Soft Handover in WCDMA System

Prepared byRiyaz Ahmad Khan

riyazahmad79@gmail.com

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 to be 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. This scheme 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 twobase stations, and similarly the downlink signal is sent by at least two base stations.

Macro-diversity in the DLIn the downlink the UE can receive, at most, as many macro- diversity components as it 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 new transmission may also increase the system interference. If too many base stations are used in an SHO, the system interference level increases instead of decreasing and preserving the usefulness of an SHO.

Macro-diversity in ULIn the uplink the effects of macro-diversity are only positive, as the more base stations 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 can probably 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 theSHO branches are about equal. If one of the participating base stations isclearly 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 soft Handover.

2. If the SIR increases means the transmit power can be decreased3. 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 power even 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 transmission2. Reduces the transmit power requirement for each link used.3. UE at the boundary among several cells uses the minimum transmit power on

either 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 more interference to the radio environment in the downlink direction.

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

Gain in Downlink transmission power due to Soft Handover

Positive value = gain, Negative value = lossGain 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 controlcommands).

Best results are achieved when signal levels from two Node B’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.

For large signal value difference we can see that transmission power even is raised,because of signaling errors on downlink (power controlcommands).

Typical handover parameters

Window to add Window to drop1–3 dB 2–5 dB

Effect of “No Soft Handover”Without SHO, a communicating base station would have to transmit at ahigher power level to reach the UE, which would probably increase theoverall 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 moresignal 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 (a UTRAN base station has the curious name Node B), which saves the transmission capacity between Node Bs and the 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 transportresources than most other types of HOs in WCDMA systems

Soft Handover in HSDPA1. Only on the control channel(R’99) and not on the dedicated channel2. 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 same RNC (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:In the downlink (DL), the mobile receives both signals for maximal ratio combining.

Maximum ratio combining is assumed to be applied in the mobile terminal. 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 fromactive BSs. Thus, we realize maximal ratio combining by summing measured SIR values

Combining method in the uplink The mobile code channel is detected by both BSs (2-way SHO), and is routed 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 getdiscarded.

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:

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

Where;Hystfor1A = 0INTRARELTHDFOR1ACSNVP=6 ~ (3 dB)

Means: If the difference between source and neighbour cell is less than or equal to 3 dB for a set time[TRIGTIME1A=D320], then the neighbour 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 in monitored set is going to enter the reporting Range. The reporting range is 3 dB down below the best cell A in active set .

Reporting Range Plot for 1A

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 in Active 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 combines the functionality of Event 1a and 1b by replacing an Active Setcell.

The triggering condition occurs if:NBR + CIO(NBR)≥ Worst Server(In AS) + CIO(Worst Server in AS)+H1c/2

ÞNBR – 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 the Active Set

D: NBR Cell in the monitored set

2dB

D: NBR Cell in MS is 2dB stronger than the worst cell in the Active Set.

E: Hysteresis line 2dB above the worst cell.

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. H1d 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 SetOR Monitored Set is approching to the best cell in the Active Set.

Summary of Parameter Settings:

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 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 dropReason 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?

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 the drop 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 an RNC.

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.2Softer+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 Window_add for a hexagonal cell grid with three sector sites, and two different cell radii.

1. soft handover overhead increases approximately linearly when Window-add and Window-drop are increased. 2. For the same soft handover parameter settings, the soft handover overhead is typically larger 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 the small cell grid typically cansynchronize to many Node Bs.

Etisalat Setting for Window -Add

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