Handoff in CDMA (Types, Mechanism, and It’s Parameters)

salam.

This is a little post about handoff in CDMA. Maybe you can use this article as reading material in your spare time

Handoff : In Cellular Telecommunication, user frequently moves from a certain point to another point in the service area. When it moves from a certain cell/sector (A) to another cell/sector (B), it have to be transfered from A to be B without break the connection (whether it is idle or dedicated state)

So, how many kind of handoff does CDMA have? Below, we are going to discuss it

Types of Handoff in CDMA:

1. Intersector / SofterHandoff.
   • Mobile station communicates with two sectors of the same cell site. It means that user moves from a sector to another sector in the same cell site.

2. Intercell / soft handoff.

 

• Mobile station communicates with two or three sector of different cell sites. It means that user move from a cell to another cel, and in the border site it attached to two (two way soft handoff ) or more (three way soft handoff) sectors from different cell sites. Primary cell site can send the forward control message, and other cell remaining are called secondary sites. In case 3 way handoff, it could be ended when one of secondary site is dropped by system (become two way handoff). Soft handoff use more network resources than softer handoff.

  

3. Soft-softer handoff

 

• Mobile station communicates with two sectors of one cell and one sector of another cell. This kind of handoff consume network resources equals with the resource for softer handoff and two way handoff altogether.

 

4. Inter-frequency / Hard handoff

• This handoff usually called brake before make handoff. Because mobile station terminate it’s old connection and then connect it’s new connection. Handoff occur when user move from a cell site to another cell site with different frequency or different MSC.
  

  

So… after we have discussed the types of handoff, now we’re going to discuss about handoff parameter in CDMA. Maybe this topic a little bit rough for your spare time hehehe…

Pilot Sets

The Pilot Term, means the pilot channel identified by a pilot sequence and a frequency assignment. Pilot is correlated with forward traffic channels CDMA. Each Pilot have a PN code as it’s identity. There is 64 chips offset between PN’s in the adjacent Pilots. The Pilots identified by the mobile is categorized in to four sets continuously:

 

• Active set, contains a batch of pilot in active condition, which is associated with forward traffic channel signed to the mobile. As we know that CDMA receiver in Mobile Station have 3 Rake Finger, each finger can handle 2 Pilot in active set.
• Candidate set, is a set of pilot which has sufficient signal strength so the traffic transmitted on it can be demodulated, but not yet in the active set(Max 6 Pilot).
• Neighbor set, contains neighbor pilots candidate for handoff (Max 20 Pilot). Especially for neighbor set, we can configure on the BSS Parameter.
• Remaining set, is a set of possible pilots remaining besides Active, Candidate, and Neighbor set.

So.. how it can be classified in to 4 kind of sets? To find the answer… let’s take a look in to picture below,

Hmm… maybe if I give a little information, it will be more useful. The picture above explain about handoff threshold mechanism. As you can see there are areas pointed with number, (1),(2) … Below I’ll try to give some brief explanation:

(1). Mobile station measures that Pilot signal strength is bigger than T_ADD (Pilot Detection Threshold). MS sends PSMM (Pilot Strength Measurement Message) and then transfer the Pilot to candidate set.

(2). HDM (Handoff Direction Message) is sent by BTS to MS using the Pilot to be added to in active set.

(3). After MS receive HDM (Handoff Direction Message) and obtain the new traffic channel, now Pilot is in the active set and MS sends HCM (Handoff Completion Message)

(4). After Pilot signal strength drops below T_DROP (Pilot Drop Threshold), MS starts the T_TDROP (Drop Timer Threshold)

(5). T_TDROP expires, MS send PSMM to BTS.

(6). BTS sends HDM, without related pilot to the mobile.

(7). After MS Receives HDM, Pilot becomes in to neighbor set. MS send HCM to BTS.

(8). NLUM (doesn’t include the Pilot) is received by MS, and Pilot become in to remaining set.

After we have discussed about Pilot Sets, now let’s talk about Search Window and it correlation to Pilot Sets. ^_^

Search Window (SRCH_WIN)

1. SRCH_WIN_A (active and candidate)
   • search window that MS uses for active and candidate Pilots tracking.
   • should be large enough to catch all usable multipath signal components from BTS, to anticipated the propagation environment. In the same time, it should be as small as possible to maximize searcher performance.
2. SRCH_WIN_N (neighbor)
   • search window that MS uses for neighbor set Pilots tracking.
   • larger than SRCH_WIN_A, because beside it has to be large enough for it’s own BTS, it is also should be large enough to catch all usable multipath from neighbor cells.
3. SRCH_WIN_R (remaining)
   • search window that MS uses for remaining set Pilots tracking.
   • at least equals with SRCH_WIN_N

Well, there’s a lot more if we want to observe more about handoff and power control in CDMA. Maybe if I have some spare time to write it I’ll post it here. I’m sorry if I filch your spare time :p

Salam…hopefully useful. (adapted from CDMA95.10)