Technically Crazy: CDMA Hard Handoff

What is Hard Handoff?

The hard handoff process here concerns the D-to-D handoff, in which a handoff between CDMA carriers occurs. D-to-D handoff is carried out via the handoff direction message and the process is known as break-before-make. In break-before-make, the call is interrupted before the acquisition of the new carrier. If the acquisition is successful, the call continues; otherwise, the call is interrupted. Hard Handoff can be possible between different system , Different BSC/MSC where direct ATM link is not established and also between CDMA to Analog system (D-to-A).

Advantages of Hard Handoff

● Continue the call beyond the current network.
● Provide expanded service.
● Overlay and integration of new CDMA networks with the existing networks.

How to Implement Hard Handoff?

The different methods for implementing hard handoff are listed below.

1) Pilot Beacon
§ A pilot beacon cell is a signal which can be seen by arriving mobiles from the other system on their own frequency, inducing them to request handoff as soon as it is appropriate
§ When mobiles request soft handoff with the beacon, the old system steps in and instructs the mobiles to do inter system hard handoff to the real cell which the mobiles are approaching on the other system

2) RTD Triggered
§ The out looking sector of cell is tagged in the site database as a “border sector”. Whenever a mobile is served exclusively by a border sector, the system continuously monitors that mobile’s round trip delay (RTD).
§ When the mobile’s RTD passes upward through a data filled threshold, the system steps in and orders a hard handoff to previously defined sector on the other system/same system

3) Ec/Io Triggered
§ Out looking sectors of border cells are tagged as “border sectors” in the system database
§ Whenever a mobile is served exclusively by a border sector, the system frequently interrogates the mobile with pilot measurement request messages
§ When the mobile’s reports the boundary sector’s Ec/Io is below a preset threshold, the system immediately commands a hard handoff to a previously defined sector on the other system/same system.

4) Enhanced Hard Handoff

§ EHHO is a Quality based HHO Trigger, it allows the call to remain on overlay sector(s) as long as possible & only hand down to underlay when call quality is impacted.

§ User configured trigger conditions are defined from many parameters like FER, Forward Traffic Gain or Eb/No. We can set threshold for individual parameters.

How Hard Handoff Works?

The base station directs the mobile station to perform a CDMA-to-CDMA hard handoff by sending an Extended Handoff Direction Message , a General Handoff Direction Message or a Universal Handoff Direction Message in which the mobile station is transitioned between disjoint sets of base stations, different Frequency Assignments, or different frame offsets.

Different Handoff Messages.

Forward Traffic Channel Handoff Messages
1. Pilot Measurement Request Order:
2. Analog Handoff Direction Message:
3. Neighbor List Update Message
4. Extended Handoff Direction Message:
5. Candidate Frequency Search Request Message:
6. Candidate Frequency Search Control Message:
7. Extended Neighbor List Update Message:
8. Supplemental Channel Assignment Message:
9. General Handoff Direction Message:
10. Periodic Pilot Measurement Request Order:
11. Universal Handoff Direction Message:
12. Mobile Assisted Burst Operation Parameters Message:
13. Extended Supplemental Channel Assignment Message:
14. Forward Supplemental Channel Assignment Mini Message:
15. Reverse Supplemental Channel Assignment Mini Message:

Reverse Traffic Channel Handoff Messages
1. Pilot Strength Measurement Message or Extended Pilot Strength Measurement Message:
2. Handoff Completion Message or Extended Handoff Completion Message:
3. Candidate Frequency Search Report Message:
4. Periodic Pilot Strength Measurement Message:
5. Pilot Strength Measurement Mini Message:

Message Flow for the Hard handoff.