With the rapid development of mobile communication technologies and the customers’ needs, mobile phones have been developed from the first generation-analog system to the second one-digital mobile communication, which is the cellular mobile phone we use in present.
Besides the voice communication, the mobile phones are taken as a communication tool in which people can transmit images, send and receive e-mail, and have mobile Internet. In order to develop the mobile phones into multi-media and data communications, its third generation has been produced. With the growth of mobile Interne, moreover, the fourth generation of the mobile communication is being developed to provide high-speed data rate transmission under the high speed motion. The following section explores the development of mobile communication technologies [1].
The first generation (1G) of mobile communication technologies is analog frequency modulation among which American’s Advanced Mobile Phone System (AMPS) is regarded as the most successful. However, AMPS has defects, such as poor secrecy, limited system capacity, no data information transmission and higher volumes. The second generation (2G) adopts digital modulation technique, speech coding, and digital signal processing to provide higher secrecy, system capacity, spectrum efficiency, and the transmission of the low-speed data information. In order to offer more various services to customers, International Telecommunication Union (ITU) enacted the concept of third generation system (3G).
Different from voice services provided by the second generation mobile communication systems, the third generation mobile communication systems can provide the data transmission rate up to 2Mbps, and can dynamically adjust the data rate based on the user’s demands. Besides, the third generation mobile communication systems can support different types of services including voice, data transmissions, streaming video transmissions, and video phone. Users can get the huge resources from Internet anytime, anywhere through 3G networks. Both circuit-switched and packet-switched modes are supported by 3G systems.
Circuit-switched mode is used for real-time service such as voice and video services while packet-switched mode is used for non-real time data transmissions. 3G systems are able to
adjust the modes dynamically based on the service quality requirements [1].
Next generation wireless communication systems are moving toward integrating more than two networks, and users can choose the network according to the signal quality of the network and his/her requirements anytime, anywhere [1-3]. Owing to the developments of multimedia, people want to watch favorite programs on the go. Therefore, mobile communication technologies are developing toward high data rate transmissions. Users can not only transmit/receive voice and video but also get services from Internet to obtain useful information and multimedia services. The fourth generation mobile communication is developing toward this direction.
Multi-path fading channel effect is a problem that wireless communication systems must solve. If a modulated signal is transmitted, multiple reflected waves of the transmitted signal will arrive at the receiving antenna from different directions with different propagation delays and different phases. These reflected waves are combined together constructively or destructively giving rise to received signal fading at the receiver site depending on the phases of the reflected waves. Besides, inter-symbol interferences (ISI) might be generated owing to the superposition of reflected signals which have different delays. A equalizer must be used to solve ISI problem in the traditional single carrier systems with high speed transmissions. As the transmission rate increases, equalizer design becomes more complex. Multi-carrier systems like OFDM systems which data are transmitted by orthogonal sub-carriers aim to solve this problem. The symbol duration of the OFDM systems is longer than that of traditional single carrier systems. A cyclic prefix is added to the front of each symbol to reduce inter-block interferences made by multi-path channel effects. Because all sub-carriers in OFDM systems are orthogonal and partially overlapped, the spectrum efficiency is higher than the traditional single carrier systems. Currently, OFDM-based systems are IEEE 802.11a, HyperLAN 2, asymmetric digital subscribe loop (ADSL), and IEEE 802.16-based WiMAX system.
Internet protocol (IP) based Network is mutual in the wire-line networks, and the most popular Internet uses IP-based networks. Because IP-based networks are not related to the
radio access techniques, IP network is treated as the most important techniques to integrate heterogeneous networks so that they can inter-network with each other [3]. Compared with circuit-switched networks, IP-based network is more suitable for packet-based applications such as multimedia, data packets transmission, etc [2]. These applications are also the developing trends of future wireless communication systems. Future wireless communications are integrated by IP networks, and users can choose one of the usable networks according to the network conditions at that time.
In the wireless communication environments, the signals on the same frequency band will interfere with each other. Therefore, the spectrum, a very scare resource which should be used efficiently, causes the birth of the cellular wireless communication system. However, the adjacent BSs cause co-channel interferences to the users if they use the same frequency band.
Therefore, the concepts of frequency reuse which can reduce co-channel interferences and efficiently reuse the frequency band become very important. Besides, to use scare radio resources more efficiently, fractional frequency reuse (FFR) cell structure [4-7] has been proposed. When the users are cross to the serving BS, the SINR is larger than the SINR of the user in the vicinity of cell border. Thus, under the condition that the required SINR is met, the BS can serve the user using the sub-channels with the smallest frequency reuse factor to increase system capacity. This is the main functionality of the fractional frequency reuse technology.
Handoff technique is one of essential mechanisms to support user mobility in multi-cellular communication systems. The use’s signal quality is influenced by not only the distance between the user and the serving BS but also the obstacles in the signal propagation path. To maintain uninterrupted services, the handoff mechanisms should be initiated at the right moment such that the user can communicate with the BS having better signal quality.
Currently, the development of wireless communication is toward IP-based network systems, and both network layer handoff and link layer handoff should be taken into consideration in handoff mechanisms. The network layer handoff processing time depends on the network topology and states. In [8], authors used neighborcasting to achieve low latency handoff.
Before link-layer handoff, the MN notifies the old foreign agent (FA) to forward duplicated
packets to all neighboring FAs. Network-layer handoff latency can be reduced significantly. In [9], authors introduced mobile initiated tunneling handoff. Before disconnecting from the old AP, a mobile can initiate a handoff by sending the old foreign agent (FA) a handoff request containing the new FA’s information got from the mobile pre-trigger. The methods proposed by these two papers are to perform the network layer handoff in advance which handoff users’
packets can be forwarded to possible new FAs to reduced the network layer handoff latency.
In [10], the authors gave a survey of mobility management techniques in next generation IP-based wireless systems. Hierarchical network architecture is introduced in this paper. This network structures can avoid any signaling traffic to Home Agent (HA) as long as the mobile is within a regional network. An explicit proactive handoff scheme with motion prediction is introduced by [11]. A mobile can anticipate a handoff from the link layer trigger, and use locally stored movement patterns to dynamically predict the next subnet. As a result, handoff latency and packet loss rate is reduced.
Link layer handoff aims to adjust mobile’s transmit power, timing, and carrier frequency in OFDMA systems. To reduce the service disruption time, the concepts of scanning with association have been proposed in IEEE 802.16e-2005 [12]. During the normal operation, the serving BS may allocate time intervals to MS for the purpose of MS seeking and monitoring suitability of neighbor BSs as targets for handoff. The time during which the MS scans for available BS will be referred to as a scanning interval. MS can perform initial ranging with neighboring BSs in the scanning interval. Association is an optional initial ranging procedure occurring during scanning interval with respect to one of neighbor BSs. The function of association is to enable the MS to acquire and record ranging parameters and service availability information for the purpose of proper selection of HO target and/or expediting a potential future handover to a target BS. Consequently, the time required to perform link layer handoff can be reduced, and the service disruption time is also reduced.
Next generation wireless systems are envisioned to have an IP-based infrastructure with the support of heterogeneous access technologies. Next generation wireless systems also call for the integration and interoperation of heterogeneous networks. To achieve high data rate transmission and high spectrum efficiency, OFDM seems to be one of important technologies.
Fractional frequency reuse (FFR) cell structure can be used to improve scare radio resources utilization. For this reason, IP-based OFDMA system with factional frequency reuse cell structure is chosen as the system model in this thesis. The network layer handoff effect and link layer signal quality are jointly considered in designing handoff mechanisms in this thesis.
How the fast base station mechanism gets the benefits of fractional frequency reuse cell structure to improve performances is introduced in this thesis.
The rest of this thesis is organized as follows. In Chapter 2, we describe the system models.
Chapter 3 describes the different kinds of handoff mechanisms. Chapter 4 presents the signaling procedures. Fast base station switching with factional frequency reuse is presented in Chapter 5. Chapter 6 describes the simulation environments and results. Finally, conclusions are presented.