Soft and Softer Handover in WCDMA

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Soft Handover in WCDMA System

24th March 2010



Contents1. Soft Handover Concepts – Definition

2. Macro diversity

3. Advantages

4. Disadvantages

5. Soft Handover Events

6. Soft Handover Event Triggering and related parameters

7. Soft Handover Factor(Ratio)



Definition 1:

Soft handover refers to the process that allows a Mobile connection tobe served simultaneously by several cells, adding and dropping them as

needed. This feature is possible in WCDMA because all cells use the

same frequency and are separated only by codes.

Definition 2:

Soft handover means that the radio links are added and removed in a

way that the UE always keeps at least one radio link to the UTRAN. Soft

handover is performed by means of macro diversity, which refers to the

condition that several radio links are active at the same time.





Macro-diversity:

Q: What is the difference between SHO and macro-diversity?

A:Note the difference in these concepts: a SHO is a procedure. Once it is performed,the result is a macro-diversity situation.

Signals are transmitted over the air interface on the same frequency,

from several base stations separated by considerable distances. Thisscheme is called the soft handover (SHO). In a SHO all the participating

base stations use the same frequency, and the result is a macro-diversity

situation.

In macro-diversity the mobile’s transmission is received by at least two base stations, and similarly the downlink signal is sent by at least twobase stations.



Macro-diversity in the DLIn the downlink the UE can receive, at most, as many macro- diversity components asit has fingers in its RAKE receiver. Thus the more RAKE fingers the UE has, the better

performance it has, providing that all fingers find a separate diversity component.However, from the system point of view this case is not so clear. Each newtransmission may also increase the system interference. If too many base stations areused in an SHO, the system interference level increases instead of decreasing andpreserving the usefulness of an SHO.

Macro-diversity in ULIn the uplink the effects of macro-diversity are only positive, as the more basestations that can receive the signal from a UE, the better the probability that some of them will receive it successfully. This does not generate more transmissions or

interference. Indeed, the opposite is true, as the UE transmission power level canprobably be lower if macro-diversity is used.



Huawei’s implementation of Macro-diversity in RNC:

MDC Module in DSP(DPUb)

MDC: Macro Diversity Combining (MDC) processes the uplink

combining and downlink distribution of the macro diversity for a

UE during soft handover, thus improving the transmission quality.



Best Time of gain during Soft Handover:

The gain from macro- diversity is highest when the path losses of the

SHO branches are about equal. If one of the participating base stations is

clearly stronger than the others, then macro-diversity cannot provide

Much gain.



Soft Handover Combining Gain:

1. Effective Signal to Interference Increases when UE is in softHandover.

2. If the SIR increases means the transmit power can be decreased

3. If Transmit power can be decreased means less interference

Description:

On the DL, the UE can combine the different received signals to increase

the reliability of demodulation. By combining the signals from different

links, the effective SIR increases, which reduces the transmit powereven when compared to the power required over the best link only. This

is termed soft combining gain.



On the UL, if macro-diversity gain is observed, the same is not always

true for the soft combining gain. If the cells in soft handover do not

belong to the same Node B, it is not possible to combine the signals

before they are demodulated.

Instead, all the demodulated frames are sent to the RNC, which decides

which one to use. This process still provides a gain compared to a single

link, since it increases the probability of having at least one link without

error. This is the selection gain, also a macro-diversity gain.



Soft handover advantages:

1. It increases the reliability of transmission

2. Reduces the transmit power requirement for each link used.

3. UE at the boundary among several cells uses the minimum transmit power oneither link

Soft handover drawbacks:

Since information must be sent over multiple links, that repetitiondecreases the efficiency of resource utilization.

More transmitted signals may mean more energy in the air, which means moreinterference to the radio environment in the downlink direction.

The control procedure in the UTRAN has to be very clever indeed to meet theconflicting demands of mobility and low interference levels. SHO branches should beadded to a connection only when the estimated resulting total interference level isless than it would be without the SHO.



Gain in Downlink transmission power due to Soft HandoverPositive value = gain, Negative value = loss

Gain in TX Power

Relative path Loss between two Node B cells

For large signal value difference

we can see that transmission

power even is raised,

because of signaling errors on

downlink (power control

commands).

Best results are

achieved when signal

levels from two NodeB’s has equal values

Gain= 2.5 dB

When the signal of

two cells are more

than 4.5 dB apart, it

starts to have negative

impact.



Gain in uplink transmission power due to Soft HandoverPositive value = gain, Negative value = loss

Gain in TX Power

Relative path Loss between two Node B cells

Best results are

achieved when signal

levels from two Node

B’s has equal values

When the signal of

two cells are more

than 5 dB apart, it

starts to have negative

impact.



Typical handover parameters

Window to add Window to drop

1 –3 dB 2 –5 dB



Effect of “No Soft Handover”

Without SHO, a communicating base station would have to transmit at a higher power level to reach the UE, which would probably increase the

overall system interference level. Additionally, if a UE is in SHO, the

connection is not lost altogether if one branch gets shadowed.



Soft Handover Zone(Overlap Zone):

A SHO is typically employed in cell boundary areas where cells have

overlap.

It has many desirable properties. In the cell edges, a UE can collect more

signal energy if it is in SHO than if it has only a single link to a base

station.



Softer handover

Softer handover is a special case of soft handover where the radio links that are added

and removed belong to the same Node B (i.e. the site of co-located base stations from

which several sector-cells are served.

A softer HO is a HO between two sectors of a Node B. From a UE’s point of view, it is

just another SHO.

The difference is only meaningful to the network, as a softer HO is an internal

procedure for a Node B ,which saves the transmission capacity between Node Bs andthe RNC (a UTRAN base station controller).

The uplink softer HO branches can be combined within the Node B,which is a faster procedure, and uses less of the fixed infrastructure’s transport resources than most other types of HOs in WCDMA systems



Soft Handover in HSDPA

1. Only on the control channel(R’99) and not on the dedicated channel

2. The HS-DSCH does not support soft handovers



Soft Handover Combining Methods:

During soft handover, a mobile simultaneously communicates with two

(2-way SHO) or more cells belonging to different Node Bs of the sameRNC (intra-RNC) or different RNCs (inter-RNC).

Two active power control loops participate in soft handover, one for

each BS.

In the softer handover situation, a mobile is controlled by at least two

sectors under one BS, the RNC is not involved and there is only one

active power control loop.

In UL, the combining is done in Node B itself and not the RNC.

Combining method in DL:



Combining method in DL:

In the downlink (DL), the mobile receives both signals for maximal ratio

combining.

Maximum ratio combining is assumed to be applied in the mobileterminal. Therefore, to a user inside the soft handover zone, the

received Eb/I0 is the sum of the Eb/I0 from all the BSs in the active set.

In the downlink, we model macro diversity to sum together the signal received from

active BSs. Thus, we realize maximal ratio combining by summing measured SIRvalues

Combining method in the uplink

The mobile code channel is detected by both BSs (2-way SHO), and isrouted to the RNC for selection combining.

In the uplink, selection combining among active BSs takes place to use the frame withthe highest average SIR for statistics collecting purposes, while the other frames get

discarded.





Intra-frequency reporting events for the FDD mode:

1A. A primary CPICH enters the reporting range.

1B. A primary CPICH leaves the reporting range.

1C. A non-active primary CPICH becomes better than an active primary CPICH.

1D. Change of best cell.

Simplest way to define SHO Event:

Any Modifications to the active sets is referred as soft handover Event.

Event 1A:



Event 1A:

Serving(Best Cell) – [NBR + CIO(NBR)] <= 3 dB

Where;Hystfor1A = 0

=6 ~ (3 dB)

Means: If the difference between source and neighbor cell is less than or equal to 3 dB for a set

time[TRIGTIME1A=D320], then the neighbor will be added to the active set.

CIO Consideration:

This parameter can be defined at cell level and per Neighbour relation.

+ve value of CIO will help trigger the Soft Handover event 1 A earlier than if we have the CIO=0.

-ve value of CIO will introduce delay in the Soft Handover event 1 A .



3dB

NBR cell B which is inmonitored set is going

to enter the reporting

Range. The reporting

range is 3 dB down below

the best cell A in active set .

Reporting RangePlot for 1A

Event 1B:



Event 1B:

Serving(Best) – [Serving(old) + CIO(old) ]>= 6 dB

Where;

Hystfor1B = 0INTRARELTHDFOR1BCSNVP=12 ~ ( 6dB)

Means: If the difference between best cell and 2nd or 3rd best cell in active set is greater than or

equal to 6 dB for a set time[TRIGTIME1B=D640], then the 2nd or 3rd best cell will be removed

from the active set.

CIO Consideration:

This parameter can be defined at cell level and per Neighbour relation.

-ve value of CIO will help trigger the Soft Handover event 1 B earlier than if we have the CIO=0.

+ve value of CIO will introduce delay in the Soft Handover event 1 A .



6dB

Reporting Range

Plot for 1B

NBR cell B which is inActive set is going

to leave the reporting

Range. The reporting

range is 6 dB down below

the best cell A in active set .

Event 1C:



Event 1C:

Event 1c combines the functionality of Event 1a and 1b by replacing an Active Set

cell.

The triggering condition occurs if:

NBR CIO(NBR)≥ Worst Server(In AS) CIO(Worst Server in AS)H1c/2

NBR – Worst Server(In AS) ≥ 2 dB *Considering the CIO values to be 0+

Means: If the New cell in the “monitored Set” is stronger than “Worst Cell in the Active Set” by 2 dB for a

time period of 640 msec [TRIGTIME1C=D640], Event 1C will be triggered.

Where:

NBR: is the measurement value of the cell in the reporting range.

CIO(NBR) : is Cell offset of the cell in the reporting range.

Worst Server(In AS): is the measurement value of the worst cell in the active set.

CIO (Worst Server in AS) is Cell offset of the worst cell in the active set.

• H1c is the hysteresis parameter for Event 1c = 8 ~ 4 dB



A: Best Cell in the Active Set

B: 2nd Best Cell in the Active Set

C: Worst Cell in theActive Set

D: NBR Cell inthe monitoredset

2dB

D: NBR Cell in MS is 2dBstronger than the worst cell in

the Active Set.

E: Hysteresis line 2dBabove the worst cell.

Event 1D:



Event 1D:

M(Not best)+ CIO(Not Best) ≥ M(Best) CIO(Best) H1d/2

Where:

MNotBest is the measurement value of a cell that is not on the list of the best cells.

CIONotBest is the cell offset of a cell that is not on the list of the best cells. The offset is not used.

MBest is the measurement value of the best cell in the active set. CIONotBest is the cell offset of

the best cell. The offset is not used. H 1d is 1D hysteresis, the hysteresis value of event 1D.

Means: If the New cell either in “Active Set” OR “Monitored Set” is stronger than “Best Cell in

the “Active Set” by 2 dB for a time period of 640 msec *TRIGTIME1D=D640+, Event 1D will be

triggered.

H1d = Hystfor 1d = 8 ~ 4dB



2dB

A: Best Cell in the Active SetC: Hysteresis line 2dB

above the best cell.

B: A new cell either in Active Set

OR Monitored Set is approachingto the best cell in the Active Set.



Summary of Parameter Settings:

General Discussions:



General Discussions:

Q: What is the function of CIO?A: The sum of this parameter value and the actual measured value is used in UE event

evaluation. In handover algorithms, this parameter is used for moving the border of a cell. It is

configured according to the actual environment.

Q: What is difference between Event 1C and 1D?A: In event 1C, the monitored set cell is compared with “Worst Cell in Active Set” whereas in 1D,

The “Monitored set” cell OR “Active Set Cell” is compared with “Best Cell in the Active Set”.

Q: Which all events from 1A, 1B, 1C & 1D are part of soft handover?

A: All.

Q: Consider our Active Set size is 3. Is it possible if there are only two cells in the Active Set , and

event 1C can be triggered?

A: No.

Q: Why the Measurement Report is not sent in UL likewise in GSM and



Q y t e easu e e t epo t s ot se t U e se GS a d

A: In WCDMA, the OTA(Over the air transmission) is designed to be kept minimum and thus

keeping the interference level minimum.

Q: Who calculates the Intra-frequency Events(UE, Node B or RNC)?

A: Events are calculated in UE based on Thresholds, Hysteresis and CIO sent in MC.

Q: What is the signaling procedure during the soft Handover?

A:

UE RNC

Event 1x

Active Set Update

Active Set update complete

Failing to reach to the UE will lead to call drop

Reason for Fail: DL interference

Failing to reach to the RNC will lead to call dropReason for Failure: UL Interference

Q: What is the difference between SHO and Multipath?



Q: What is the difference between SHO and Multipath?

A: An important difference between a multipath component and a SHO branch is that each SHO

branch is coded with a different spreading code, whereas multipath components are just time-

delayed versions of the same signal.

Q: Why the threshold for event 1B should be more than event 1A?A: This prevents the premature removal of base stations from the active set. The value of thedrop threshold 1B is 6dB and for 1A is 3dB in our case.

Soft Handover Ratio - RNC:



It tells the consumption of extra network resources due to soft handover in one RNC

Means: It is nothing but a simple ratio of No. of Radio Links used over the No. of UEs in anRNC.

Formula:Soft Handover Ratio(RNC)=[(A1+B1*2+C1*3+D1*4+F1*5+F1*6)/(A1+B1+C1+D1+E1+F1)-1]*100%

A1:Number of UEs with 1 RL;

B1:Number of UEs with 2 RLs;C1:Number of UEs with 3 RLs;

D1:Number of UEs with 4 RLs;

E1:Number of UEs with 5 RLs;

F1:Number of UEs with 6 RLs.

Soft Handover

Ratio(RNC)={[VS.SHO.AS.1+(VS.SHO.AS.2Softer+VS.SHO.AS.2Soft)*2+(VS.SHO.AS.3Soft2Softer+VS.SHO.AS.

3Soft+VS.SHO.AS.3Softer)*3+VS.SHO.AS.4*4+VS.SHO.AS.5*5+VS.SHO.AS.6*6]/(VS.SHO.AS.1+VS.SHO.AS.2

Softer+VS.SHO.AS.2Soft+VS.SHO.AS.3Soft2Softer+VS.SHO.AS.3Soft+VS.SHO.AS.3Softer+VS.SHO.AS.4+VS.

SHO.AS.5+VS.SHO.AS.6)-1}*100%

The soft handover overhead versus the soft handover parameter



The soft handover overhead versus the soft handover parameter

Window_add for a hexagonal cell grid with three sector sites, and two

different cell radii.

1. soft handover overheadincreases approximately linearly

when Window-add and

Window-drop are increased.

2. For the same soft handover

parameter settings, the soft

handover overhead is typicallylarger for the scenario with

small cells, compared to large

cells. This behavior is observed

because UEs in the large cell

grid can only synchronize to a

few Node Bs, while UEs in thesmall cell grid typically can

synchronize to many Node Bs.

Etisalat Settingfor Window -Add

Documents

Soft and Softer Handover in WCDMA