Inter-technology handover has become an important area of research to achieve ubiquitous connectivity especially with the emergence of 4G concept where IP-based mobile and wireless data networks will converge together and inter-technology handover will become a necessity to provide seamless and better services in terms of quality and capacity to the end users roaming between these technologies. The 2.5G network GPRS is only able to support very low data rate, while 3G network like UMTS is one step ahead of GPRS network and can provide high mobility with wide area coverage. It can support low to medium data rate which is still insufficient to carry data-intensive applications. The recently invented mobile WiMAX (IEEE 802.16e) was standardized to support mobility to the end user with wider coverage and higher data rate. However, WiMAX still has a critical drawback and it is about the standard does not fully define the complete network infrastructure. Hence, it is still a far future to deploy the WiMAX network on its own. But it can be interworked with the existing networks. This chapter will provide an overview of three technologies of interest and the handoff techniques within these networks.
2.2 GPRS Network
GPRS is a best-effort packet switched mobile data service that can provide data rates of 56-114 kbit/s [1]. It is a technology between the 2G and 3G generations of mobile telephony and often addressed as 2.5G. GPRS is integrated into [2] and newer releases and was originally standardized by ETSI, but now by the 3GPP.
2.2.1 GPRS Network Architecture
The main components of the GPRS network architecture is shown in Figure 2.1.
Figure 2.1 GPRS Core Network
Mobile Switching Centre (MSC): Abbreviated as MSC the mobile switching center connects calls by switching the digital voice data packets from one network path to another (also called routing). The MSC also provides the information that is needed to support mobile service subscribers, such as user registration and authentication information.
Gateway GSN (GGSN): It can be considered as the heart of the GPRS network. It is a gateway between GPRS network and other external packet data networks, like IP networks, the Internet and X.25 networks. It maintains the routing necessary to tunnel the Protocol Data Units (PDUs) to the SGSN and converts the GPRS packets coming from the SGSN into the appropriate PDP format and sends them out on the corresponding packet data network. Conversely, PDP addresses of incoming data packets are converted to appropriate format of the destination user. It is in charge of assigning IP address. It also performs authentication, charging functions, subscriber screening, IP Pool management and address mapping, QoS and PDP context enforcement. In essence, it carries out the role in GPRS equivalent to the Home Agent in Mobile IP.
Serving GSN (SGSN): It facilitates access to network resources in aid of the user and executes the packet scheduling policy involving different QoS classes. It is accountable for launching PDP context with the GGSN upon activation. It also mediates mobility management, logical link management, and authentication and charging functions.
2.2.2 GPRS Handover
Handover in GPRS network can be categorised from different perspectives. According to the handover from one network element to another it can be categorised in the following types [1]:
Intra-cell handover within the same BSC: There are normally several cells within the same BSC. Hence, the user roaming between these cells needs make this type of handover.
Intra-cell handover not within different BSC: While the user is roaming between the cells of different BSCs, i.e different IP-subnets, the user needs to change the link layer registration that forces a break-before-make type of handover.
Intra-MSC handover: There are normally several BSCs within the same MSC. Hence, this type of handover includes both type of handover discussed above.
Inter-MSC handover: When the handover is required from MSC to MSC the HLR information should be updated about the change and normally this type of handover is of break-before-make.
According to the function location, the handover in GPRS can be further categorised into following types:
Mobile initiated
Network initiated
Mobile assisted
The user can have three different mobility management states:
Idle- MS is not attached to GPRS
Standby- Subscriber is attached to GPRS mobility management, MS performs RA and cell selection locally, reports RA changes and Data, signalling or page response move the MS to READY. While the Detach procedures moves the state to Idle
Ready- Information on cell selection is reported which may be done locally or by network control
2.3 UMTS Network
UMTS is one of the 3G mobile telecommunications technologies, which is also being developed into a 4G technology. It is specified by 3GPP and is part of the global ITU IMT-2000 standard. The most common form of UMTS uses W-CDMA as the underlying air interface but the system also covers TD-CDMA and TD-SCDMA. Being a complete network system, UMTS also covers the radio access network (UTRAN), the core network (MAP) as well as authentication of users via USIM cards.
Unlike EDGE (IMT Single-Carrier) and CDMA2000 (IMT Multi-Carrier), UMTS requires new cell towers and new frequency allocations. However, it is closely related to GSM/EDGE as it borrows and builds upon concepts from GSM [22]. Further, most UMTS handsets also support GSM, allowing seamless dual-mode operation. Therefore, UMTS is sometimes marketed as 3GSM, emphasizing the close relationship with GSM and differentiating it from competing technologies. UMTS, using W-CDMA, supports maximum theoretical data transfer rates of 21 Mbit/s (with HSDPA), although at the moment users in deployed networks can expect a transfer rate of up to 384 kbit/s for R99 handsets, and 7.2 Mbit/s for HSDPA handsets in the downlink connection [24].
2.3.1 UMTS Network Architecture
UMTS specifies the UMTS Terrestrial Radio Access Network (UTRAN), which is composed of multiple base stations, possibly using different terrestrial air interface standards and frequency bands. Figure 2.2 shows the basic components of UMTS core network.
Figure 2.2 UMTS Core Network
Radio Network Controller (RNC): It is responsible for autonomous radio resource management (RRM) and organizes the functionalities of Node-B connected to it. The data is to be sent to the user are encrypted in it before sending. It also assists in Soft Handover for the user moving from one cell to another, controlling outer loop power and admission control, packet scheduling, and maintaining security functions, controlling dynamic radio bearer and so on.
Node-B: It is the fixed transceiver placed in different geographical area to establish communication between the user and the network. It uses WCDMA technology and requires higher power to operate than that of GPRS BS. This is because of using high frequency. Using of high frequency also degrades the cell size.
2.3.2 UMTS Handover
There are following categories of handover in UMTS:
Hard Handover: Hard handover means that all the old radio links in the UE are removed before the new radio links are established. Hard handover can be seamless or non-seamless. Seamless hard handover means that the handover is not perceptible to the user. In practice a handover that requires a change of the carrier frequency (inter-frequency handover) is always performed as hard handover.
Soft Handover: Soft handover means that the radio links are added and removed in a way that the UE always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed.
Softer handover: Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B (i.e. the site of co-located base stations from which several sector-cells are served. In softer handover, macro diversity with maximum ratio combining can be performed in the Node B, whereas generally in soft handover on the downlink, macro diversity with selection combining is applied.
Generally we can distinguish between intra-cell handover and inter-cell handover. For UMTS the following types of handover are specified:
Handover 3G -3G (i.e. between UMTS and other 3G systems)
FDD soft/softer handover
FDD inter-frequency hard handover
FDD/TDD handover (change of cell)
TDD/FDD handover (change of cell)
TDD/TDD handover
Handover 3G - 2G (e.g. handover to GSM)
Handover 2G - 3G (e.g. handover from GSM)
The most obvious cause for performing a handover is that due to its movement a user can be served in another cell more efficiently (like less power emission, less interference). It may however also be performed for other reasons such as system load control.
The different types of air interface measurements are:
Intra-frequency measurements: measurements on downlink physical channels at the same frequency as the active set. A measurement object corresponds to one cell.
Inter-frequency measurements: measurements on downlink physical channels at frequencies that differ from the frequency of the active set. A measurement object corresponds to one cell.
Inter-RAT measurements: measurements on downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one cell.
Traffic volume measurements: measurements on uplink traffic volume. A measurement object corresponds to one cell.
Quality measurements: Measurements of downlink quality parameters, e.g. downlink transport block error rate. A measurement object corresponds to one transport channel in case of BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only).
UE-internal measurements: Measurements of UE transmission power and UE received signal level.
UE positioning measurements: Measurements of UE position. The UE supports a number of measurements running in parallel. The UE also supports that each measurement is controlled and reported independently of every other measurement.
2.4 WiMAX Network
WiMAX is a telecommunications technology that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access. The technology provides up to 10 Mbit/s broadband speed without the need for cables. The technology is based on the IEEE 802.16 standard (also called Broadband Wireless Access). The name "WiMAX" was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard. The forum describes WiMAX as "a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL".
WiMAX is a possible replacement candidate for cellular phone technologies such as GSM and CDMA, or can be used as an overlay to increase capacity. It has also been considered as a wireless backhaul technology for 2G, 3G, and 4G networks in both developed and poor nations.
2.4.1 WiMAX Network Architecture
The WiMAX Forum has proposed an architecture that defines how a WiMAX network can be connected with an IP based core network, which is typically chosen by operators that serve as Internet Service Providers (ISP); Nevertheless the WiMAX BS provide seamless integration capabilities with other types of architectures as with packet switched Mobile Networks.
The WiMAX forum proposal defines a number of components, plus some of the interconnections (or reference points) between these, labelled R1 to R5 and R8 [4]:
SS/MS: the Subscriber Station/Mobile Station
ASN: the Access Service Network
BS: Base station, part of the ASN
ASN-GW: the ASN Gateway, part of the ASN
CSN: the Connectivity Service Network
HA: Home Agent, part of the CSN
AAA: Authentication, Authorization and Accounting Server, part of the CSN
NAP: a Network Access Provider
NSP: a Network Service Provider
It is important to note that the functional architecture can be designed into various hardware configurations rather than fixed configurations. For example, the architecture is flexible enough to allow remote/mobile stations of varying scale and functionality and Base Stations of varying size - e.g. femto, pico, and mini BS as well as macros.
2.4.2 WiMAX Physical and MAC Layer
WiMAX is a term coined to describe standard, interoperable implementations of IEEE 802.16 wireless networks, similar to the way the term WiFi is used for interoperable implementations of the IEEE 802.16 Wireless LAN standard. However, WiMAX is very different from Wi-Fi in the way it works.
Physical layer: The original version of the standard on which WiMAX is based (IEEE 802.16) specified a physical layer operating in the 10 to 66 GHz range. 802.16a, updated in 2004 to 802.16-2004, added specifications for the 2 to 11 GHz range. 802.16-2004 was updated by 802.16e-2005 in 2005 and uses scalable SOFDMA as opposed to the OFDM version with 256 sub-carriers (of which 200 are used) in 802.16d. More advanced versions, including 802.16e, also bring multiple antenna support through MIMO [35]. See: WiMAX MIMO. This brings potential benefits in terms of coverage, self installation, power consumption, frequency re-use and bandwidth efficiency. 802.16e also adds a capability for full mobility support. The WiMAX certification allows vendors with 802.16d products to sell their equipment as WiMAX certified, thus ensuring a level of interoperability with other certified products, as long as they fit the same profile.
Most commercial interest is in the 802.16d and 802.16e standards, since the lower frequencies used in these variants suffer less from inherent signal attenuation and therefore give improved range and in-building penetration [36]. Already today, a number of networks throughout the world are in commercial operation using certified WiMAX equipment compliant with the 802.16d standard.
MAC layer/data link layer: In Wi-Fi the MAC uses contention access - all subscriber stations that wish to pass data through a WAP are competing for the AP's attention on a random interrupt basis. This can cause subscriber stations distant from the AP to be repeatedly interrupted by closer stations, greatly reducing their throughput [37].
In contrast, the 802.16 MAC uses a scheduling algorithm for which the subscriber station needs to compete only once (for initial entry into the network). After that it is allocated an access slot by the base station. The time slot can enlarge and contract, but remains assigned to the subscriber station, which means that other subscribers cannot use it. In addition to being stable under overload and over-subscription, the 802.16 scheduling algorithm can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control QoS parameters by balancing the time-slot assignments among the application needs of the subscriber stations [37].
2.4.3 WiMAX Handover
WiMAX handover is the process where a mobile station migrates from air-interface of one base station to another air-interface provided by another base station. In order to provide the session continuity of IP layer during handover, the IP handover between different IP subnets is available. This is a transparent way for higher-level connections. Standard Mobile IPv6 is often unacceptable to real-time traffic such as VoIP because of latency. To override this Mobile IPv6 Fast Handover protocol (FMIPv6) has been proposed to improve the handover latency by predicting and preparing the impending handover. Cooperation between link layer and IP layer is necessary.
Three handoff methods are supported in IEEE 802.16e-2005; one is mandatory and other two are optional.
Hard Handover (HHO)
Fast Base Station Switching (FBSS) and
Macro Diversity Handover (MDHO)
The mandatory handoff method is called the hard handover (HHO) and is the only type required to be implemented by mobile WiMAX initially. HHO implies an abrupt transfer of connection from one BS to another. The two optional handoff methods supported in IEEE 802.16e-2005 are fast base station switching (FBSS) and macro diversity handover (MDHO).
Hard Handover (HHO): When MS stays in link, it listens to L2 (link-layer) messages. A BS periodically broadcasts a Neighbour Advertisement Message (MOB_NBR-ADV) for identification the network and to define the characteristics of the neighbour BS.
After that MS is able to scan the neighbour BS and measure the signal parameters. For future handover MS can perform ranging and association procedures. The handover is divided into two steps: handover preparation and handover execution.
Handover preparation. Either MS or serving BS may initiate the handover. When MS initiates the handover, it sends MOB_MSHO-REQ message. The serving BS replies with MOB_BSHO-RSP message containing recommended BSs after negotiation with candidate BSs. When BS initiates handover, it sends MOB_BSHO-REQ message only.
Handover execution. After handover preparation, handover execution is following. When the target BS is finally selected and only switching links remains, MS sends MOB_HO-IND message. After sending this message all communication between MS and serving BS discontinues. As soon as MS switches the link, it shall execute ranging with target BS. This means the MS can acquire the timing, power and frequency adjustment information of the target BS. Then MS negotiates basic capabilities, performs authentication and finally registers with the target BS. Since this time, the target BS starts to serve the MS; it becomes serving BS. Communication with MS via new serving BS is available now. If MS moves to different IP subnet, it should re-establish IP connection; IP handover should be performed as mentioned above.
Fast Base Station Switching (FBSS): An FBSS handover begins with a decision for an MS to receive/transmit data from/to the anchor BS that may change within the diversity set. An FBSS handover can be triggered by either MOB_MSHO-REQ or MOB_BSHO-REQ messages. When operating in FBSS, the MS only communicates with the anchor BS for uplink and downlink messages (management and traffic connections). The MS and BS maintain a list of BSs that are involved in FBSS with the MS. This is the FBSS diversity set. The MS scans the neighbour BSs and selects those that are suitable to be included in the diversity set. Among the BSs in the diversity set, an anchor BS is defined. An FBSS handover is a decision by an MS to receive or transmit data from a new anchor BS within the diversity set. The MS continuously monitors the signal strength of the BSs of the diversity set and selects one of these BSs to be the anchor BS. Transition from one anchor BS to another, i.e. BS switching, is performed without exchange of explicit handover signalling messages. An important requirement of FBSS is that the data are simultaneously transmitted to all members of a diversity set of BSs that are able to serve the MS. The FBSS supporting BSs broadcast the DCD message including the H_Add Threshold and H_Delete Threshold. These thresholds may be used by the FBSS-capable MS to determine if MOB_MSHO-REQ should be sent to request switching to another anchor BS or changing diversity set.
Macro Diversity Handover (MDHO): An MDHO begins with a decision for an MS to transmit to and receive from multiple BSs at the same time. An MDHO can start with either MOB_MSHO-REQ or MOB_BSHO-REQ messages. When operating in an MDHO, the MS communicates with all BSs in the diversity set for uplink and downlink unicast traffic messages. The use of this transmission diversity is not the same in the two different communications:
For a downlink MDHO two or more BSs provide synchronised transmission of MS downlink data such that diversity combining can be performed by the MS.
For an uplink MDHO, the transmission from an MS is received by multiple BSs such that selection diversity of the information received by multiple BSs can be performed.
The BSs involved in an MDHO or equivalently a member of an MS MDHO diversity set must use the same set of CIDs for the connections that have been established with the MS. The same MAC/PHY PDUs should be sent by all the BSs involved in the MDHO to the MS.
The decision to update the diversity set and the process of anchor BS update begin with notifications by the MS (through the MOB_MSHOREQ message) or by the BS (through the MOB_BSHO-REQ message).
