What is Handoff?
Handoff is the process of changing the channel associated with the current connection while a call is in progress. It is often initiated either by crossing a cell boundary or by a deterioration in quality of the signal in the current channel.
Poorly designed handoff schemes tend to generate very heavy signaling traffic
and, thereby, a dramatic decrease in quality of service (QoS)
HANDOFF DECISION
There are numerous methods for performing handoff, at least as many as the kinds of state information that have been defined for MSs, as well as the kinds of network entities that maintain the state information. The decision-making process of handoff may be centralized or decentralized (i.e., the handoff decision may be made at the MS or network) From the decision process point of view, one can find at least three different kinds of handoff decisions.
In a network-controlled handoff protocol, the network makes a handoff decision based on the measurements of the MSs at a number of BSs. In general, the handoff process takes 100–200 ms. Information about the signal quality for all users is available at a single point in the network that facilitates appropriate resource allocation. Network-controlled handoff is used in first-generation analog systems such as AMPS (advanced mobile phone system), TACS (total access communication system), and NMT (advanced mobile phone system).
Handover scenarios in GSM systems
Intracell handover
The easiest type of handover is intracell handover where either the physical channel or the associated timeslot configuration is changed. This may become necessary if the connection on a physical channel is impaired. To evaluate connection quality, the mobile phone continuously transmits the measured RXLev (receive level measured by the telephone) and RXQual (bit error ratio determined) values to the base station. If the base station wants to hand over the telephone to another physical channel, all it needs to do is to inform the telephone about the new channel number and the new timeslot configuration. The telephone changes directly to the new channel and is able to maintain both its previous settings for timing and the base station parameters.
Intercell handover
If the mobile phone moves from one cell to another during a call, it must be handed over to the new cell. If the neighbour cell is time-synchronous with the current cell, the base station is able to effect a finely synchronized intercell handover. In this case, the mobile phone is transmitted on the new physical channel in the neighbour cell. Moreover, the mobile phone must be informed about the vital parameters of the new cell.
The mobile phone then optionally transmits four access bursts on the new channel. Compared to the normal bursts, these are shortened which is why they cannot cause interference with other calls even if the timing is slightly incorrect. If necessary, timing is corrected in a next step and the call continued. If the two cells with time offset are synchronous, the base station will effect a pseudo-synchronized or presynchronized intercell handover. This handover is similar to the finely synchronized intercell handover, but differs in that the mobile phone is provided with information about the time offset. Usually, however, a non-synchronized intercell handover takes place. In this case, the mobile phone transmits up to 64 access bursts on the new channel by means of which the new base station determines the timing and hands it over to the mobile phone. The mobile phone then reestablishes the call connection with the correct timing.
The base station requires the mobile phone’s help in order to know the new cell to hand it over to. By means of the neighbour cell list, the base station informs the mobile phone about the RF channels for the BCCH that are used by the neighbour cells. The mobile phone now cyclically measures the RF level on these channels and transmits the measurement results to the base station. Based on this information, the base station determines the point in time at which the mobile phone is handed over to which cell. Changing the physical
channel both for the call and for the BCCH information is key to intercell handover.
Handoff is the process of changing the channel associated with the current connection while a call is in progress. It is often initiated either by crossing a cell boundary or by a deterioration in quality of the signal in the current channel.
Poorly designed handoff schemes tend to generate very heavy signaling traffic
and, thereby, a dramatic decrease in quality of service (QoS)
HANDOFF DECISION
There are numerous methods for performing handoff, at least as many as the kinds of state information that have been defined for MSs, as well as the kinds of network entities that maintain the state information. The decision-making process of handoff may be centralized or decentralized (i.e., the handoff decision may be made at the MS or network) From the decision process point of view, one can find at least three different kinds of handoff decisions.
- Network-Controlled Handoff
In a network-controlled handoff protocol, the network makes a handoff decision based on the measurements of the MSs at a number of BSs. In general, the handoff process takes 100–200 ms. Information about the signal quality for all users is available at a single point in the network that facilitates appropriate resource allocation. Network-controlled handoff is used in first-generation analog systems such as AMPS (advanced mobile phone system), TACS (total access communication system), and NMT (advanced mobile phone system).
- Mobile-Assisted Handoff
- Mobile-Controlled Handoff
Handover scenarios in GSM systems
Intracell handover
The easiest type of handover is intracell handover where either the physical channel or the associated timeslot configuration is changed. This may become necessary if the connection on a physical channel is impaired. To evaluate connection quality, the mobile phone continuously transmits the measured RXLev (receive level measured by the telephone) and RXQual (bit error ratio determined) values to the base station. If the base station wants to hand over the telephone to another physical channel, all it needs to do is to inform the telephone about the new channel number and the new timeslot configuration. The telephone changes directly to the new channel and is able to maintain both its previous settings for timing and the base station parameters.
Intercell handover
If the mobile phone moves from one cell to another during a call, it must be handed over to the new cell. If the neighbour cell is time-synchronous with the current cell, the base station is able to effect a finely synchronized intercell handover. In this case, the mobile phone is transmitted on the new physical channel in the neighbour cell. Moreover, the mobile phone must be informed about the vital parameters of the new cell.
The mobile phone then optionally transmits four access bursts on the new channel. Compared to the normal bursts, these are shortened which is why they cannot cause interference with other calls even if the timing is slightly incorrect. If necessary, timing is corrected in a next step and the call continued. If the two cells with time offset are synchronous, the base station will effect a pseudo-synchronized or presynchronized intercell handover. This handover is similar to the finely synchronized intercell handover, but differs in that the mobile phone is provided with information about the time offset. Usually, however, a non-synchronized intercell handover takes place. In this case, the mobile phone transmits up to 64 access bursts on the new channel by means of which the new base station determines the timing and hands it over to the mobile phone. The mobile phone then reestablishes the call connection with the correct timing.
The base station requires the mobile phone’s help in order to know the new cell to hand it over to. By means of the neighbour cell list, the base station informs the mobile phone about the RF channels for the BCCH that are used by the neighbour cells. The mobile phone now cyclically measures the RF level on these channels and transmits the measurement results to the base station. Based on this information, the base station determines the point in time at which the mobile phone is handed over to which cell. Changing the physical
channel both for the call and for the BCCH information is key to intercell handover.
Intersystem handover
If the mobile phone leaves a cell and no new cell can be found in the same system, the base station can hand over an appropriately equipped mobile phone to a cell in another system. These intersystem handovers are highly complex because two technically disparate systems must be combined with each other. Basically, there are two handover options from WCDMA to GSM: In the case of blind handover, the base station simply transmits the mobile phone with all relevant parameters to the new cell. The mobile phone changes “blindly” to the GSM cell, i.e. it has not yet received any information about the timing there. It will first contact the transmitted BCCH channel, where it tries to achieve the frequency and time synchronization within 800 ms. Next, it will switch to the handed-over physical voice channel, where it will carry out the same sequence as with the non-synchronized intercell handover.
For the second type of handover from WCDMA to GSM, the compressed mode is used within the WCDMA cell; in this mode, transmission and reception gaps occur during the transmission between base station and mobile phone. During these gaps, the mobile phone can measure and analyze the nearby GSM cells. For this purpose, the base station, similar to the GSM system, provides a neighbour cell list, and the mobile phone transfers the measurement results to the base station. The actual handover in the compressed mode is basically analogous to blind handover.
There is, of course, an intersystem handover from GSM to WCDMA. A special neighbour cell list for WCDMA cells was established in GSM to support this handover.
