With the introduction of digital cellular mobile networks, such as 3G, mobile telephone users experienced the freedom of traveling and always being connected. Even with a call in progress it is now possible to move from one radio cell into another without the call being interrupted. The action of switching a call in progress from one radio channel to another to secure the continuity of an established call is named handover. Idea is to remain connected.
Soft and softer handover
With soft handover functionality the handset can communicate simultaneously with two or more cells in two or more base stations. This flexibility in keeping the connection open to more than one base station results in fewer lost calls, which is very important to the operator. To achieve good system performance with a frequency re-use of 1 and power control, soft and softer handover is required.
Soft and softer handover enables the handset to maintain the continuity and quality of the connection while moving from one cell to another. During soft or softer handover, the handset will momentarily adjust its power to the base station that requires the smallest amount of transmit power and the preferred cell may change very rapidly. The difference between soft and softer handover is that during soft handover, the handset is connected to multiple cells at different base stations, while during softer handover, the handset is connected to multiple cells at the same base station. A drawback with soft handover is that it requires additional hardware resources on the network side, as the handset has multiple connections. In a well-designed radio network, 30–40 % of the users will be in soft or softer handover.
Soft handover:
At the boundaries of the cell UE usually receive strong interference from other neighboring cells. This interference can be reduced by soft handover procedure. In this way UE is connected to more than one NodeB’s. All the NodeB’s use the same frequency in an SHO but they are not same as multi path components as all the signals in SHO use different spreading codes whereas multipath components are just the time delayed versions of the same signal.
Intra frequency intra BTS soft(er) handover:
During softer handover a mobile station is in the overlapping cell coverage area of two adjacent sector of a base station. The communication between mobile station take place concurrently via TWO air interface channels, one for each sector separately. This requires the use of two separate DL scrambling code, so that MS can distiguish the signals. In the DL two signals are received in the mobile station by means of RAKE processing. In the UL the code channel of the mobile station is received in the each sector, the routed to the same baseband RAKE receiver and combined there. During softer handover only one power control loop is active.
Intra frequency intra RNC soft handover:
During soft handover, a mobile station is in the overlapping cell coverage area of two sectors belonging to different base stations. Seen from the mobile station there are very few differences between softer and soft handover. However, in the UL direction soft handover differs significantly from softer handover: the code channel of the mobile station is received from the both base stations, but the received data is then routed to the RNC for selective combining (MDC).
Intra frequency inter RNC soft handover:
Inter-RNC soft handover is similar handover than intra-RNC soft handover. The main difference is that the soft handover takes place between two BTS controlled by separate RNCs. The following terms are releated to the inter-RNC soft handover:
S-RNC, Serving RNC : Serving is a role an RNC can take with respect to a specific connection between an MS and RAN. There is one serving RNC for each MS that has a connection to RAN. The serving RNC is in charge of the radio connection between an MS and RAN. The serving RNC terminates the Iu for this MS.
D-RNC, Drifting RNC : Drifting is a role an RNC can take with respect to a specific connection between an MS and RAN. The drifting RNC supports the serving RNC with radio resources when the connection between the RAN and the MS needs to use the cells controlled by this RNC.
C-RNC, Controlling RNC : Controlling is a role an RNC can take with respect to a specific set of WCDMA BTSs. The controlling RNC has the overall control of the logical resources of its BTSs. There is only one controlling RNC for any BTS.
Relocation:
Serving radio network subsystem (SRNS) relocation is a common thing which use to happen as often in which the routing of a UE connection in the UTRAN changes. If a UE is in an SHO and all participating Node Bs belong to the same radio network controller (RNC), then the signals will be combined in this RNC and sent further to the serving mobile services switching center (MSC). If the SHO exists between sectors of the same Node B (softer handover), then the combining will be performed in Node B. Relocation is a procedure when status of SRNC changes from the first RNC to second RNC. Relocation is done when UE completely out of way from the first RNC i.e. previously DRNC.
Anchoring mode of RNC:
Following things happens during anchoring:
n All the User plane traffic transferred from DRNC to CN through the SRNC. SRNC acts as the anchoring point. Combining can be done at DRNC side too which further help in saving transport capacity at Iur interface. Although SRNC can request all the signals to route through it and perform combining.
n RRC Messages are transferred to/from UE through IUR from the DRNC.
n What if the MS moves to the RNC not in the neighboring RNC list of the SRNC
n SHO not possible, and hence HHO attempt is tried, if not successful, call drop criteria is applied, if it is satisfied call drops.
n Emergency calls are allowed, even if they cause extra interference to the system.
Features of SHO:
If UE is at the edge of the cell and it is in SHO, it do not need to send the signal at higher power than in case it is having only one link to node-B. So there will be very less intereference to the other cells due to lesser power transmission of signals. But as more signals in the air, it will lead to more interference , so SHO branches should be added only if the resulting intereference level is less than what is without SHO.
Hard Handover:
It is basically an interfrequency handover. It is normally called as the break before make connection. So there is a short disconnection of RT bearers while switching from the one cell to other or from one system(RAT) to another. So there is a disconnection of the UE during the call.
Intra frequency inter RNC hard handover:
Intra -frequency inter-RNC hard handover is a special case of inter-RNC soft handover. It is based on the same measurement events which are usually applied to soft handover. It is also based on the decision of the handover control algorithm of RNC when the averaged CPICH power difference between the neighbour cell(n) and the best active set cell exceedes the maximum allowed difference, which is defined by RNP parameter, and the RNC is not abel to perform an inter-RNC handover between the cells. Intra-frequency hard handover causes a short disconnection of a real-time radio access bearer. Intra-frequency hard handover takes place when Iur interface between the RNCs is not available.
Inter System handover:
Inter-operability between various networks is quite essential. When WCDMA was standardized, a key aspect was to ensure that existing investments could be re-used as much as possible. One example is handover between the new (WCDMA) network and the existing (GSM) network, which can be triggered by coverage, capacity or service requirements. Handover from WCDMA to GSM, for coverage reasons,is initially expected to be very important since operators are expected to deploy WCDMA gradually within their existing GSM network. When a subscriber moves out of the WCDMA coverage area, a handover to GSM has to be conducted in order to keep the connection.
The 3GPP specifications for UMTS include the functionality to inter-operate with the GSM network. This scenario is built upon UEs capable of supporting both UMTS and GSM radio access standards.
The serving RNC (SRNC) takes the decision of how to perform a handover to the 2G system based upon measurements reported by the UE. A 3G to 2G handover is always a hard handover.
Handover between GSM and WCDMA can also have a positive effect on capacity through the possibility of load sharing. If for example the numbers of subscribers in the GSM network is close to the capacity limit in one area, handover of some subscribers to the WCDMA network can be performed. Another function that is related to inter-system handover is the compressed mode. When performing handover to GSM, measurements have to be made in order to identify the GSM cell to which the handover will be made. The compressed mode is used to create the measurement periods for the handset to make the required measurements. This is typically achieved by transmitting all the information during the first 5 milliseconds of the frame with the remaining 5 milliseconds being used for measurements on the other systems.
Compressed Mode:
During inter-frequency handover the UE’s must be given time to make the necessary measurements on the different WCDMA carrier frequency. 1 to 7 slots per frame can be allocated for the UE to perform this inter frequency (hard handover). These slots can either be in the middle of the single frame or spread over two frames. There are three methods by which we can apply compressed mode:
a) SF/2 method: This will increase the data rate so bits will get sent twice as fast.
b) Puncturing bits. This will remove various bits from the original data and hence reduce the amount of information that needs to be transmitted. Its ommitted from 3GPP due to loss of data bits in this.
c) The higher layer scheduling could also be changed to use less timeslots for user traffic.
SF/2 method:
The spreading factor (SF) can be reduced by 2 during one compressed radio frame to enable the transmission of the information bits in the remaining time slots of the compressed frame. This method is not supported for SF=4. On the downlink, UTRAN can also order the UE to use a different scrambling code in a compressed frame than in a non-compressed frame. If the UE is ordered to use a different scrambling code in a compressed frame, then there is a one-to-one mapping between the scrambling code used in the non-compressed frame and the one used in the compressed frame.
Spreading factor = chip rate/symbol rate
A chip is a bit in a code word, which is used to modulate the information signal. As they do not represent any information so we call them chips. Every second, 3.84 million chips are sent over the radio interface. However, the number of data bits transmitted during the same time period is much smaller. The ratio between the chip rate and the symbol rate is called the spreading factor.
So, Spreading factor/2 = chip rate/(symbol rate *2)
Which further corresponds to (chip rate/2) * symbol rate
So we are halving the code bits rate, which will effectively lead to decrease in signal rate. So gaps will be created. In theory we could have a spreading factor of one, that is, no spreading at all.
HLS method:
Compressed frames can be obtained by higher layer scheduling. Higher layers then set restrictions so that only a subset of the allowed TFCs is used in a compressed frame. The maximum number of bits that will be delivered to the physical layer during the compressed radio frame is then known and a transmission gap can be generated. Note that in the downlink, the TFCI field is expanded on the expense of the data fields and this shall be taken into account by higher layers when setting the restrictions on the TFCs. Compressed mode by higher layer scheduling shall not be used with fixed starting positions of the TrCHs in the radio frame.
In compressed frames, TGL slots from Nfirst to Nlast are not used for transmission of data. As illustrated in figure 11, the instantaneous transmit power is increased in the compressed frame in order to keep the quality (BER, FER, etc.) unaffected by the reduced processing gain. The amount of power increase depends on the transmission time reduction method. What frames are compressed, are decided by the network. When in compressed mode, compressed frames can occur periodically, as illustrated in figure below, or requested on demand. The rate and type of compressed frames is variable and depends on the environment and the measurement requirements.
a) From 3GPP TS 25.212.
b) Introduction to 3G mobile communications: juha korhonen
c) Basic concepts of WCDMA Radio Access Network : A White paper Ericcsson
