A Unified Architecture and Key Techniques for Interworking between ...

Wireless Pers Commun (2008) 45:67–90 DOI 10.1007/s11277-007-9400-2

A Unified Architecture and Key Techniques for Interworking between WiMAX and Beyond 3G/4G Systems Mugen Peng · Wenbo Wang

Published online: 2 October 2007 © Springer Science+Business Media, LLC. 2007

Abstract The trend of inter-connection among a multitude of different wireless access networks presents the great and bright potential business opportunities for communication operators. However, such a trend also creates a huge challenge for the network designers to manage the different radio access technologies in a cooperative mode. Therefore, this paper proposes two advanced functional architectures to support the functionalities of interworking between WiMAX and beyond 3G/4G systems, i.e., Radio Control Server (RCS) and Access Point (AP) based centralized architectures. Both control and user planes are defined in these two architectures. This paper further describes the corresponding key techniques for the interworking. These techniques can be partitioned into four categories: the key mechanisms in Generic Link Layer (GLL), the key mechanisms in Multi-Radio Resource Management (MRRM), the end-to-end Quality of Service (QoS) provision, and the heterogeneous paging. For the mechanisms in GLL, Multi-radio transmission diversity (MRTD) and Multi-Radio Multi-Hop networking (MRMH) schemes are researched. Meanwhile, Radio Access Technology (RAT) selection and load balancing schemes in aspect of MRRM are presented to improve the performance of interworking. In particular, the heterogeneous paging procedures for the convergence of WiMAX and beyond 3G/4G are proposed and evaluated, which can decrease the transmission load and save the power consumption. Keywords Beyond 3G/4G · WiMAX · Interworking · Generic Link Layer (GLL) · Multi-Radio Resource Management (MRRM)

1 Introduction Mobile communication is continuous one of the hottest areas developing at a booming speed, with advanced techniques emerging in all the fields of mobile and wireless

M. Peng (B) · W. Wang Wireless Signal Processing & Network Lab, Beijing University of Posts and Telecommunications, P.O. Box 93, No.10 Xi Tu Cheng Road, Beijing 100876, China e-mail: [email protected]

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communications. The third generation (3G) mobile communication systems are being deployed. However, it has several limitations: (1) 3G performance may not be sufficient to meet requirements of future high-performance applications like multi-media packet service, full-motion video, wireless teleconferencing, and etc. The advanced networking technology to improve the 3G capacity is necessary. (2) Multiple standards for 3G systems make them difficult to roam and interoperate across different access networks. Global mobility and service portability should be considered. (3) 3G is designed primarily for the wide area. The hybrid networks that utilize both the access point based short range wireless network and the cell or base-station based wide area network are designed jointly to promote their limitations and provide higher performance. (4) The wider frequency bandwidth is used to support higher transmission bit rate, while the maximum frequency bandwidth is not beyond 5MHz. (5) The much more efficient modulation schemes that are created by recent researchers can not be retrofitted into the 3G infrastructure. Attempting to address these limitations, many researchers began to design the next generation of mobile communications, i.e., beyond 3G (B3G) or the forth generation (4G) wireless mobile networks. B3G/4G is a simple initiative in the academic research and design labs. Currently it is insufficient to reach the promised performance and throughput, and it therefore encounters troubles in getting deployed. As the evolution from 3G, B3G/4G system has more advantages from the fully new defining mobile system, in which enough scalability and satisfaction for many requirements of future mobile systems can be completed [1]. This study is focused on the next generation mobile communications systems, so called B3G (also named as Supper 3G and even 4G, in which the long term evolution (LTE) of 3G is also included). It aims to achieve the seamless inter-operation of the heterogeneous networks such as WLAN, 3G, Digital Broadcasting (DVB), etc. Such a seamless inter-operation is expected to enable subscribers and their machines to access the information anywhere, anytime, with any medium in a cost-effective and secure manner. This paper introduces the key mechanisms of the interworking between WiMAX and B3G/4G. These key mechanisms will promote the innovative schemes to facilitate the system performances on the aspect of interworking. Our proposed mechanism would solve some problems: (1) Interworking protocol between fixed and mobile networks, (2) Mobility among heterogeneous access networks, (3) Mobile users and network agents, and (4) Reconfigurable terminals and networks. The remainder of this paper is organized as follows. In Sect. 2, the necessity of interworking between WiMAX and B3G/4G is introduced, and our work is different from the published papers and ongoing projects are emphasized. Section 3 proposes two novel protocol architectures for supporting the interworking. The key techniques for protocol implementation are further examined in Sect. 4. Finally, a summary is drawn in Sect. 5.

2 Background of Researching Interworking Driven by the necessary to address the aforementioned limitations of 3G wireless systems and support context-rich multimedia services and applications, WiMAX (Worldwide Interoperability for Microwave Access) and B3G/4G wireless systems are envisioned to provide high data rates in both downlink and uplink directions. Since 3G downlink data rates at the present do not exceed 2 Mbit/s and 384 kbit/s at pedestrian and vehicular scenarios respectively, WiMAX and B3G/4G systems are expected to attain data rates of 50 Mbit/s or higher. IEEE 802.16 standards and their associated industry consortium, WiMAX, promise to deliver high data rates over large areas in a large number of users. Especially, IEEE 802.16e offers a mobile and quickly deployable alternative to cabled access networks, such as fiber

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optic links, coaxial systems using cable modems, and digital subscriber line (DSL) links. Because WiMAX systems have the capability to address broad geographic areas without the costly infrastructure requirement to deploy cable links to individual sites, the technology may prove less expensive to deploy and should lead to more ubiquitous broadband access [2]. For the B3G/4G systems, the recent unfaltering advances pertaining to the underlying key technologies, such as networking, multiplexing, packet scheduling, multi-antenna, and transmitting-receiving techniques in the physical layer, have been making 3G evolution to B3G/4G systems realizable at a steady pace. In addition to providing high data rates, supporting global roaming and multiple classes of service with variable end-to-end quality of service (QoS) requirements across heterogeneous wireless systems are the key features of [3]. Though 3G system is being improved, its performances, including the transmission bit rate, the service provision, the spectrum efficiency and etc., are still in an inferior position compared with WiMAX, thus B3G/4G is expected to be enhanced at all factors. B3G/4G is believed to be a long term technology evolution for keeping the continuous growth of mobile communication industry, whose requirements are presented to take into account not only the data rate and capacity, but also the latency, spectrum, networking and etc. [4]. In both WiMAX and B3G/4G systems, the radio access network (RAN) and the core network will base on the packet switch, and a pure end-to-end IP architecture is conceivable. On the other way, a multitude of Radio Access Technologies (RATs) can co-exist with its strengths and weaknesses in terms of capacity, cost, achievable data rates, flexibility, scalability, mobility, etc. While most of them are continuously evolving to support the higher bit rates, lower delay, affordable cost and flexible architecture. By using different devices or networks, users nowadays have the capability to support the numerous services they want with enough satisfaction. Users expect to access any affordable service available in a seamless way while moving around the world. However, there is still no single RAT that can fully support all services and satisfy user requirements. Operators expect to effectively integrate a number of different RATs within the same RAN in a most cost effective way to deploy networks or at least make different RATs interwork. In order to complete the aforementioned requirements, new challenges and requirements on the suitable RAN network architectures are imposed. It is important to note that a well-defined interworking architecture will accelerate the creation of enriched services through the cooperation of different RANs. As far as operation aspects is concerned, WiMAX interworking with B3G/4G will be a possible case in the future, where the mobile operators provide the same infrastructure for the both radio access networks. WiMAX will be mainly applied to the metropolitan area or the hot-point area for packet services on demand, while B3G/4G focuses on the seamless coverage in a wide area with the support of high-speed mobility. Another fact is that the development of WiMAX is prior to B3G/4G. At present, the standards of mobile WiMAX have been established and some manufactures can supply the corresponding productions. As for the technology aspects, WiMAX and B3G/4G have many homologies, such as the network architecture and the techniques in the physical levels including OFDM and adaptive multiple antenna techniques (e.g. MIMO), diverse QoS guarantees in the MAC layer, and so on. Meanwhile, the interworking between WiMAX and B3G/4G can balance the load with each other. All these technical factors promote the cooperation of the two heterogeneous access networks. Since WiMAX and B3G/4G systems will both play a key role in the future generation of mobile data networks, the interworking between WiMAX and B3G/4G systems is very important and deserves the deep research. The interworking techniques and architectures on the radio access network (RAN) for WiMAX and B3G/4G will impact the development

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of hybrid mobile packet networks capable of ubiquitous data services and high data rates. Unfortunately, most ongoing research work and published papers focused on the interworking of IP based core networks. In [5], the interconnection at the session negotiation level, using SIP, COPS/Go, and diameter protocols/interface to provide session negotiation with QoS are introduced. In [6], a policy framework for resource management in a loosely coupled cellular/WLAN integrated network is presented. In [7], some approaches on cooperation between multiple RATs in a multi-radio environment were presented, where different levels of cooperation have been researched based on two concepts: generic link layer and multi-radio resource management. However, the protocol architecture for the RAN based interworking are not researched, and the interworking scenario for the WiMAX and B3G/4G convergence is not specified. In this paper, the interworking and some corresponding key mechanisms focused on the RAN based interworking are investigated. The advantages of RAN based interworking include: (1) the transmission time delay decreases and the QoS can be guaranteed in the wireless ubiquitous environments; (2) the modification of protocol structure on the core network can be neglected; and (3) the terminal with dual-modes of both B3G/4G and WiMAX can seamlessly roam and be supplied with the optimum service quality. Accurately, a new air interface sub-layer over the conventional medium access control (MAC) sub-layer in the Node B/ Access Point (AP) is defined to complete the interworking functions.

3 Protocol Architectures In particular, the architecture of RAN based interworking between WiMAX and B3G/4G systems should meet the following objectives: • Provide cooperation between WiMAX and B3G/4G networks without compromising the impartiality of each network. • Create an architecture that provides the cost-effective solutions with high spectral efficiency to deliver a wide range of services to mobile users. • Define the architecture components to enable the cooperation between WiMAX and B3G/4G in a low cost and high efficient performance. • Define the main capabilities, requirements and functions of the inter-network architecture components. • The architecture should be flexible enough to support new network concepts without introducing too many nodes and/or interfaces. The evolution of 3G system architecture and new trends in RAN architecture are considered and referenced in the design of interworking architecture between WiMAX and B3G/4G. After the introduction of the IP transport in R4 and R5, 3GPP TSG RAN group studied the UTRAN architecture evolution item [8,9] to improve the radio performance and transport layer utilization, and this work continues in release 7. In [8], several UTRAN architecture enhancement proposals are presented, which are based on: (1) Separation of the control and user planes, (2) Redefinition of UTRAN functionalities and separation of the functional entities for cell, multicell and user sides, (3) Enhancement of Node B functionality, i.e., moving part of functionalities in the RNC to the evolved Node B (eNodeB), which includes: the cell specified radio resource management, soft handover management and radio processing (MAC, RLC and PDCP), user data handling, and etc. In order to be compatible with the current existing and future evolution architectures of the mobile cellular systems, two architectures supporting the convergence of WiMAX and

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Fig. 1 RCS based centralized architecture

B3G/4G in the RAN with necessary logical nodes and interfaces are proposed. The two architectures are designed in response to the different levels of interworking, and each of them enables the combinations of several RATs within a single RAN and allows a flexible deployment of network nodes and the interconnecting transport networks. The combination not only includes the general functions of different RATs but also defines a Generic Link Layer (GLL) for generalizing some common functions, such as sequencing the data packets, compressing the header in the higher layer, providing the segmentation and retransmission functionality, and completing Multi-Radio Resource Management (MRRM) functionalities. 3.1 Radio Control Server (RCS) based centralized architecture Figure 1 shows the RCS based centralized architecture, which consists of the following logical nodes: • User Terminal (UT): this logical node consists of all functionalities necessary for an end user to access either WiMAX or B3G/4G network. • Relay Node (RN): it consists of forwarding functionality in order to extend the network’s coverage area and simplify the network planning. • Base station (BS): it is a pure WiMAX Access Point (AP). • Radio Control Server (RCS): it is used to control the BSs in WiMAX, while it is similar with Radio Network Control (RNC) and used to control Node Bs in B3G/4G. • Multi-Radio Control Server (MRCS): this node is defined to control and coordinate the heterogonous RCS for interworking. • Bearer Gateway (BG): this node acts as Access Router (AR), assigning the IP address, etc. It specifics the user plane functions for the RATs in both WiMAX and B3G/4G systems. In this proposed architecture, WiMAX and B3G/4G RANs cooperate in a loose mode based on the RCSs and MRCSs. The new introduced RCSs and MRCSs will play an important role for the cooperation of the two different RATs. Actually, MRCS and BG are two different logical nodes, and they can be located in the same communication entity. MRCS is used to complete the functionalities in the control plane, while BG domains the user plane. The radio interface protocol stack in the control plane is illustrated in Fig. 2. The interface between UT and RN is defined as Ur, the interface between UT and BS/Node B is defined as Uu, and the interface between RN and BS/Node B is defined as Ub. Note that UT not only can directly communicate with BS/Node B, but also can communicate with each other via RN. The interface between RCS and BS/Node B is defined as Iub,

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Fig. 2 Interface protocol architecture in the control plane

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and the interface between RCS and MRCS is defined as Ium. The Generic Link Layer (GLL) is defined above (or within) MAC layer (L2) and below Radio Resource Control (RRC) layer. It is noticed that the GLL entity in BS/Node B node is optional in the loose cooperation scenario. In RRC layer, the MRRM performs the control functions of the radio connection and manages the radio resource for different RATs. TNL (Transport Network Layer) is used to carry the radio interface protocols between the wired infrastructure nodes [10]. The radio interface protocol in the user plane is described as Fig. 3. The interface between different communication entities in the user plane is the same as that in the control plane. However, RCS and MRCS are not concerned here because user and control planes are separated. Bearer Gateway (BG) is necessary to convey data formats from B3G/4G or WiMAX to the IP core network and vice versa. Consequently, GLL should be involved in this node. The interface between BS/Node B and BG is defined as Iud. IP packets are transmitted between the BG and the Node B/BS via the Layer two Tunnel (L2T) which is based on some specific tunneling protocol. The functions and interfaces of the communication entities are further explained as follows. (1) User Terminal (UT): The user terminal may connect to one BG, one or several RNs, and one or several BSs/Node Bs. It also has connections to the Core Network (CN) servers (e.g. AAA and charging servers) and Services Servers (e.g. streaming and MMS servers). Moreover, UTs include the GLL functionalities with capabilities to access multi-systems. An UT may be controlled simultaneously by several RCSs if macro diversity is applied. (2) Relay Node (RN): RN can be classified into homogeneous and heterogeneous types. The homogeneous RN is a network element with relaying capabilities via the same radio technology for both RN-BS/NodeB and RN-UT connections. A heterogeneous RN will use the different radio access technologies for the connections between the RN-BS/NodeB and

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Fig. 4 AP based centralized architecture

RN-UT. RN may involve GLL or not, which depends on the functions of UTs. Note that RN is mainly considered as a transceiver to extend the coverage area and to improve the network capabilities. (3) Base Station (BS): This refers to a pure WiMAX BS with all the relevant characteristics. BS is controlled only by the serving RCS, and is terminated in RCS related to the control plane and the BG related to the user plane. Apart from the receiving control commands, BSs have to provide the static and dynamic state information to the RCS for better control purposes. (4) Node B: This is a pure BS for B3G/4G systems with all the relevant characteristics. Node B is controlled by only one RCS, and terminated in the RCS related to the control plane and the BG related to the user plane. The functions of Node B are similar with BS. (5) Radio Control Server (RCS): RCS controlling both BSs and Node Bs are defined. RCSs perform the management functionalities according to the intrinsic characteristics of its serving network. Based on received information from BSs or Node Bs, RCSs perform functionalities such as intra-Network handover, load balance, and etc. RCSs provide necessary information to MRCS. Actually, RCS will be the main node to perform RRM functionalities. (6) Multi-Radio Control Server(MRCS): MRCS is the node that manages the function of interworking and controls the BG. The suitable vertical handover is judged and executed in this entity. MRCS performs MRRM functionalities. When a user moves from one RAT to another, the RCS relocation functionality occurs, which is managed by MRCS. MRCS handles the resource lending or borrowing between RATs. In a word, MRCS is capable of steering the traffic among the RATs towards optimal distribution via the efficient radio resource management. (7) Bearer Gateway (BG): The GLL functionalities should be incorporated in this node as the packet format conversion between WiMAX and B3G/4G systems. BG will provide the unified interface to the Core IP Network (CIPN). Therefore, BG is terminated in GLL on CIPN side. Meanwhile, BG performs some important functions such as buffering and ciphering. 3.2 Access Point (AP) based centralized architecture Figure 4 shows the alternative proposed architecture for the interworking between WiMAX and B3G/4G, which consisting of the following logical nodes:

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Fig. 5 Control plane protocol terminations

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• User Terminal (UT): this logical node consists of all functionalities necessary for the end user to access either WiMAX or B3G/4G system. • Relay Node (RN): this entity performs the relay function in order to extend the coverage area and improve the network capacity. • Radio Access Technology Access Point (RAT-AP): it is a new communication entity, which combines the functionalities of both Node Bs in B3G/4G and the BSs in the WiMAX systems. In order to manage the heterogeneous traffic flows, the new sub-layer, named as GLL, should be specially designed. • Radio Control Server (RCS): this is a general controller of RAT-AP, performing both RRM for the intra-RAT and MRRM for the inter-RAT. • Access Router (AR): AR assigns the IP address, carries out the routing, and etc. In this proposed architecture, WiMAX and B3G/4G RANs cooperate in a tight mode based on the RCSs and ARs. RAT-AP provides both WiMAX and B3G/4G access capabilities. AR is independent of any RATs. Figure 5 shows the radio interface protocol in the control plane. The interfaces between UT and RN, UT and RAT-AP, RN and RAT-AP are defined as Ur, Uu, and Ub respectively. Note that UT not only can communicate directly with the serving BS/Node B, but also can communicate with RN. The interfaces between RCS and RAT-AP, BS/Node B and AR are defined as Iub and Iud respectively. GLL is defined above the L2 and below Radio Resource Control (RRC) layer. In RRC layer, the MRRM is defined to control the radio resource and balance the load of the different RATs. TNL incorporates the tunnel transmission technology. The radio interface protocol in the user plane is described as Fig. 6. The interface between different communication entities in the user plane is the same as that in the control plane. (1) User Terminal (UT): UT not only can access the unique serving BG, but also can connect to one or several RAT-APs. UT will build the connection between the Core Network (CN) servers (e.g. AAA and charging servers) and services servers (e.g. streaming and MMS

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servers) via RN/RAT-AP. UTs can access multi-system and are controlled by several RCSs simultaneously via GLL functionalities. (2) Relay Node (RN): The function of RN in the AP based centralized architecture is the same as that in the RCS based centralized architecture, which is used to extend the coverage and provide the diversity gain for improving the capacity. (3) Radio Access Technology Access Point (RAT-AP): WiMAX BSs and B3G/4G Node Bs are integrated into the RAT-AP via the GLL in this architecture. RAT-AP is controlled by the unique serving RCS, and is terminated in the RCS related to the control plane and the Bearer Gateway (BG) related to the user plane. RAT-AP provides the static or dynamic channel and network information to the serving RCS. (4) Radio Control Server (RCS): Each RCS performs both RRM and MRRM functionalities throughout the network. One RCS always controls a certain number of RAT-APs. Based on received information from RAT-APs and AR, RCS can perform some functions such as horizontal or vertical handover, load balance, and etc. RCSs are proposed to exchange information through AR to avoid the mesh connections between RAT-APs. As RAT-APs involve the functions of GLL, there’s no necessary to adopt the BG as in Fig. 1. Consequently, relocation for different RAT-APs or different ARs will be executed in the communication entity of RCS. (5) Access Router (AR): AR allocates the IP address and distributes the traffic flows to the destination, which can be regarded as the gateway between the radio access network and packet core network and is not different from that in the RCS based centralized architecture. 3.3 Features of two proposed architectures In order to be compatible with the current existing network architecture, RCS and AP based centralized interworking architectures for the heterogonous RANs are presented in the aforementioned contents. For the RCS based centralized architecture, MRCS and BG as the new communication logical entities are defined to complete the functions of network control and traffic format conversion respectively in the convergence of heterogeneous RANs. This interworking architecture is available for the conventional cellular system architecture, where the base station control (BSC) or radio network control (RNC) entities have been designed for completing the RRM functions. For the RCS based centralized architecture, the current existing communication entities will be kept unchanged, and some new communication entities involving MRCS and BG will be defined, and the corresponding interfaces to connect with RSC and Node B/BS will be specified. For the AP based centralized architecture, there is no new communication entity to be designed or defined, while the old communication entity (Node B/BS) should be redesigned, i.e., the interworking between the heterogonous access networks is executed in the conventional Node B/BS (which is renamed as RAT-AP). The advantage of this architecture is that the interworking can provide a higher data rate and lower latency. The disadvantage is that the RAT-AP should be redesigned and the corresponding protocol and interfaces should be standardized again. Meanwhile, the deployed BS/Node B for different RANs should be rebuilt. The difference between these two proposed architectures lies on the communication entity which implementing the interworking functions. The AP based architecture has better performances, while bring out the huge modification of Node B/BSs to RAT-AP. The RCS based architecture has degraded interworking performances due to the longer time latency and inefficient scheduling of inter-RAN, while the modifications only exist on the RCS, which is more convenient to implement.

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The performance evaluations and comparisons for these two interworking architectures are out of this paper’s scope, which is related to the bearing service, the communication mechanism and signal design between the core network and radio access networks, and etc. This paper will only focus on the common key interworking mechanisms, the corresponding key schemes, and the necessary protocol designs specified for these two architectures.

4 Key Mechanisms There are many functionalities ranging from the network security to the radio resource management, and from the context provisioning to the inter-RAT handover in the interworking scope. In particular networks, there are two main entities that will play a fundamental role in the conceptual design: GLL and MRRM [11]. GLL is used to convert the traffic flows coming from the heterogeneous RAN into the native packet format in the serving RAN. Meanwhile, the heterogeneous paging and end-to-end QoS guarantee as the key schemes of MRRM for the heterogeneous networks are important. 4.1 Key Mechanisms in GLL Generic Link Layer (GLL) as an additional communication sub-layer provides the universal data processing for multi-RATs and is identified as a toolbox of functions that can be readily adapted to the characteristics of both legacy and new (as yet unforeseen) radio access technologies. More specifically, GLL has the following functions: (1) Provide a unified interface to the upper layers, acting as a convergence layer for multi-RATs, hiding the heterogeneity of the underlying multi-RAT environments. (2) Control and possibly complement the RLC/MAC functionalities of the multi-RATs in order to maximize the system performance while efficiently utilizing the radio resources managed by the MRRM. (3) Provide a modular architecture that readily caters for the integration and cooperation of different types of legacy and new RATs. (4) Provide the potential capability to support the novel concepts proposed by MRMM, such as the dynamic scheduling over multi-RANs and multi-radio macro diversity. (5) Provide the channel and network information to the higher layers for supporting the efficient inter-RAT mobility management [7]. The functions and protocol structures of these two architectures are almost similar. However, some difference should also be specified. For example, the main functions of GLL are executed in RAT-AP for the AP based centralized architecture, while they are implemented in BG for the RCS based centralized architecture. In Fig. 7, both RANs and terminals have installed the GLL logical sub-layer to support the interworking between different RANs. GLL provides the unified link layer processing, offers a generic interface towards higher layers and an adaptive capability to the underlying RATs. It is a generic toolbox of link layer functions, which provides a unified interface to the higher layers and facilitates the efficient interworking between diverse RATs. The proposed GLL enables two novel concepts. The first one, named as Multi-Radio Transmission Diversity (MRTD), implies the sequential or parallel process of multiple RATs for the traffic flow’s transmission. The second, termed as Multi-Radio Multi-Hop networking (MRMH), implies the different wireless links via the heterogeneous RATs reach the same destination over the multi-hop routing path.

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Fig. 7 GLL structure for supporting interworking

4.1.1 Multi-Radio Transmission Diversity (MRTD) A major concern in the wireless communication system is how to control transmission error caused by the channel fading, the interference and noise. More error packets require more packet retransmissions (e.g., ARQ) or more longer and powerful error correction codes (e.g., FEC), which are at the cost of increasing of packets delay and decreasing of transmission rates. In the multi-RATs, one method to increase the transmission reliability is to efficiently utilize the Multi-Radio Transmission/Reception Diversity (MRTD) scheme. MRTD is defined as a well-defined split of data flows (on IP or MAC PDU level) between two communicating entities over more than one RAT. Depending on the level of integration between the different RATs and the location of GLL, two MRTD options are defined: IP level and MAC level [12]. Since IP level MRTD is far from the air interface, this paper only focused on the MAC level MRTD, which is described as Fig. 8. ……

Fig. 8 Structure of MAC level MRTD

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Fig. 9 Processing flow of proposed IPMRTD scheme

Fig. 10 Processing flow of proposed CPMRTD scheme

When referring to the scheme of determining the optimal multi-RATs for the transmissions, the MRTD schemes can be classified into switched and parallel types. For the switched MRTD, user’s data, equivalent in size to the payload of MAC PDUs, is transmitted via only one RAT at any given time [13]. Successive MAC PDUs may be transmitted via different RATs. In order to achieve higher diversity gain, the parallel MRTD is preferred. For the parallel MRTD, the simultaneous transmission of the same data over multi-RATs to the same destination is allowed. At the reception, the received packets from different RATs can be combined to achieve the diversity gain, and the corresponding combining strategies may use the selective combining, equal gain combing, or maximum ratio combining. Moreover, inspired by HARQ schemes, it can transmit the same packet but with different FEC codes or incremental redundancy bits on different RATs. If the received packets are detected error, these error packet blocks are not discarded, but are combined with the other transmission blocks over the different RATs to recover the original information. Note that this type of HARQ is based on the error detection of GLL, while traditional HARQ performs the combination on the physical layer. Two improved parallel MRTD strategy, named as incremental parallel MRTD (IPMRTD) and complementary parallel MRTD (CPMRTD), are proposed. For the IPMRTD scheme, described as Fig. 9, one RAT is used to transmit the basic data flow named as “prime sequence”, which is coded and has a certain capability of self-correction. The other RAT will be used to send the additional parity bits known as redundancy bits calling “incremental sequence”. At the reception, the prime sequence is directly forwarded to the data sink if it is error free. Otherwise, the prime and incremental sequences are combined together for decoding, which will bring out the diversity gain [14]. The basic concept of CPMRTD described as Fig. 10 is to transmit the same message with different FEC codes, any received packet that is detected error is self-decodable. Moreover, the packets that are detected error are not discarded, but are combined with the complementary transmission provided by the other RAT to recover the transmitted message. Note that if the final decoding failed when employing the IPMRTD or CPMRTD scheme, the HARQ scheme related to the physical layer can be applied which only need to retransmit the incremental redundancy bits (in IPMRTD) or another CPC coding sequence from the same coding family (in CPMRTD), to further provide the reliable transmission.

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Fig. 11 Architecture for MRMH

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4.1.2 Multi-Radio Multi-Hop (MRMH) In order to extend the coverage and improve the performance, the relay node (RN) is utilized in both WiMAX and B3G/4G systems. For the interworking mechanisms, the heterogeneous RN is deployed to complete the multi-RATs’ convergence. Actually, the MRMH scheme based on the heterogeneous RN can provide the multi-RAT’s diversity gain, in which the GLL acted as new logical layer will domain the functions of MRMH schemes. If the communication occurs in the same RAT, such as the link between WiMAX SS and WiMAX BS, the GLL will not be effective. However, if it is necessary to communicate in the different RATs, such as between B3G/4G Node B and WiMAX SS, the GLL can transfer the packet format of the different RAT to the native format. RN has the capability of heterogeneous communications and should have at least two communication modes in both the MAC and PHY layers, which is described as Fig. 11. For the proposed both architectures, the heterogeneous RN is configured for the packet relay between UTs and BSs over different RATs. In order to complete the functions of RNs, some technological problems should be solved, including the QoS mapping, the heterogeneous PDU format conversion, the packet scheduling between different RATs, and etc. As shown in Fig. 11, when the B3G/4G MAC PDUs (denoted as B3G-PDU) pass through the relay node and arrive at the WiMAX SS, they will be transferred to the WiMAX PDUs (denoted as WiMAX-PDU) in the GLL. How to guarantee the QoS over the multi-hop link is a great challenge for GLL. Considering there are 2-hops in the whole link, the end-to-end QoS is divided into two parts: first-hop and second-hop. If end-to-end QoS guarantees are necessary for the heterogeneous link, the given QoS metrics in the first and second hop should be considered jointly when B3G-PDUs are converted into WiMAX-PDUs, i.e., the end-to-end QoS should be mapped into the QoS guaranteeing in both the fist and the second hops. In addition to the QoS mapping, there are still other key problems with regard to implement the MRMH schemes, in which the function of heterogeneous RN is the key. Some open issues for completing the MRMH include:

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• RN completes the conversion of segmentation sizes over the different RAT, in which the challenge is how to reorder the sequence of segmentation over the multi-hop link. • The capacity in the multi-hop network is typically determined by the bottleneck hop and the weakest link. So for the bottleneck node and the bad links, the flow control mechanism should be utilized to avoid the extensive data buffering and reduce the amount of packet data that needs to be recovered in cases where the routing path changes. To facilitate the prioritization of traffic packets with high QoS, a priority based queuing scheme is required. • MRMH can be combined with MRTD to obtain higher diversity gain and improve the link quality over the multi-hop link. 4.2 Key Mechanisms in Multi-Radio Resource Management (MRRM) To efficiently use the radio resources in a multi-RANs, it is important to adopt the efficient RRM mechanisms between the different RATs. In order to avoid multiple interfaces between controller nodes, the same interface is preferred to be applied between them. Thus, the general load and radio resource parameters exchange over the interface is readily achieved. The MRRM should handle the radio resource management in the case of multi-hop network or multi-RANs, i.e., coordination should happen between the different administrative domains. MRRM is the functionality in the control plane. At the system level, MRRM performs the spectrum assignment, power allocation, handover control, load control and congestion control across two or more RANs. At the session level, MRRM coordinates decisions on different associated flows, where the event of executing MRRMs can be triggered either by system administration or by the session/flow arrival. The MRRM is divided into two logical parts to be built on the existing intrinsic RRM functions: (1) RAT coordination functions: The scope of these generic functions spans over the available RATs and typically includes functions such as dynamic RAT addition and removal, inter-MRRM communication, RAT selection, inter-RAT handover, congestion control, load sharing, adaptation of the allocated resources in a coordinated manner, and etc. (2) Network-complementing RRM functions: These technology-specific functions are particularly designed for one or more RATs. However, these functions do not replace the existing RRM functions but rather complement them. These functions may provide missing and complement inadequate RRM functions to the underlying RAT, e.g., providing admission control, congestion control, intra-RAT handover. They are responsible for the RAT-specific interaction and act as an adaptation function towards the network-intrinsic RRM functions. Hence, MRRMs appropriately translate the packet format and command to support the effective interaction. In order to integrate the characteristics of radio transmission, the burstiness of data traffic and the multimedia service’s QoS over multi-radio networks explicitly, the cross-layer RRM should be considered jointly. However, in order to be compatible with the current air interface protocol, this paper will not consider the impacts of cross-layer design on the interworking protocol. For the RCS based centralized architecture, the MRRM can be located in the MRCS. While for the AP based centralized architecture, the MRRM will be mainly incorporated in the RCS, and some adaptive scheduling algorithms can be transferred to the RAT-AP. Actually, if mobile WiMAX and B3G/4G systems shift most functions from RCS to RAT-AP, the MRRM will be dominated by RAT-AP. There is huge difference about the signaling and protocol related to the MRRM algorithms between these two proposed architectures. The detailed signaling and protocol definitions are strictly related to the standardization of both WiMAX and B3G/4G, which is out of this

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Fig. 12 Cost function based RAT selection scheme

AHP User’s User’s irement requirement requ

calculate weights AHP calculate weights

Trigger

Network condition

Calculate Cost function

Access to network

Larger the better

normalization Smaller the better

Information Collecting

Information Processing

Making decision

paper’s scope. In this paper, only the MRRM algorithms which are suitable for these two proposed architectures are investigated. The following parts present a non-exhaustive list of the most important RRM algorithms in the convergence of multi-RANs. 4.2.1 Mobility Management and Broadcasted Signaling The mechanisms of mobility management have higher complexity when considering the multi-radio coverage situations than the signal radio network. It remains important to update the location of the mobile user when it traverses the coverage areas of the different RAN covering. Except for the paging and location update signaling, the broadcasted information in each RAN will be extended and some new information has to been complemented, such as multi-RANs capabilities, QoS offering, cost information etc. Actually, the complementary information will consume the non-trivial scarce radio resource. Therefore, an efficient and low power consuming signaling for the mobility management and the enhanced broadcast procedures should be designed, which is used to minimize the consumption of the radio capacity and extend the battery life of the mobile UT. 4.2.2 Radio Access Technology (RAT) Selection Future devices can incorporate more than one communication modes and have the capability of solving the service request either from the user or from the network. The technological solutions should be transparent to the end user. Therefore, one of the principle research challenges involved in the heterogeneous networks is the network selection problem, in which the appropriate access network from those available multi-RANs is determined when users are reachable through several RANs. A perfect RAN selection scheme should provide the capability to communicate anywhere and anytime with high QoS and less cost, but also can improve the transmission quality of the wireless channel and utilization of spectrum. A cost function-based RAT selection algorithm is proposed in Fig. 12. The goal of the RAT selection algorithm is to optimize a pre-defined cost function to minimize the consumed resources and/or “minimal price”, maximize the achievable QoS level, and improve the spectrum efficiency. The RAT selection algorithm is flexible for many scenarios through regulating the weight factors. The implementation of the RAT selection algorithm can be divided into three stages: information collection, parameters processing, and the RAT decision. In the first stage, the available inputting information used in the RAN selection procedure is collected. This information consists of the radio propagation conditions, the load situation per RAN, the required QoS level according to the service property, the achievable QoS level per RAN, the consumed radio resources and the corresponding charge per RAN, etc.

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In the second stage, it is to calculate the weight of each parameter in the pre-defined cost function. The weight factors reflect the dominances of the particular requirements with respect to the user. AHP [15] as a mathematical-based technology to analyze complicated problems and assist in finding the best solution by synthesizing all deciding factors, is responsible to derive the weights of QoS parameters based on the user’s preference and service application. Furthermore, these QoS parameters are normalized. Since these parameters have different characteristics, the normalization of the data is performed according to two methods: largerthe-better, and smaller-the-better. In the last stage, based on the prepared parameters and information, the cost function can be calculated for each user-network pair. The cost function is pre-defined as: CostFunc = Wse × S E + Wc × Cost + Wα × α + Wβ × β + Wγ × γ

(1)

where the parameter S E is the spectrum efficiency and Cost represents the cost of transmitting a data unit in the specific network. α and β are the required bit rate and BER of the specific service. γ is the required GOS (Grade Of Service) for the serving network. The network with the maximum value of the cost function will be selected to access. 4.2.3 Load Balancing Balancing the load between multi-RANs allows for the better utilization of radio resources and the improvement of systems’ capacity. Many intelligent algorithms have been proposed to balance the load between different RATs. Here, a theoretical framework for evaluating the performance of dynamic load balancing strategies is discussed. A scenario of two RATs having the overlapped coverage area is discussed in this paper. For simplicity, it is supposed that the two networks have the same capacity C, and each service utilizes the single unit of radio resource. Based on the load balance strategy, the user or the service of one RAT can be transferred to the other. In our analysis, it is assumed that call requests arrive according to the Poisson process and call arrival rates in RAT 1 and 2 are denoted as λ1 and λ2 respectively. The serving duration time in both RANs is exponentially distributed with the parameter µ. By applying the multi-dimensional Markov chain to model the load state of both RANs, the blocking probability between the two interworking networks can be derived as the following content. Assuming p(0, 0) is the idle-state probability and s(i 1 ; i 2 ) is the state which both RANs experience, and the probabilities of all the states are derived and satisfied : P {s(i 1 ≤ c; i 2 ≤ c)} + P {s(i 1 > c; 0 ≤ i 2 ≤ 2c − i 1 ) ∩ s(0 ≤ i 1 ≤ 2c − i 2 ; i 2 > c)} +P {s(i 1 ≤ c; i 2 ≥ 2c − i 1 + 1) ∩ s(i 1 ≥ 2c − i 2 + 1; i 2 ≤ c)} = 1

(2)

The expression of each element in formulation (2) depends on the load balance strategy. When a “simple borrowing” scheme [16] is employed, the call blocking probability of network i(i = 1, 2), denoted as Pbi , is expressed as: ⎧ ∞ ∞ ∞ c ⎨ cc T1 i1 cc T2 i2 Pbi = p(0, 0) × + ⎩ c! c c! c i 1 =c i 2 =c i 2 =0 i 1 =2c−i 2 ⎫ i1 i2 Tc Tc⎬ T2 T1 (2c − i 2 )2c−i2 − 1 2 (3) × (2c − i 2 )! 2c − i 2 i2 ! c! c! ⎭

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Fig. 13 Comparison of blocking probability

Where T1 = λ1 /µ and T2 = λ2 /µ are the traffic intensities of network 1 and 2 respectively. Figure 13 describes the call blocking probability in different traffic intensity, i.e., it shows the comparison of blocking probability in the case of using the load balancing strategy (Pbi) and the case of not using any load balancing strategy (Pbs). According to the curves in Fig. 13, it can be seen that in terms of blocking probability, the case of using load balancing outperforms the case without using load balancing scheme, and the difference between them increases as the traffic intensity increases. 4.3 End-to-End QoS Provision The end-to-end QoS provision as the key performance factor has been investigated extensively in the homogeneous network, especially emphasizing on the traffic classification and the packet scheduling algorithm on different protocol layers. However, in the heterogeneous network, the end-to-end link may cover more than one hop and cross the different RANs. Meanwhile, the capability and the strategy of QoS provision for different RANs are different and are not easy to predict for the sender. Therefore, in order to provide the better end-to-end QoS guarantee in the heterogeneous network, the unified evaluation metrics of QoS capability is necessary. The QoS configurations of the whole network should be mapped into several QoS sub-configurations according to the number of hops. For the RCS based centralized architecture, the end-to-end QoS guarantee is executed in the MRCS. For the AP based centralized architecture, RAT-AP will domain the QoS provision if the RCS capabilities are degraded. Though the executing entity of the QoS provision is different from these two proposed architectures, there is no much difference about the QoS guaranteeing mechanisms and the corresponding algorithms. Considering the specific MRMH scenario showed by Fig. 14, there is a relay node connecting RAT-1 BS and RAT-2 UT. When the RAT-1 MAC PDUs (denoted as RAT-1 PDUs) pass through the relay node to the RAT-2 MAC PDUs (denoted as RAT-2 PDUs), each of them will be processed through the GLL in the relay node. Actually, RAT-1PDUs will be segmented and reassembled in several RAT-2 PDUs according to the PDU size in the different RANs. The overall packet loss and time delay are not only determined by the RAT-1 link but also the RAT-2 link. How to evaluate the packet loss in the different RANs is the key point to evaluate the performance of the whole network. The packet loss probability in the second hop considering of the segmentation and reassembly is derived and mapped to first link or sender (BS).

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Fig. 14 Segmentation and reassembly in MRMH

For simplicity, assuming the RAT-1 PDU can be divided into N (N > 1) RAT-2 PDUs, which are labeled from 0 to N − 1. The loss probability of the ith RAT-2 PDU is independent of others, defined as pi . The probability of successful transmission containing the continuous transmission of N RAT-2 PDUs in the second hop can be described as: PW = 1 −

N −1

(1 − pi ).

(4)

i=0

Because of the related fading characteristics of wireless channel, the loss of different PDUs is relative, and a Markovian model is adopted where the probability of a packet loss depends only on whether the previous packet is lost. Let Mi represents the event that the ith B3G/4G-PDU is lost, then: (5) P Mi |Mi−1 = αp, P Mi |Mi−1 = p where α > 1 and 0 < αp < 1, and α represents the relativity of the channel conditions in the time intervals for the transmission of two continuous RAT-2 PDUs. The larger α is, the more similar the channel conditions are. Then, the probability of the ith RAT-2 PDU delivering correctly can be calculated as: P Mi = P Mi |Mi−1 P Mi−1 + P Mi |Mi−1 P Mi−1 (6) = (1 − αp)P Mi−1 + (1 − p)P Mi−1 The steady-state probability that the RAT-2 PDU is delivered correctly can be derived from (6), denoted as β. β=

1 − αp 1 + p − αp

(7)

Finally, according to (4), the overall packet loss probability can be obtained: PW = 1 − β N

(8)

That is the capability of packet loss provided by the second hop which has an impact on the previous hop. The time delay mapping can be analyzed in a similar way. 4.4 Heterogeneous Paging In order to locate a user terminal (UT) in case of a new incoming call or any kind of service under the manageable situation, it is necessary to have the registration procedure of the whereabouts of the UTs. Unfortunately, the available coverage area, where the paging information can be received successfully, is often limited due to the link budget. Nevertheless,

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Fig. 15 Heterogeneous paging procedure. Notes:. ➀ W: WiMAX. ➁ PR: paging request. ➂ HLR: Home Location Register. ➃ HP: Heterogeneous Paging . ➄ HPI: HP Indicator

due to the very large number of subscribers, the number of messages related to the location management procedures will be huge, especially in heterogeneous networks in which the UT has more than one mode and it could not work on all the modes simultaneously because of the huge energy consumption which has great impact on the mobile phone. The location updating and paging signals will be much more complex than that in the single system. Most of the solutions offered so far take IP based paging approach [17], assuming only one air interface in the terminal. This poses two problems. First, many systems consumes a lot of power even just to listen to paging signals. Secondly, for a multiservice terminal, there is ambiguity as to which of the air interfaces to use while paging the terminal. In the interworking of GSM/UMA (Unlicensed Mobile Access) [18] and 3G/WLAN [19] scheme, the dual-mode UT has to always turn on the two radio interface simultaneously and scan RF criteria to trigger handover. Moreover, UT is always switching to UMA mode when in the overlapping area, the manual switching will introduce a long delay and inconvenience, and the automatic switching is not always necessary when there is not communication in UMA, which may cause increasing energy consumption and reduce throughput. 4.4.1 Novel Heterogeneous Paging Scheme On the analysis of heterogeneous paging issues, there is a pair of contradiction for paging the terminal with multi-mode in the heterogeneous network environment: a) UT working on dual modes simultaneously will result in more location updating signals and consuming a lot of energy even just listening to the paging signals. b) UT working on one fixed mode will result in less location updating signals but being passively connected to the network infrastructure, which is not efficient for listening to the paging messages from the other network. A novel heterogeneous location updating and paging scheme based on the previous proposed interworking architectures is proposed to solve the aforementioned problems. The proposal makes the terminal without traffic to send or transmit in a dormant status, which provides a power-efficient way and keeps the network signal minimum. Figure 15 illustrates the paging procedures, and the scheme can be described briefly as: (1) When the UT with dual modes travels in the overlapping heterogeneous cells, it only works on only one mode (based on preference, lower cost and so on) and executes the location update procedure in the corresponding system;

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(2) When the paging request message comes from the homogenous system, the paging procedure will trigger the location information as usual. (3) When the paging request message comes from the heterogeneous system (i.e., work on the energy saving mode for the heterogeneous system), a HPI (heterogeneous paging indicator) message is added in the paging channel and the paging information is sent via the other system’s radio interface which is in the working status. (4) After getting the HPI message, the UT automatically turn on the other mode, then searches for the available access point (AP)/Node B to get access into the heterogeneous network to start the communication. Note that the HP procedure is mostly determined by both the mobility management in the core network and the transmission mechanism between UT and AP/BS/Node B in the air interface. For the two proposed architectures, the difference mainly focuses on the responsible entities, i.e., RCS and MRCS respectively, which implied that the heterogeneous paging in these two architectures is almost the same except that BS/Node B is used for sending the paging information in the RCS based centralized architecture, while RAT-AP is adopted in the AP based centralized architecture.

4.4.2 Performance Analysis Signaling Reduction: In this proposed energy saving scheme, the UT only accesses one radio interface and there is no communication with the other heterogeneous system, which means that when the UT locates the paging area (PA) boundary of another heterogeneous RAN, it does not execute the location updating procedure, so the signaling load can be lessened. The advantage of the proposal is that the UT registers in one system and can communicate with the two systems simultaneously, while adopting the conventional mechanism, the UT has to register both systems. It also can be seen that the signaling load reduction is related with the overlapping ration of these two heterogeneous RANs. Energy Saving: With the conventional method in [20], the UT working only in the energy saving mode is in the standby state when there is no communication. If there is still no paging message arrived after a fixed time (assuming the length is fixed m, to predigest the problem), the UT may enter the deep energy saving state (named as idle state) to save more energy until the heterogeneous paging message arrives to wake it up. For our proposed scheme, the UT will turn off when there is no active communication, so there is only one mode to work anytime. Assuming that the arrival of the heterogeneous paging messages follows the Poisson distribution (denoted as λ), and the inter-arrival time (standby, sleep, idle, turnoff) follows the exponential distribution with the average value 1/λ. Besides, it is assumed that the traffic interval (communication interval) is predigested and fixed (denoted as L). Figure 16 shows the differences between the proposal and the conventional schemes in the aspect of the active state and the energy saving state. In our scheme, parameters E s , E p , E i and E a denote the energy consumption units per unit of time in the standby, sleep, idle state interval and active state interval respectively. The parameters Eˆ s , Eˆ p , Eˆ i and Eˆ a denote the energy consumption of each interval. According to the assumption, the length of the standby mode(X = S − L) follows the exponential distribution with mean 1/λ. The count of the active state follows the Poisson distribution. The energy consumption of active and standby states can be denoted as Eˆ a and Eˆ s respectively

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Fig. 16 Energy saving states and active states comparison

after K times of state exchange. Eˆ a =

K λk e−λ k=0

Eˆ s = E s K

k!

k L Ea

(9)

+∞

+∞ (s − L)λ e−λs ds = E s K sλ e−λs ds 0

0

+∞ Lλ e−λs ds = E s K /λ − E s K L = E s K (1/λ − L) −E s K

(10)

0

The energy consumption with the energy saving mode is Eˆ a , and the energy consumption when there is no communication in the standby state with the energy saving mode is Eˆ a + Eˆ s . Comparing the energy consumption in the standby state with that in the energy saving mode, the energy consumption saving ratio in our proposed method can be described as: K K λk e−λ λk e−λ ˆ ˆ ˆ µ1 = E a /( E a + E s ) = kLEa kLEa + Es K(1/λ − L) k! k! k=0 k=0 K K λk λk −λ −λ = Ea Le (11) k/ Ea Le k + Es (1/λ − L)K k! k! k=0

k=0

When K → ∞, µ1 stands for the average ratio of energy consumption saving. µ1 = E a L/(E a L + E s (1/λ − L))

(12)

In the same way, the energy saving ratio in the standby, idle, sleep states can be deduced. Given the fixed value of E a ,E s , L (assuming that E a = 8, E s = 5, L = 1, E p = 3, E i = 1,), the energy saving ratio decreases with the parameter λ becoming bigger. Meanwhile, the performance of our proposal has a huge performance gain when the traffic is not heavy and system load is low. The energy saving ration of the proposal and the conventional mechanisms is presented in Fig. 17, in which the performance comparisons in three states (such as standby, idle, sleep) are shown.

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Fig. 17 Energy consumption saving ratio in three states

Fig. 18 Energy consumption of the proposal and general scheme

According to the analysis and simulation results in Fig. 17, it can be seen that the less λ is, the lower µ is (which means our proposal provides higher performance gain). The parameter λ means the frequency of the service arrival, and the less the service arrival ratio, the more time the heterogeneous mode will be in the status of power off. Given the energy consumption ratio of active to standby state is 8:5, Fig. 18 shows the result of energy consumption of proposed scheme to energy consumption of using standby and idle state in energy saving mode when there is no communication. In this figure, m is the length of standby window for UT waiting to enter more energy saving state (idle state). According to the figure, it can be seen the energy consumption of the proposed scheme is less than general scheme. Moreover, the less λ is, the smaller value of the ratio of energy consumption of our proposed scheme to the conventional scheme is.

5 Summary Each RAT has its advantages and disadvantages with respect to the capacity, cost, achievable data rates and mobility. Since only a RAT is incapability of fully supporting all services and satisfying the user requirements, different RATs can be integrated to definitely improve the capabilities and performances.

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This paper proposed two architectures to make different RATs cooperation, which makes subscribers be able to access the multi-RANs seamlessly with high transmission quality. Based on the proposed interworking architectures for the Multi-RATs, the key problems and corresponding solutions are also presented, which includes four parts: key mechanisms in GLL, key mechanisms in MRRM, end-to-end QoS provision, and heterogeneous paging. Note that the performance comparisons of these two proposed architectures are not focused and only some general key mechanisms for both architectures are researched in this paper. Some special schemes available for these two different interworking architectures should be further investigated and their performance evaluation should be presented in the future work. The vertical handover control scheme for the multi-RANs should be further researched. Since the IEEE 802.21 standard attempts to give a solution of media independent handover recently, further work should consider some definitions in IEEE 802.21 standard. The performance of cooperative diversity between different RANs should be evaluated by the simulator. How to incorporate the cooperative techniques into the multi-RANs and improve the diversity gain of interworking is a interesting research hot spot. Acknowledgements Supported in part by the National Natural Science Foundation of China under Grant No. 60572120 and No. 60602058. Sponsored in part by the national advanced technologies researching and developing programs. (China 863 programming, NO.: 2006AA01Z257).

References 1. Hui, S. Y., & Yeung, K. H. (2003). Challenges in the migration to 4G mobile systems. IEEE Communications Magazine, 41(12), 54–59. 2. Ghosh, A., Wolter, D. R., Andrews, J. G., & Chen, R. (2005). Broadband wireless access with WiMAX/802.16: Current performance benchmarks and future potential. IEEE Communications Magazine, 43(2), 129–136. 3. Varshney, U., & Jain, R. (2001). Issues in emerging 4G wireless networks. Computer, 34(6), 94–96. 4. Chatterjee, S., Fernando, W. A. C., & Wasantha, M. K. (2003). Adaptive modulation based MC-CDMA systems for 4G wireless consumer applications. Consumer Electronics, IEEE Transactions, 49(4), 995– 1003. 5. Xu, F., Zhang, L., & Zhou, Z. (2007). Interworking of Wimax and 3GPP networks based on IMS [IP Multimedia Systems (IMS) Infrastructure and Services]. IEEE Communications Magazine, 45(3), 144– 150. 6. Song, W., Zhuang, W., & Cheng, Y. (2007). Load balancing for cellular/WLAN integrated networks. Network, IEEE. 21(1), 27–33. 7. Sachs, J., Muñoz, L., & Aguero, R. (2004). Future wireless communication based on multi-radio access. WWRF 11th, Oslo, Norway, Jun 10–11, 2004. 8. 3GPP TR 25.897. Feasibility on the evolution of UTRAN architecture, Release6, v0.3.1, 08/2003. 9. 3GPP TR 25.882. 3GPP system architecture evolution: Report on technical options and conclusions, Release7, v1.0.0, 03/2006. 10. Sachs, J. et al. (2005). Future wireless communication based on multi-radio access, WRRF internal reports, 2005 11. Niebert, N., Prytz, M. et al. (2005). Ambient networks: A framework for future wireless internetworking, VTC 2005-Spring. 2005 IEEE 61st Volume 5, 30 May–1 June 2005, pp. 2969–2973. 12. Dimou, K., Agero, R. et al. (2005). Generic link layer: A solution for multi-radio transmission diversity in communication networks beyond 3G. IEEE, VTC Fall 2005. 13. Koudouridis, G. P., Karimi, H. R., & Dimou, K. (2005). Switched multi-radio transmission diversity in future access networks. IEEE, VTC Fall 2005. 14. Kallel, S. (1995). Complementary punctured convolutional codes and their application. IEEE Transactions on Communication, 43, 2005-2009. 15. Saaty, T. L. (2000). Fundamentals of decision making and priority theory with the analytic hierarchy process. USA: RWS Publications.

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16. Kahwa, T. J., & Georganas, N. D. (1978). A hybrid channel assignment scheme in large-scale, cellular-structured mobile communication systems. IEEE Transactions on Communication, COM-26(4), 432–438. 17. Zhang, X., Gomez, J., & Campbell, A. T. (2002). P-MIP: Paging extensions for mobile IP paging. ACM Journal on Mobile Networks and Applications, 7, 127–141. 18. UMA (Unlicensed mobile access) specifications protocol (stage 3) R1.0.2 (2004-11-05). 19. 3GPP TS 23.234 v6.9.0. Technical specification group services and system aspects; 3GPP system to Wireless Local Area Network (WLAN) interworking system description. 20. IEEE Std 802.16e-2005. Part 16: Air interface for fixed and mobile broadband wireless access systems.

Author Biographies Mugen Peng received his PhD from Beijing University of Posts and Telecommunications (BUPT) in 2005. Presently, he is on the staff of BUPT and has done some research projects on the radio resource management algorithms, network planning and optimisation for 3G/B3G networks. His current research interests focus on the key techniques of wireless broadband access networks, key radio resource management algorithms for wireless multi-hop networks, and the cooperative theory in the wireless multi-hop network. He has published more than 60 technical papers and translated and edited 5 books until now.

Wenbo Wang received his BS an MS and a PhD from BUPT in 1986, 1989 and 1992, respectively. Now he is a Professor and PhD supervisor of BUPT. He is also the Dean of School of Telecommunications Engineering of this University. His research interests include essential technologies for 3G and B3G wireless communication systems, digital signal processing and wireless broadband networks. He has published more than 100 technical papers and authored 7 technical books.

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