Soft Hand Off CDMA

8/6/2019 Soft Hand Off CDMA

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Handoff is an essential component of mobilecellular communication systems.

Mobility causes dynamic variations in link quality

and interference levels in cellular systems.

This change is known as a handoff.


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Hard handoff: A definite decision is made onwhether to handoff or not.

Soft handoff: A conditional decision is made on whether to hand off.


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Implementing hard handoffs is potentiallydifficult in power-controlled code-divisionmultiple access (CDMA) systems.IS-95 CDMA standard(the only major cellularstandard) implements soft handoff as apower control & interference-reductionmechanisms.


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1. To eliminate near-far effect2. To conserve transmit power.

What is power control?

Power control ensures that each user receivesand transmits just enough energy to properlyconvey information while interfering with otherusers no more than necessary.

Minimizing the transmitted power of a portableuser unit maximizes the interval between batterycharges.


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A system with power control attempts todynamically adjust transmitter powers whilein operation.Power control is the main tool used in IS-95to combat the near-far problem.


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Provide a reference signal for all MSs thatprovides the phase reference for COHERENTdemodulation.4-6 dB stronger than all other channelsUsed to lock onto other channels.Contains no information except the RFcarrier.

Spread using the PN spreading code toidentify the BS. (512 different BS*64 offsets).


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Using thresholds:

1 . Add threshold for adding a newconnection.2. Drop threshold for deleting a connection

(i.e. if signal goes below threshold and stays

there for some specified time)3. Timer


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Active set: The set of base stations with which a user iscommunicating at any given time. IS-95 allows up to six pilots inthe active set, with two pilots sharing one RAKE finger.Candidate set: This set contains the pilots that are not currentlyin the active set. However, these pilots have been received with

sufficient signal strength to indicate that the associated forwardtraffic channels could be successfully demodulated. Maximumsize of the candidate set is six pilots.Neighbour set: This set contains neighbour pilots that are notcurrently in the active or the candidate set and are likelycandidates for handoff. Neighbours of a pilot are all the sectors/cells that are in its close vicinity. The initial neighbour list is sent

to the mobile in the System Parameter message on the pagingchannel. The maximum size of the neighbour set is 20.Remaining set: This set contains all possible pilots in the currentsystem, excluding pilots in the active, candidate, or neighboursets.


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There are four handoff parameters.T_ADD, T_COMP, and T_DROP relate to the

measurement of pilot and T_TDROP is a timer.

Add Threshold (T_ADD): Any pilot that is strongbut is not in the HDM is a source of interference.This pilot must be immediately moved to theactive set for handoff to avoid voice degradationor a possible dropped call. T_ADD affects thepercentage of mobiles in handoff. It should below enough to quickly add useful pilots and highenough to avoid false alarms due to noise.


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Com parison Threshold (T_COMP): It has effect onhandoff percentage similar to T_ADD. It shouldbe low for faster handoff and should be high toavoid false alarms.Drop Threshold (T_DROP):

It affects thepercentage of mobiles in handoff. It should below enough to avoid dropping a good pilot thatgoes into a short fade. It should be high enoughnot to quickly remove useful pilots in the active

or candidate set. The value of T_DROP should becarefully selected by considering the values of T_ADD and T_TDROP.


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Drop Timer Threshold (T_TDROP): It shouldbe greater than the time required to establishhandoff. T_TDROP should be small enoughnot to quickly remove useful pilots. A largevalue of T_TDROP may be used to force amobile to continue in soft handoff in a weakcoverage area.


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Soft handoff reduces/eliminates the ping - pong effectcommon in hard handoff.This results in:- Less load on the network from handoff signalling

and overhead.- Smoother user communications without the clicks

typical of hard handoff when speech transmissions arestopped momentarily during handoffs.

With soft handoff, there is no hysteresis margin , resultingin less delay and equivalent to instantaneousmacroscopic selection diversity. This is accomplished byinstantaneous switching to the best base station signalduring a soft handoff (uplink), and avoids the additionalinterference associated with handoffs with hysteresis.


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Hence: Keeping base station separations and (base station and user) transmitter powersfixed, the overall uplink interference is reduced,

leading to:a) better communication quality for a given number of users.b) more users (i.e., greater capacity) for the same required Ec/I0 (ratio of received

energy per chip to total received spectral density).c) smaller required uplink transmitter powers, further reducing uplink interference. Keeping required outage probability and base station separation fixed, the system

fade margins are reduced. This leads to smaller required downlink transmitterpowers and downlink interference.

Keeping the same required outage probability and fade margins, base stationseparations increase.

Soft handoff imposes fewer time constraints on the network.

There is a longer mean queuing time to get a new channel from the target basestation, so this helps reduce blocking probability or probability of dropped calls.


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Needs extra modems - typically 30% to 50% of callsmay be in soft handoff at any time.Additional network resources are used during a softhandoff. These resources thus become unavailablefor use elsewhere.

Soft handoff is more complex.Downlink interference (to other users) increases whensoft handoff is in progress, since several basestations are transmitting what would otherwise betransmitted by one base station. This can add to theuplink interference too, if the same frequency is usedfor uplink as for downlink. The interferenceincreasing effect should normally be slight, if it isassumed that only a small fraction of the duration of a typical call is spent in soft handoff.


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Soft handoff is an intriguing technology. It promises better performance than hardhandoff.Because of the complexity of soft handoff,

another main area of research on soft handoff is on the different trade-offs involved in thehandoff parameter settings.The quantitative trade-offs between the

various advantages and disadvantages of softhandoff need to be further investigated, as dothe parameter settings.


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Soft Hand Off CDMA

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