Handoff%20Management

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Chapter 4

• Handoff Management
• Radio Link Transfer

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Introduction

• There are several alternatives for classifying
  link transfer procedures.
• hard handoff-oriented or
• soft handoff-oriented.

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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.

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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.

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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.

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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.

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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.

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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 networks.

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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).

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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).

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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).

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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).

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Intersystem handoff between two PCS networks.

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

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Handoff effect

• holding time 60 seconds.
• 0.5 inter-BS handoffs,
• 0.1 inter-BSC handoffs, and
• 0.05 inter-MSC handoffs.

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4.2 Hard Handoff
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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.

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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.

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MCHO
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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.

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• 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.

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• 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.

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• 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.

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• 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.

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4.2.2 MAHO/NCHO Link Transfer
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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.

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• 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.

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• 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.

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• 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.

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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.

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Subrating procedure
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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.

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• Steps 3 and 4. An existing caller, MSeXsting,
  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.

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• 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.

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• 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.

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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

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4.3 Soft Handoff

• Before we discuss soft handoff, we first
  introduce the code division multiple access
  (CDMA) 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.

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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.

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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.

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• 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.

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• 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.

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• 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.

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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.

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Dropping a BS
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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.

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• 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.

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• 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

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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.