Access Handoff Process

As mentioned above, the mobile can only perform access handoff when it is in the system access state. Recall from Chapter 6 that the mobile enters the system access state typically after some action has been taken, either by the mobile or by the base station. Figure 8.6 shows the substates within the system access state. It turns out that the mobile can only perform access handoff when it is in two of the substates: page response substate and mobile station origination attempt substate.

As Figure 8.6 shows, the mobile enters the page response substate after receiving a general page message. While it is in the page response substate, the mobile responds by transmitting a page response message. The page response message may be transmitted on either an R-ACH which is associated with an F-PCH, or an R-EACH which is associated with an F-CCCH. On the other hand, the mobile enters the mobile station origination attempt substate after originating a call. While it is in

the mobile station origination attempt substate, the mobile transmits an origination message, and the origination message may be transmitted on either an R-ACH which is associated with an F-PCH or an R-EACH which is associated with an F-CCCH.

The standard specifies that the mobile must perform an access handoff in the page response substate or in the mobile station origination attempt substate if one of the following occurs [3]:

• _{The mobile loses the F-PCH or F-CCCH while waiting for a response from the}

base station, and the mobile is not already in the middle of an access attempt.

• _{The mobile loses the F-PCH or F-CCCH after receiving a message but before}

responding to that message, and the mobile is not already in the middle of an access attempt.

Recall that an access attempt is the process of sending a message to the base sta- tion and receiving an acknowledgment for the message [5]. In performing an access

8.5 Access Handoff 137 IS Update overhead information substate MS idle state MS control on the traffic channel state Page response substate MS message transmission substate PACA cancel substate Registration access substate MS origination attempt substate MS order/ message response substate IS IS IS IS IS Receives general page message Originates call Receives general page message Receives general page message Originates call or reoriginates PACA call Receives general page message

IS: Idle state

handoff, the mobile chooses as its target sector one with the best pilot strength. In addition, the chosen target sector should have a pilot strength that is greater than T_ADD as well [3].

How a mobile determines whether or not it has lost the F-PCH or F-CCCH is described in Chapter 9.

8.6

Access Probe Handoff

Access probe handoff occurs when a mobile, during an access attempt, stops sending access probes to the current base station and starts sending access probes to a new base station. Since an access probe handoff occurs during an access attempt, the mobile is in the system access state while performing an access probe handoff (see Figure 8.5). Similar to access handoff, access probe handoff can only take place in the page response substate or the mobile station origination attempt substate (see Figure 8.6).

Typically, a mobile may perform an access probe handoff in the page response substate or in the mobile station origination attempt substate if the follow- ing occurs:

• _{The mobile loses the F_PCH or F-CCCH, and the mobile has not performed}

more than MAX_NUM_PROBE_HO access probe handoffs during the cur- rent access attempt.

The parameter MAX_NUM_PROBE_HO is the maximum number of times that a mobile can perform an access probe handoff. The parameter is meant to pre- vent the mobile from performing an excessive number of access probe handoffs dur- ing an access attempt.

In performing an access probe handoff, the mobile first completes the transmis- sion of the current access subattempt to the current base station, and then it begins to transmit the next access subattempt to the new base station [3]. An access subat- tempt in IS-2000 is equivalent to a complete access attempt in IS-95-A. As Figure 8.7 shows, several access subattempts make up one complete access attempt, and a mobile can transmit one complete access subattempt to one and only one base sta- tion. At the end of the current access subattempt, the mobile either performs an access probe handoff (and thereby starts to transmit another access subattempt to a new base station) or terminates the access attempt. Therefore, if the mobile does not perform any access probe handoff, then an access attempt consists of only one access subattempt [6].

Figure 8.7 shows that an access attempt can contain up to (1 + MAX_NUM_PROBE_HO) access subattempts since the mobile cannot perform more than MAX_NUM_PROBE_HO access probe handoffs during an access attempt. Within each access subattempt, there can be up to MAX_RSP_SEQ access probe sequences if the mobile is transmitting a response message, or up to MAX_REQ_SEQ access sequences if the mobile is transmitting a request message.

And of course within each access sequence, there can be up to (1 + NUM_STEP) access probes. Each access probe is basically a single R-ACH or R-EACH transmis- sion3_{described previously in Chapter 4.}

8.7

Concluding Remarks

Readers should recognize now that the different handoff features described in this chapter are meant to improve system performance, and in the case of soft handoff system capacity as well. In terms of soft handoff, using the new drop threshold and add threshold enables a mobile to drop pilots that are not and add pilots that are value-added relative to the other pilots in the active set.

In terms of idle, access entry, and access handoffs, they afford the mobile an opportunity to switch the monitoring of overhead channels from one base station to another at different points of the state transitions. This serves to minimize the prob-

8.7 Concluding Remarks 139 Access attempt Access subattempt 1 Access subattempt M Access probe sequence 1 Access probe sequence 2 Access probe sequence N (M = 1+MAX_NUM_PROBE_HO) (N= MAX_REQ_SEQ) (N= MAX_RSP_SEQ) or Access probe 1 _{(P = 1+NUM_STEP)} Access probe 2 Access probe P time time time

ability of missing a message from the base station. In terms of access probe handoff, this feature gives the mobile, in the midst of an access attempt, the ability to transmit an additional set of access subattempt to a different base station having perhaps a stronger pilot; this helps improves the probability of achieving a successful access by the mobile.

References

[1] ANSI/TIA/EIA-95-B, Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems, Telecommunications Industry Association, March 1999.

[2] Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech House, 1998. [3] TIA/EIA/IS-2000.5-A, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread

Spectrum Systems, Telecommunications Industry Association, March 2000.

[4] TIA/EIA-98-C, Recommended Minimum Performance Standards for Dual-Mode Spread Spectrum Mobile Stations, Telecommunications Industry Association, December 1999. [5] TIA/EIA/IS-2000.3-A, Medium Access Control (MAC) Standard for cdma2000 Spread

Spectrum Systems, Telecommunications Industry Association, March 2000.

[6] TIA/EIA/IS-2000.4-A, Signaling Link Access Control (LAC) Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000.

C H A P T E R 9