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Inter-network spectrum sharing

xG networks are envisioned to provide opportunistic access to the licensed spectrum using unlicensed users. This setting enables multiple systems being deployed in overlapping locations and spectrum as shown in Fig. 13. Hence, spectrum sharingamong these systems is an important research topic in xG networks. Up to date, inter-network spectrum sharing has been regulated via static frequency assignment among different systems or centralized allocations between different access points of a system in cellular networks. In ad-hoc networks, only the interference issues in the ISM band has been investigated focusing mostly on the coexistence of WLAN and Bluetooth networks. Consequently, intra-network spectrum sharing in xG networks poses unique challenges that have not been considered before in wireless communication systems. In this section, we overview the recent work in this research area.

Centralized inter-network spectrum sharing

As a first step for the coexistence of open spectrum systems, in [34], the common spectrum coordination channel (CSCC) etiquette protocol is proposed for coexistence of IEEE 802.11b and 802.16a networks. The reason we do not consider this work as a complete solution for xG networks is that it necessitates modifications in users using both of the networks. More specifically, each node is assumed to be equipped with a cognitive radio and a low bit-rate, narrow-band control radio. The coexistence is maintained through the coordination of these nodes with each other by broadcasting CSCC messages. Each user determines the channel it can use for data transmission such that interference is avoided. In case channel selection is not sufficient to avoid interference, power adaptation is also deployed. The evaluations reveal that when there is vacant spectrum to use frequency adaptation, CSCC etiquette protocol improves throughput by 35–160% via both frequency and power adaptation. Another interesting result is that when nodes are clustered around IEEE 802.11b access points, which is a realistic assumption, the throughput improvement of CSCC protocol increases. In addition to the competition for the spectrum, competition for the users is also considered in [35]. In this work, a central spectrum policy server (SPS) is proposed to coordinate spectrum demands of multiple xG operators. In this scheme, each operator bids for the spectrum indicating the cost it will pay for the duration of the usage. The SPS then allocates the spectrum by maximizing its profit from these bids. The operators also determine an offer for the users and users select which operator to use for a given type of traffic. When compared to a case where each operator is assigned an equal share of the spectrum, the operator bidding scheme achieves higher throughput leading to higher revenue for the SPS, as well as a lower price for the users according to their requirements. This work opens a new perspective by incorporating competition for users as well as the spectrum in xG networks.

Distributed inter-network spectrum sharing

A distributed spectrum sharing scheme for wireless Internet service provides (WISPs) that share the same spectrum is proposed in [36], where a distributed QoS based dynamic channel reservation (D-QDCR) scheme is used. The basic concept behind D-QDCR is that a base station (BSs) of a WISP competes with its interferer BSs according to the QoS requirements of its users to allocate a portion of the spectrum. Similar to the CSCC protocol, the control and data channels are separated. The basic unit for channel allocation in D-QDCR is called Q-frames. When a BS allocates a Q-frame, it uses the control and data channels allocated to it for coordination and data communication between the users. The competition between BSs are performed according to the priority of each BS depending on a BSs data volume and QoS requirement. Moreover, various competition policies are proposed based on the type of traffic a user demands. Although thorough evaluations are not provided in [43], the D-QDCR scheme serves an important contribution for inter-network spectrum sharing. The inter-network spectrum sharing solutions so far provide a broader view of the spectrum sharing solution including certain operator policies for the determination of the spectrum allocation. A major problem for the existing solutions in the xG network architecture is the requirement for a common control channel.

Intra-network spectrum sharing

A significant amount of work on spectrum sharing focuses on intra-network spectrum sharing, where the users of an xG network try to access the available spectrum without causing interference to the primary users.

Cooperative intra-network spectrum sharing

A cooperative local bargaining (LB) scheme is proposed in [37] to provide both spectrum utilization and fairness. The local bargaining framework is formulated based on the framework in [38,39]. Local bargaining is performed by constructing local groups according to a poverty line that ensures a minimum spectrum allocation to each user and hence focuses on fairness of users. The evaluations reveal that local bargaining can closely approximate centralized graph coloring approach at a reduced complexity. Moreover, localized operation via grouping provides an efficient operation between a fully distributed and a centralized scheme. Another approach that considers local groups for spectrum sharing is provided in [21], where a heterogeneous distributed MAC (HD-MAC) protocol is proposed. A potential problem in the solution provided in LB [37] is that a common control channel may not exist in xG networks or can be occupied by a primary user. In [21], it is shown that for a given topology, very limited number of common channels exist for each of the users in a network. However, when local neighbors are considered, a node shares many channels with its neighbors.

Based on this observation, a clustering algorithm is proposed such that each group selects a common channel for communication, and distributed sensing and spectrum sharing is provided through this channel. Moreover, if this channel is occupied by a primary user at a specific time, the nodes reorganize themselves to use another control channel. The performance evaluations show that the distributed grouping approach outperforms common control channel approaches especially when the traffic load is high. The notion of busy tones, which are mainly used in some ad-hoc network protocols, is extended to the xG networks in [40] with the dynamic open spectrum sharing MAC (DOSS-MAC) protocol. As a result, when a node is using a specific data channel for communication, both the transmitter and the receiver send a busy tone signal through the associated busy tone channel. In order to further eliminate control channel communication, FFT-based radio and the noncoherent modulation/demodulationbased radio designs are proposed which theoretically enable receivers to detect the carrier frequency and the bandwidth of a signal without any control information. In addition to spectrum allocation, transmit power determination is also included in the spectrum sharing protocol in [41]. In this work both single channel and multi-channel asynchronous distributed pricing (SC/MC-ADP) schemes are proposed, where each node announces its interference price to other nodes. Using this information from its neighbors, a node can first allocate a channel and in case there exist users in that channel, then, determine its transmit power. As a result, this scheme can be classified as a hybrid of underlay and overlay techniques. While there exist users using distinct channels, multiple users can share the same channel by adjusting their transmit power. Furthermore, the SC-ADP algorithm provides higher rates to users when compared to selfish algorithms where users select the best channel without any knowledge about their neighbors’ interference levels. Finally, it is shown that under high interference, the proposed algorithm outperforms underlay techniques. So far, we have presented distributed solutions where a fixed infrastructure is not assumed. In [42], dynamic spectrum access protocol (DSAP), which is a centralized solution for spectrum sharing in xG networks, is presented. The dynamic spectrum access protocol (DSAP) proposed in this work enables a central entity to lease spectrum to users in a limited geographical region. DSAP consists of clients, DSAP server, and relays that relay information between server and clients that are not in the direct range of the server. Moreover, clients inform the server their channel conditions so that a global view of the network can be constructed at the server. By exploiting cooperative and distributed sensing, DSAP servers construct aRadioMap.

Non-cooperative intra-network spectrum sharing

An opportunistic spectrummanagement scheme is proposed in [43], where users allocate channels based on their observations of interference patterns and neighbors. In the device centric spectrum management scheme (DCSM), the communication overhead is minimized by providing five different system rules for spectrum allocation. As a result, users allocate channels according to these rules based on their observations instead of collaborating with other users. In case more than one node chooses the same channel in close proximity, random access techniques are used to resolve the contention. The comparative analysis of this scheme with the cooperative schemes show that rule-based spectrum access results inslightly worse performance. However, the communication overhead is reduced significantly. A spectrum sharing protocol for ad-hoc xG networks, (AS-MAC), is proposed in [27]. AS-MAC exploits the RTS-CTS exchange and Network Allocation Vector (NAV) concepts of the IEEE 802.11 MAC protocol in an open spectrum setting. Moreover, a common control channel is used such that transmitter receiver handshake is initiated through this channel. In this work, the xG network is assumed to coexist with a GSM network. Each node first listens to the broadcast channel of the GSM network as well as the control channel of the xG network, and each node then constructs its NAV and selects channels accordingly. In addition to the spectrum allocation methods, a transmitter receiver andshake method is proposed in [26] as a part of a cross-layer decentralized cognitive MAC (DC-MAC) protocol. In the transmitterreceiver handshake method, each user is assigned a set of channels is continuously monitored by the user. A transmitter selects one of those channels and initiates communication. The actual data channel selection is then performed through this initial handshake channel.