Handoff

IDEN utilizes MAHO to assist in the handoff process. The
handoff can either be mobile- or base station-initiated depending on
the parameter settings. Handoffs are only possible with
interconnection calls. However, for a dispatch, the location information
supplied in the response also includes the neighbor list from cells that
are on the beacon channel list. Therefore, if the DLA is set up
incorrectly, it is possible that the subscriber will need to reacquire the
system if it moves outside of the coverage area of the sites in the list.

The mobile-assisted handover (MAHO) process is as follows:

1. The mobile monitors information on BCCH as to which cells to
monitor for inclusion in MAHO list.
2. The mobile continues to monitor SQE, the Receive Signal Strength
Indicator (RSSI) for the primary serving channel, and the channels in
the MAHO list.
3. If the subscriber detects trouble in the primary service or a better
neighbor cell, the mobiles sends a sample of its measurements.
4. The subscriber signals in the ACCH with an SQE measurement.
5. MSC/BSC/EBTS finds a new server to handover to and allocates a TCH
for this process.
6. MSC/BSC/EBTS senses a handover command on ACCH with the initial
power setting, channel, and TCH to tune to.
7. MS changes to an assigned channel.
8. MS uses the random access procedure (RAP) to get its timing
information from the target EBTS.

9. The channel changes to TCH and conversation continues.
Lastly, SQE is used extensively in various cell site selection decisions and
is based primarily on the outbound RSSI measurements of the serving cell
as well as for neighboring cells which are potential handover candidates.
SQE is very similar to C/(I  N) in the range of 15 to 23 dB. The Dispatch
system involves the key components of the iDEN system.

The Dispatch system basically has three primary service offering or
functions:

■ Private
■ Talk Group
■ Call Alert (twiddle)

Whereas Private Dispatch is where the originating call uses PTT
between one subscriber unit and another (classic two way), Call Alert is
used to notify a subscriber that a voice communication is desired. However,
talk groups involve a more extensive look.
Service areas (SAs) define talk groups, shown in Figure 3-39. The SA is
used for dispatch group calls. When a dispatch call takes place, a single
voice channel slot is used in any coverage area for a cell when one or more
members of the call group are in that coverage area. Fleets are assigned to
the same group and a mobile can be grouped into several talk groups in
order to communicate between specific groups that comprise the entire
fleet.
As briefly stated, a mobile can be grouped into several talk groups used
to communicate with a group of mobiles in the fleet at the same time or all
of the mobiles. For example, let’s say there is a fleet for all of N.Y. City, but
the subscriber only wants to talk with the Queens fleet. The mobile for the
Queens fleet is assigned their own talk group, which is part of the overall
fleet group. In doing so, a mobile can be part of numerous talk groups.
To help clarify, or further confuse the situation, a call-flow diagram for
dispatch calls is shown in Figure 3-37.

Looking at the flow chart in Figure 3-37, the following text better
explains some of the sequences:

1. Push to Talk (PTT) dispatches a call request.
2. The call request packet is routed to the DAP.
3. The DAP recognizes subscriber units’ group affiliation and tracks the
group members’ current location area.
4. DAP sends a location request to each group member location area to
obtain the various subscribers’ cell/sector location information.
5. The subscriber units in the group responds with their current
cell/sector location information.
6. The DAP instructs the originating EBTS with packet routing
information for all group members.
7. Call voice packets are received by the PD and then are replicated and
distributed to the group’s end node.

For interconnections, another portion of the iDEN system is utilized after
the radio access. The general sequence of events for an interconnection call
is the same, whether it is for a 3:1 or 6:1 call, with the exception of the
amount of TCHs assigned.

Therefore, the interconnection sequence for a mobile-to-land call is listed
here in brevity:
1. Call initiation
2. RAP on PCCH
3. DCCH assigned
4. Authentication
5. Call setup transaction
6. TCH assignment
7. Conversation
8. Call termination request via ACCH
9. Call is released

Figure 3-38 is a call flow diagram for a mobile land interconnection call
sequence that should help bring the components together. It is interesting
to note the differences between the interconnection call diagram and those
for the dispatch sequence.

The interaction of sharing resources for radio access for both interconnections
and dispatches involves the establishment of dispatch and interconnection
location areas, referred to as DLA and ILA. The DLA and ILA
are usually designed independently but have interactions that require joint
considerations to be made for the selection of both the DLA and ILA boundaries. The DLA and ILA boundaries are in addition to BSC boundaries;
however, the ILA or DLA needs to be inclusive of the EBTSs, which are connected
to a the BSC.

An example of a DLA boundary is shown in Figure 3-39, which shows a
total of four location areas associated with dispatch. Each location area is
then folded into an SA. Keeping in mind the dispatch discussion regarding
SAs, the design engineer must take care not only during the selection of
location areas, but in what constitutes the service area. The location area is
where the dispatch call is broadcast when the service area defines which
location areas are possible for inclusion in the dispatch call.

Figure 3-40 is the corollary to the DLA boundaries and shows the Interconnection
Location Areas (ILAs) for the same sample system. The ILA is
used for call delivery and paging for the subscriber unit. The ILA boundaries
should not be set up such that the subscriber units regularly transition
from one ILA to another, increasing the amount of overhead signaling
required to keep track of the mobile.

In looking at Figures 3-39 and 3-40, the differences between the ILA and
DLA boundaries become evident. Next, Figure 3-41 shows the composite
view of both ILA and DLA boundaries. 130