UMTS WCDMA Handover: hard, soft, softer, inter-RAT

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One of the key aspects of any cellular telecommunications systems is the handover from one cell or sector to another.

Within 3G UMTS there are several forms of handover including hard handover, soft handover, a form of handover called softer handover and a handover from one radio access technology to another called inter-RAT handover.

It is key that all these handovers work seamlessly because any issues will result in dropped calls and this is a major source of user annoyance.

It is worth noting that the two terms handover and handoff have the same meaning. UMTS handover tends is the terminology that tends to be used within Europe, whereas UMTS handoff is more likely to be used within North America.

UMTS handover types

Within UMTS it is possible to define a number of different types of UMTS handover or handoff. With the advent of generic CDMA technology, new possibilities for effecting more reliable forms of handover became possible, and as a result one of a variety of different forms of handover are available depending upon the different circumstances.

For purely inter W-CDMA technology, there are three basic types of handover:

• Hard handover:   This form of handover is essentially the same as that used for 2G networks where one link is broken and another established.
• Soft handover:   This form of handover is a more gradual and the UE communicates simultaneously with more than one Node B or base station during the handover process.
• Softer handover:   Not a full form of UMTS handover, but the UE communicates with more than one sector managed by the same NodeB.
• UMTS inter RAT handover:   This form of handover occurs when mobiles have to change between Radio Access Technologies, e.g. UMTS to / from GSM or UMTS to / from 4G LTE, etc..

Each of the different types of handover is used on different occasions dependent upon the conditions. Further details of each type of UMTS handover are given in the individual sections below.

UMTS hard handover

The name hard handover indicates that there is a "hard" change during the handover process. For hard handover the radio links are broken and then re-established. Although hard handover should appear seamless to the user, there is always the possibility that a short break in the connection may be noticed by the user.

The basic methodology behind a hard handover is relatively straightforward. There are a number of basic stages of a hard handover:

1. The network decides a handover is required dependent upon the signal strengths of the existing link, and the strengths of broadcast channels of adjacent cells.
2. The link between the existing NodeB and the UE is broken.
3. A new link is established between the new NodeB and the UE.

Although this is a simplification of the process, it is basically what happens. The major problem is that any difficulties in re-establishing the link will cause the handover to fail and the call or connection to be dropped.

UMTS hard handovers may be used in a number of instances:

• When moving from one cell to an adjacent cell that may be on a different frequency.
• When implementing a mode change, e.g. from FDD to TDD mode, for example.
• When moving from one cell to another where there is no capacity on the existing channel, and a change to a new frequency is required.

One of the issues facing UMTS hard handovers was also experienced in GSM. When usage levels are high, the capacity of a particular cell that a UE is trying to enter may be insufficient to support a new user. To overcome this, it may be necessary to reserve some capacity for new users. This may be achieved by spreading the loading wherever possible - for example UEs that can receive a sufficiently strong signal from a neighbouring cell may be transferred out as the original cell nears its capacity level.

3G UMTS soft handover

Soft handover is a form of handover that was enabled by the introduction of CDMA. Soft handover occurs when a UE is in the overlapping coverage area of two cells. Links to the two base stations can be established simultaneously and in this way the UE can communicate with two base stations. By having more than one link active during the handover process, this provides a more reliable and seamless way in which to perform handover.

In view of the fact that soft handover uses several simultaneous links, it means that the adjacent cells must be operating on the same frequency or channel as UEs do not have multiple transmitters and receivers that would be necessary if they were on different frequencies.

When the UE and NodeB undertake a soft handover, the UE receives signals from the two NodeBs and combines them using the RAKE receiver capability available in the signal processing of the UE.

In the uplink the situation is more complicated as the signal combining cannot be accomplished in the NodeB as more than one NodeB is involved. Instead, combining is accomplished on a frame by frame basis. The best frames are selected after each interleaving period. The selection is accomplished by using the outer loop power control algorithm which measures the signal to noise ratio (SNR) of the received uplink signals. This information is then used to select the best quality frame.

Once the soft handover has been completed, the links to the old NodeB are dropped and the UE continues to communicate with the new NodeB.

As can be imagined, soft handover uses a higher degree of the network resources than a normal link, or even a hard handover. However this is compensated by the improved reliability and performance of the handover process. However with around 5 to 10% of handovers falling into this category, network operators need to account for it.

Note on the RAKE receiver

A RAKE receiver is a form of radio receiver that has been made feasible in many areas by the use of digital signal processing, DSP. It is often used to overcome the effects of multipath propagation. It achieves this by using several sub-receivers known as "fingers" which are given a particular multipath component. Each finger then processes its component and decodes it. The resultant outputs from the fingers are then combined to provide the maximum contribution from each path. In this way rake receivers and multipath propagation can be used to improve the signal to noise performance.

3G UMTS softer handover

A form of handover referred to as softer handover is really a special form of soft handover. It is a form of soft handover that occurs when the new radio links that are added are from the same NodeB. This occurs when several sectors may be served from the same NodeB, thereby simplifying the combining as it can be achieved within the NodeB and not require linking further back into the network.

UMTS softer handover is only possible when a UE can hear the signals from two sectors served by the same NodeB. This may occur as a result of the sectors overlapping, or more commonly as a result of multipath propagation resulting from reflections from buildings, etc.

In the uplink, the signals received by the NodeB, the signals from the two sectors can be routed to the same RAKE receiver and then combined to provide an enhanced signal.

In the downlink, it is a little more complicated because the different sectors of the NodeB use different scrambling codes. To overcome this, different fingers of the RAKE receiver apply the appropriate de-spreading or de-scrambling codes to the received signals. Once this has been done, they can be combined as before.

In view of the fact that a single transmitter is used within the UE, only one power control loop is active. This may not be optimal for all instances but it simplifies the hardware and general operation.

Inter-RAT / Intersystem or iRAT handover

In many instances it is necessary for the UMTS radio access network to handover to the 2G GSM network. These handovers are given a variety of names including Inter-RAT handover as they are handing over between different forms of Radio Access Technology, Intersystem Handover, and UMTS / GSM Handover. These handovers may be required for one of a variety of reasons including:

• Limited UMTS coverage
• UMTS network busy whereas spare capacity is available on GSM network

The most common form of intersystem or inter-RAT handover is between UMTS and GSM. There are two different types of inter-RAT handover or iRAT handover:

• UMTS to GSM handover:   There are two further divisions of this category of handover:
  
  
  • Compressed mode handover:   Using compressed mode handover the UE uses the gaps in transmission that occur to analyse the reception of local GSM base stations. The UE uses the neighbour list provided by the UMTS network to monitor and select a suitable candidate base station. Having selected a suitable base station the handover takes place, but without any time synchronisation having occurred.
  • Blind handover:   This form of handover occurs when the base station hands off the UE by passing it the details of the new cell to the UE without linking to it and setting the timing, etc of the mobile for the new cell. In this mode, the network selects what it believes to be the optimum GSM based station. The UE first locates the broadcast channel of the new cell, gains timing synchronisation and then carries out non-synchronised intercell handover.
• Handover from GSM to UMTS :   This form of handover is supported within GSM and a "neighbour list" was established to enable this occur easily. As the GSM / 2G network is normally more extensive than the 3G network, this type of handover does not normally occur when the UE leaves a coverage area and must quickly find a new base station to maintain contact. The handover from GSM to UMTS occurs to provide an improvement in performance and can normally take place only when the conditions are right. The neighbour list will inform the UE when this may happen.

One of the benefits of using a scheme like CDMA is that handover can be effected more easily than with systems like GSM where a hard handover is always needed. Using a soft or softer form of handover it is easier to effect a successful handover. Obviously the Inter-RAT handover is the most difficult to achieve, but nevertheless it is done successfully.

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