Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-12-26
2004-03-30
Bost, Dwayne (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S436000, C455S442000, C455S452200, C370S335000, C370S441000, C370S331000
Reexamination Certificate
active
06714788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a link exclusion method for use during a soft handover in a cellular system of mobile radio communication.
2. Discussion of the Background:
In mobile radio communication, one type of which is commonly referred to as a cell phone system, mobile users communicate with a fixed network by using base stations as a relay. Each base station may communicate with any mobile user that is within its radio coverage area, which is often called a radio “cell”. When a mobile unit moves from one cell to the other, it then changes covering area and is no longer able to communicate with the base station covering its former cell.
A classical solution for not interrupting communication while a mobile changes from one cell to another cell is called “handover”. It consists in allowing a mobile that has begun communication with the network via a first base station to be connected to a second base station that will then serve to relay the communication to the network. This type of relay transfer is determined by the network based on many criteria. Most often, the criteria consist of passing on the relay from one base station, from which mobile is moving away, to a base station which it is approaching, But other criteria exist, as for example, when a cell becomes overloaded, the network may force a handover to a less overloaded cell.
It is to be noted that during handover, synchronization problems may be encountered between both links when the time comes to transfer to a new base station, which may cause undesirable effects on the communication between the mobile and the network.
When the communication between the network and the mobile is not affected from the users point of view, the handover is referred to as a seamless handover. A type of seamless handover is often used by duplicating the communication between the, mobile and second base station. Once the communication is established and the synchronization problems are resolved, the mobile is then in communication with both base stations (this is possible in overlap zones between cells), for a short moment and then the communication with the first base station is ended. This is referred to as a “hard” handover.
The procedure for establishing a hard handover may take a while and during that period of time, it is possible that the criterion which leads to a handover may change, and that a new reverse handover may occur. If the handover repeatedly occurs between two base stations a “ping-pong” phenomenon that may last which is harmful to the quality of the link, the link not being completely established with the most appropriate base station. In other respects, this type of handover procedure requires signaling between base stations, which should be limited in order to maintain useful information within the network. In order to limit the “ping-pong” effect, we may use a hysteresis, or time delay, when deciding to go ahead with a handover. But the hysteresis will then involve a delay in the decision to complete the handover which may again be harmful to the quality of the link.
Another version of handover exists which consist in preserving the former link for a certain period of time, according to a certain criterion. During that period of time, the communication is indeed reinforced by a phenomenon called macro-diversity: if one of the links suddenly weakens, for example when a mobile enters into a shade zone (corner of a building, bridge, etc.), the other link may not be affected and avoid a degradation of the communication. This type of handover is called “soft” handover, and brings a considerable profit in terms of quality of transmission in comparison to a hard handover.
A classical way of performing a soft handover consists in comparing the power level received from different base stations neighboring the mobile station. It is then decided that the mobile station be in soft handover with all the base stations so that their power level received by the mobile be in the range between the maximum power level received and that power reduced by a factor which is determined in advance. This factor may be expressed in decibels, and that is what represents the handover window. The group of base stations with which the mobile is in communication represents what is called the “Active Set” of the mobile. With other respects, since the mobile cannot support an infinite number of links, the size of the active set is limited. In this way, the impact of the soft handover is modulated by working with two parameters: the size of the active set and the handover window.
The same type of ping-pong phenomenon occurs in soft handover as well as in hard handover. However, the impact on the quality of the link is much lower since the windowing system of the soft handover is in itself a form of hysteresis. The ping-pong would eventually be located at the input/output of the handover, but the impact would remain weak, since the strongest link would always be preserved. However, in order to avoid this ping-pong effect on the input/output of a Base Telephone Station (BTS) in an active set, we may add an output criterion to the active set that may be looser, and therefore add a second hysteresis. The interest of such sophistication would then rest less in the concern of preserving the quality of the link (which from observation is by nature preserved by the actual principle of the soft handover), than the concern of reducing the signaling load in the network following a too large request for addition or withdrawal of BTS within the same active set.
In the soft handover situation, the communication is then optimized by the combination of signals coming from many different links. This combination may be done in many ways. For example, each of the links may be demodulated and decoded in an individual and, regular manner, and at each moment, the best of them is selected to effectively participate in the communication. We then refer to the “selection combining”. This technique can be compared to an instant hard handover without having to previously negotiate, since the communication is available on all the links in the active set.
An important variant of this combination technique consists of a joint demodulation of the different links. Each link then contributes to the quality of the communication in proportion to the power received. This latter technique, based on the “Maximum Ratio Combining” allows taking better advantage of the gain in diversity induced by the soft handover, and is the technique status known under the name of “softer” handover. To be effective, this technique requires that the recombination be done before the error corrector decoding, which is not always the case in the network up link.
It is to be noted that even if it is interesting from a radio communication quality point of view, soft handover is not used in all mobile radio systems. In fact, it requires a duplication of radio resources that can increase the complexity and the mobile station's ability to receive two signals coming from more than two different stations, on frequencies that are not necessarily the same. This is why in GSM (Group Speciale Mobile) for example only the seamless hard handover is useful. For other mobile radio systems, for example based on code division multiple access (CDMA), it is however possible to recover the two links from only one receiver. This is due to the fact that two adjacent cells may use the same frequency without causing one another too much harm in terms of interference, thanks to the nature of the spread spectrum signals that are used.
In such CDMA systems, limited in terms of capacity from the total level of interference, soft handover brings additional advantages. In fact, the number of links in the system becomes larger than the number of users in the system due to the soft handover, and with the same useful load, the effective load of the network becomes larger than in the case of the hard handover. This has an impact on the system capacity that, we are seeking to
Bost Dwayne
Mitsubishi Denki & Kabushiki Kaisha
Sharma Sujatha
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