Information Technology Mobile Computing Module: GSM Handovers Learning Objectives What is handover? Why handover are required? Types of handovers(hard and Soft) Types of Handovers in GSM(Intra cell, Inter cell, Inter BSC and Inter MSC) Criteria for handover and handover decision Handover mechanism Introduction Handover/Handoff is a process typical to and of significant importance in cellular systems. The goal of proper handover is to ensure undisrupted calls and seamless services. It is a key performance indicator of any cellular network and customer satisfaction.handover is a mechanism by which the call or data service is transferred from one base to other or from one channel to another in order to maintain seamless connectivity. It is often triggered by various factors like low signal strength due movement of the wireless device away from existing base station or environmental conditions, and load balancing. A handover can be hard handover as in case of GSM or soft handover as in CDMA. Decision regarding handover depends on a value called handover margin which is the threshold upto which difference between two signal strengths is tolerated and above which handover is initiated. In this module we will study handovers in context with GSM telecommunication standard. Handover In a cellular system, geographical area is divided into smaller units called cells. Each unit is served by a base station which has low power transmitter/receiver antennas. The transmission of base station goes upto a certain distance depending on power of antenna (Fig. 1). The sizes of cell depend on topography and teledensity of the area. The cell sizes are bigger in suburbs or rarely populated areas and smaller in cities or highly populated areas (Fig. 2). All the devices belonging to a cell transmit/receive via base station.
Figure 1: Cellular Systems Figure 2: Cell sizes are bigger in suburbs than in cities At any point of time the signal strength received by the mobile station from a base station or vice versa should be upto a certain threshold value and below which it would be interpreted as noise. But it not always the ideal situation. Even when in free space, when the device moves away from the base station, signal strength decreases as per square of distance from the base station owing to the isotropic nature of the radiator. Multipath propagation due to the objects in the environment, error on the wireless links, further degrades the signal strength from the current serving base station. But at the same time it may starts receiving strong signals from new/nearby cell. Therefore to avoid discontinuity of service like call,the call or data service is transferred from current serving base station to new base station. The process is called handover. Handover is a mechanism of transferring a call or a data transfer from one cell to another or from channel to another.
In effect the transfer is from one channel to another or one code to code or one time slot to another.during handover the old channel/timeslot/code is released and a new channel/timeslot/code is assigned. Situations triggering Handovers 1. Mobile device moves out of range of one cell or base stations and gets much stronger signal from another base station. It releases connection from old station and call/data service is handed to new base station. Figure 3: Channel from old base station depicted in blue and from new base station in red 2. The entities above the cell i.e. BSC and MSC continuously monitor that traffic in a cell. If they observe that traffic in a cell is high relatives to other cells, some of the mobile stations are shifted to adjacent cells with lower load. This is known as load balancing. Figure 4: Load balancing
3. The device gets low signal strength from the current serving channel due to high bit error rate on the link. In this situation, call is handed over to different channel in the same cell. The channel can be a frequency /code/time channel. Types of Handovers Hard Handover Existing radio link from the current base station must be dropped for some period of time before connecting to new base station.the mobile is able to transmit only on one frequency at a time, the connection must be broken before it can move to the new channel where the connection is reestablished. No voice or data transmission takes place during the break period.this type of handover is also known as BREAK-BEFORE-YOU-MAKE. The break is for very short duration hardly noticed by user. GSM uses hard handovers and maintains interruption of 60 ms. Figure 5: Hard over where old connection is released then new connection is established Soft Handover Soft Handover does not provide interruption or breaking of radio link for cell-to-cell transfer of a call.in CDMA whole of frequency spectrum is available to all the users for the whole time. Therefore it is possible to have neighboring cells on the same frequency and hence there is no need to break the connection during handover. This is called soft handover or soft handoff, and it is defined as a handover where a new connection is established before the old one is released. Due to this property it also known as MAKE-BEFORE-YOU-BREAK.Connection goes seamless during this type of handover 3G technologies like CDMA, UMTS uses soft handover
Figure 6: Soft Handover Handover decision Handovers are caused due to low link quality. The measures of quality of the link are signal level and bit error rate. The mobile phone and base station periodically perform measurements of downlink and uplink quality of the current link used for transmission as well as quality of channels in the neighboring cells. The difference between the two measurements initiates the handover. A threshold value for this difference is known as Handover margin. If the difference between the link qualities of two channels from same or different cells is greater than Handover margin, a handover is initiated.it can be seen in the figure that MS is moving from old base station to new base station. The quality of link from old base station is deterioting whereas from new base station is getting stronger. Difference between them is depicted by HO_margin. The value of handover margin should be carefully chosen. If it is too large, it causes cut-off or call drops and if it is too small, it will cause frequent handovers.the Ho-Margin should also be such as to avoid ping pong effect which would cause handover even if signal quality is degraded by some other factor like bad weatheretc. Figure 7: Handover margin
Figure 8: Handover decision Types of Handover GSM architecture is a hierarchical organization. There is one administrative unit called MSC. Each MSC governs number of BSC s and each BSC governs number of base stations. Accordingly handovers are divided into different categories: Inter cell intra BSC Intra cell Inter BSC Intra MSC Inter MSC Intra Cell handover:the device is within the cell boundary but the signal quality from the current frequency channel is poor than the call or data transfer is redirected to another frequency. Inter Cell Intra BSC:The mobile device moves from one cell to another cell belonging to same BSC area. This handover is controlled by BSC. It assigns a new channel from new cell to the device and releases old channel.
Inter BSC-Intra MSC: A BSC has limited number of cells. If the device moves amongst cells controlled by different BSC area handovers are controlled by MSC Inter MSC: Handover between 2 cells belonging to different MSC. Figure 9: Types of handovers BTS: Base station transceiver; BSC: Base station controller; MSC: Mobile switching center The figure above shows 4 different types of handovers Cell 1 belongs to BTS 1 which is in BSC 1 area Cell 2 belongs to BTS2 which is also in BSC 1 area, Cell 3 belongs to BTS 3 which is in BSC 2 area, Cell 4 belongs to BTS 4 which is in BSC 3 area BSC 1 and BSC 2 belongs to MSC 1 area BSC 3 belongs to MSC 2 area. The figure also shows the four handover scenario just discussed. Scenario 1 is Intra cell handover label by 1 Scenario 2 is inter cell, Intra BSC handover since device is going from cell 1 to cell 2 Scenario 3 is Inter BSC Intra MSC handover Scenario 4 Inter MSC handover First 2 type of handovers are called internal handover lasttwo are called external handover. Handover Mechanism To understand the handover mechanism let us take case of inter BSC intra MSC handover
1) MS performs measurements and sends the reports to the current BTS. The BTS also performs measurements and sends reports to the BSC. 2) Studying the reports and current traffic conditions the old BSC takes the Handover decision. If it finds that handover is required, it sends HO-request message to MSC (since inter BSC handovers are controlled by MSC) and the new BSC belongs to its area. 3) MSC sends HO-request to the new BSC and requests the resources required for handover. The resources can be frequency or code channel or time slots. If the resources are available, the BSC activated new channel at new BTS and sets the stage for arrival of MS. 4) BTS new acknowledges the channel activation BSC new acknowledges the HO-request. 5) Handover is initiated by sending HO-command message by MSC to the old BSC and is forwarded to old BTS and then to the MS. 6) The HO-Command tells MS to break the old radio link and access new link provided by new BTS. 7) The link is established by link establishment message followed by HO complete message. 8) Clear command is sent to by the MSC to release the resources at old BSC and BTS. Old BTS releases the resources and sends clear complete message to signal successful handover. Summary Figure 10: Intra-BSC Inter MSC Handover Handover is a mechanism to transfer a call or a data transfer to new channel/cell when the quality or strength of current link is low below a threshold value. The threshold value should be carefully chosen so that it neither leads to call disconnectivity nor frequent handovers and at the same time avoids ping
pong effect. Handovers can also be initiated for load balancing purpose. In hard handover, connection with old base station is released and then new connection established whereas in soft handover first connection with new base station is established then the old one is released. A handover can be intra cell, Inter cell, Inter BSC or intra MSC depending on the location of old and new cell. The entities controlling the handover depend on the type of handover.