Chapter 4

Handoff Management

Radio Link Transfer

Introduction

– There are several alternatives for classifying link transfer procedures.

– hard handoff-oriented or – soft handoff-oriented.

Hard Handoff

• For hard handoff, the mobile station (MS) connects with only one base station (BS) at a time, and there is usually some interruption in the conversation during the link transition.

• Hard handoff is typically used in TDMA and FDMA systems.

Soft Handoff

• The MS receives /transmits the same signals from/to multiple BSs simultaneously.

• The network must combine the signals from the multiple BSs in some way.

• Thus soft handoff is more complicated than hard handoff.

Outline

• Hard handoff for mobile-controlled handoff (MCHO), network-controlled handoff (NCHO), mobile-assisted handoff (MAHO), subrating under TDMA systems,

• Soft handoff for MAHO under CDMA systems or some TDMA systems with macro diversity.

4.1 Link Transfer Types

– Two operations must take place for a successful link transfer:

– The radio link must be transferred from the old BS to the new BS.

– The network must bridge the link to the new BS into the existing call and drop the link to the old BS.

Link transfer

– Link transfer can be made from one channel to another channel on the same BS or from one BS to another BS, which subtends the same controller or switch.

– In these two cases, the network operation is relatively simple.

– Alternatively, the handoff can take place between BSs whose common point is much higher in the switching hierarchy of the network, in which case, the network operation can be expensive, time-consuming, and difficult.

Link Transfer cases:

– Intracell handoff. – Intercell handoff or inter-BS handoff. – Inter-BSC handoff. – Intersystem handoff or inter-MSC handoff. – Intersystem handoff between two PCS networ

ks.

Intracell handoff.

• The link transfer is performed between two time slots or channels in the same BS. For a TDMA system, intracell handoff is also referred to as time slot transfer (TST).

Intercell handoff or inter-BS handoff.

• The link transfer is performed between two BSs attached to the same base station controller (BSC); see Figure 4.1(a).

Inter-BSC handoff.

• The link is transferred between two BSs connected to different BSCs on the same mobile switching center (MSC); see Figure 4.1(b).

Intersystem handoff or inter-MSC handoff.

• The link transfer takes place at two BSs connected to different BSCs on different MSCs; see Figure 4.1(c).

Intersystem handoff between two PCS networks.

• The link transfer is between two BSs connected to different MSCs homing to different PCS networks.

Handoff effect

• holding time: 60 seconds.

• 0.5 inter-BS handoffs,

• 0.1 inter-BSC handoffs, and

• 0.05 inter-MSC handoffs.

4.2 Hard Handoff

MCHO Link Transfer

• In MCHO, when a handoff is needed, a new radio channel is selected by the MS, and a handoff request message is transmitted by the MS to the new BS.

• The handoff can also be initiated by the network. It is, however, still the responsibility of the MS to choose the best BS.

Failure case

• In the case of a handoff failure, the MS link-quality maintenance process must decide what to do next.

• There are several possibilities. The MS may choose to:– Initiate another handoff to the "next best" channel.– Simply stay on the old channel.– Try again later.– Perform some other action appropriate for the

situation.

MCHO

MCHO message flow for inter-BS handoff

• Step 1. To initiate handoff, the MS temporarily suspends the voice conversation by sending a link suspend message to the old BS.

• Step 2. The MS sends a handoff request message through an idle time slot of the new BS to the network.

• Step 3. The new BS sends a handoff acknowledgment message and marks the slot busy. The network may check other parameters to ensure that it wishes to complete the handoff.

• Step 4. Upon receipt of the handoff acknowledgment message from the network, the MS returns to the old assigned channel by sending a link resume message to the old BS.

• Step 5. The MS continues voice communication while the network prepares for the handoff.

• Step 6. Upon receipt of a handoff request message, the new BS checks if it already controls this call. If so, it is an intra-BS handoff. The BS sends a handoff acknowledgment message and reconfigures itself to effect the handoff. If it is an inter-BS handoff, the new BS acquires the cipher key from the old BS through the MSC. This session privacy key is transferred to the privacy coder associated with the new channel.

• Step 7. The MSC inserts a bridge into the conversation path and bridges in the new BS.

• Steps 8 and 10. Finally, the network informs the MS to execute the handoff via both the old and new BSs by sending the handoff execution messages, 8 and 10, respectively.

• Step 9. The MS releases the old channel by sending an access release message to the old BS. Note that messages 8 and 9 are not exchanged if the old channel fails before the new channel is established.

• Step 11. Once the MS has made the transfer to the new BS, it sends the network a handoff complete message through the new channel, and resumes voice communication. The network can then remove the bridge from the path and free up resources associated with the old channel.

• Bridges used for handoff should be inserted as quickly as possible. Bridges used in existing switching systems (such as "loudest talker" and "additive" bridges) may be adequate. However, it is possible that specific characteristics will be required in the future, possibly necessitating specialized bridges.

• DECT follows a similar MCHO procedure except that the selected new channel and the old channel may use the same carrier frequency. In this case, the MS does not need to switch frequency; this DECT handoff is referred to as seamless handoff.

4.2.2 MAHO/NCHO Link Transfer

MAHO/NCHO Link Transfer

• Step 1. The MS transmits the radio link measurement report to the old BS. In GSM, this information is updated every 0.5 seconds.

• Step 2. When the old BS determines that a handoff is required, it sends a handoff required message to the MSC. In terms of actions on the network side, the handoff is originated by the old BS in MAHO/NCHO, whereas in MCHO, the handoff is initiated by the new BS.

• Step 3. When the MSC receives the handoff required message, it examines the list of the candidate BSs supplied by the old BS and selects the highest-ranked BS with an available channel. Then it sends a handoff request message to the new BS-the target BS for handoff.

• Steps 4 and 5. When the new BS acknowledges the request, the MSC sends the handoff command message with the information regarding the new BS and the ItF channel to the old BS.

• Step 6. The old BS commands the MS to transfer the link to the new BS.

• Step 7. The MS tunes to the new RF channel, establishes the channel to the new BS, and sends the handoff complete message to the new BS.

• Steps 8 and 9. The new BS informs the MSC of the handoff completion by the handoff complete message. The MSC then clears the link to the old BS by the clear command message.

• Step 10. The handoff procedure is complete when the old BS acknowledges the clear command message.

• In MAHO or NCHO, the handoff command to the MS-message 6 in Figure 4.3-is sent over the failing link. The handoff procedure fails if the MS does not receive this message. In MCHO, the handoff request message-message 2 in Figure 4.2-is sent by the MS to the new BS on the new, more reliable, link. As a result, the success of the handoff does not depend on any signaling message over the failing link. Another advantage of MCHO is that it is not necessary to transmit measurement information via the air interface, thus reducing the signaling overhead required to maintain the call.

4.2.3 Subrating MCHO Link Transfer

• The procedure of subrating a full-rate channel into subrated channels for a handoff request consists of three parts:

• 1.Requesting the handoff.

• 2.Subrating an existing call.

• 3.Assigning the newly created subrated channel to the MS requesting the handoff.

Subrating procedure

Subrating procedure• Step 1. When Nlshandoff detects the need for a handoff, it attempts

to seize an available traffic channel. If an idle channel is found, the link transfer follows the MCHO procedure described in Section 4.2.1. If no traffic channels are available, the MS synchronizes to a common signaling channel (CSC) and transmits a priority access request message.

• Step 2. The new BS responds with either a priority channel assignment message or a priority access acknowledgment message. In the former case, the BS has a nonbusy channel, which it can immediately make available to the MS. In the latter case, the BS does not have an available channel and is simply acknowledging the receipt of the request message. The MS must continue to monitor the CSC for a priority channel assignment message. If no channel is available within a timeout period, the handoff call is forced to terminate.

• Steps 3 and 4. An existing caller, MSeX;sting, receives a time slot transfer message commanding it to perform a time slot transfer to a subrated channel. This action frees up a subrated channel for MShandoff, the MS requesting the handoff. This message is acknowledged by the transmission of the transfer complete message. The time slot transfer message is used by the MS to command an MSC to transfer the time slot in the same BS. The new time slot could be a subrated channel of the currently used full-rate traffic channel. This same message is sent to return both calls to full-rate time slots once a traffic channel becomes available.

• Step 5. MShanaoff is informed of the newly available subrated traffic channel via the priority channel assignment message.

• Steps 6 and 7. After receiving it, the MS synchronizes to the available channel and transmits a handoff request message, which will be answered by the handoff complete message. The subrated channels are switched back to full-rate channels immediately after some occupied channels are released, as shown in steps 812 in Figure 4.4.

• Step 8. When a user, Weaving/ terminates an existing call or performs its own handoff away from the BS, it transmits an access release message and releases its channel. The released channel may be either a full-rate channel or a subrated channel. Assuming that the channel is full-rate, the channel is not made available for access this time. Instead, two subrated channels are switched back to full-rate channels, as described in the next steps.

• Steps 9-12. The released full-rate channel is assigned to either MSeXisting or Nlshandoff through the timeslot transfer and transfer complete message exchange; both of these users now enjoy full-rate transmission.

Emergency access

• It follows that this access protocol should be generalized to also include a means for emergency access. To accomplish this, the message elements of the priority access request and the priority access acknowledgment messages should include an access random number to resolve collisions and to temporarily identify the MS requesting priority access, the type

4.3 Soft Handoff• Before we discuss soft handoff, we first introduce the code division multiple access (C

DMA) direct sequence spread spectrum technology. In this approach, the information-bearing signal is multiplied with another fasterrate, wider-bandwidth digital signal that may carry a unique orthogonal code. This second signal is referred to as a pseudo-noise sequence (PN sequence). The mixed signal looks very similar to a noise signal, but contains the information signal embedded in its code. The mixing operation is called "spreading." To recover the information-bearing signal, the receiving end must use the same PN sequence to "despread" the mixed signal. Thus, CDMA allows many users to share a common frequency/ time channel for transmission, and the user signals are distinguished by spreading them with different PN sequences. Also, an MS can transmit/ receive the same information to/from several BSs if they have the same PN sequence. In other words, in a CDMA-based mobile system, an MS may simultaneously receive/send the same information from/to several BSs using multiple radio links. The signaling and voice information from multiple BSs are typically combined (or bridged) at the MSC, and the MSC selects the highest-quality signals from the BSs. Similarly, voice and signaling information must be sent from the MSC to multiple BSs, and the MS must combine the results. Thus, within the overlap area of two cells, an MS can simultaneously connect to both the old and the new BSs, and the link transfer procedure is no longer time-critical.

• The following subsections describe the procedures for adding and removing BSs with MAHO soft handoff.

4.3.1 Adding a New BS

• CDMA BSs transmit pilot signals that assist MSs to track/ synchronize the BS downlink signals. The MSs measure the strength of the pilot signals of the serving BSs, that is, the old BS and the surrounding BSs. If the pilot signal strength of a surrounding BS-the new BS-exceeds a threshold, then the link between the MS and the new BS is established. The MAHO procedure of adding a new link to an MS is described in the following steps. The message flow is illustrated in Figure 4.5.

Adding a New BS

• Step 1. The MS sends a pilot strength measurement message to the old BS, indicating the new BS to be added.

• Steps 2 and 3. The old BS sends a handoff request message to the MSC. If the MSC accepts the handoff request, it sends a handoff request message to the new BS.

• Step 4.The new BS sends a null traffic message to the MS to prepare the establishment of the communication link.

• Steps 5 and 6. The new BS sends a join request message to the MSC. The MSC bridges the connection for the two BSs, as described in Chapter 4, Section 4.2.2, so that the handoff can be processed without breaking the connection.

• Steps 7-10. The new BS sends a handoff acknowledgment message to the old BS via the MSC. The old BS instructs the MS to add a link to the new BS by exchanging handoff command and handoff complete messages.

• Steps 11-14. The old BS and the MSC conclude this procedure by exchanging the required handoff information. The quality of the new link is guaranteed by the exchange of the pilot measurement request and the pilot strength measurement message pair between the MS and the new BS.

• In soft handoff MAHO, the link between the MS and the old BS may be of good quality. On the other hand, in hard handoff MAHO, the MS and the old BS typically communicate through a failing link.

4.3.2 Dropping a BS

• If the signal strength on the link between a BS and the MS falls below a predetermined threshold, the MS requests to remove the BS. Assume that the old BS is to be dropped. The MAHO procedure of dropping an old link from an MS is described in the following steps.

Dropping a BS

Drooping a BS

• Steps 1-3. The MS sends a pilot strength message to the old BS to remove the BS with the failing link. The old BS and the MS exchange the handoff command message pair to remove the link.

• Steps 4 and 5. The old BS sends the relevant call record information to the new BS by exchanging the interface primary transfer message pair.

• Steps 6-9. The new BS and the MSC exchange the handoff information message pair to indicate the failing link to be dropped. Then the new BS and the MS exchange the pilot measurement message pair to ensure that the communication between the MS and the network can be continued after dropping the failing link to the old BS.

• Steps 10 and 11. The MSC and the old BS exchange the remove link message pair to remove the bridge between the new and the old BSs and other resources.

• In addition to soft handoff, two other types of link transfer are defined for IS-95 CDMA: softer handoff and hard handoff.– ~ In many existing IS-95 cellular systems,

a BS is designed with threesector directional antenna. Softer handoff occurs when the MS is in handoff between two different sectors at the same BS.

– Cal

Handoff Management: Radio Link Transfer

– Hard handoff occurs in IS-95 systems when the two BSs connected tc an MS are not synchronized or are not on the same frequency band.