol. The performance improvement is further evaluated by analysis and extensive simulations.

IEEE 802.11 specifies two modes of operation, an infrastructure mode where nodes communicate to/through an access point, and an ad-hoc mode, where nodes communicate with each other directly. Neither mode supports multiple hop transmissions between these nodes. In this paper we present two advantages in extending 802.11 MAC to support multiple hops in the infrastructure mode. One advantage is higher available bandwidth in a multi-rate 802.11 network. IEEE 802.11 allows hosts to select different transmission rates based on the quality of the signal received by the host. Based on performance results both from analytical modeling and simulations, we demonstrate that the total available bandwidth can be improved by using multiple hops instead of reducing the transmission rates of nodes. We present the results in terms of both the increase in total throughput of the network and the available throughput for the forwarding node. The second advantage presented is that by using multi-hop transmissions, the power of transmission at the edges of 802.11 cells can be reduced resulting in lower interference with nodes at the edges of other 802.11 cells. This leads to a more uniform coverage, with increased throughput experienced by nodes at the cell edges.