Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-12-31
2002-08-13
Chin, Vivian (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S069000, C455S439000
Reexamination Certificate
active
06434386
ABSTRACT:
BACKGROUND
The present invention relates generally to methods and systems for radiocommunications and, more particularly, to such systems in which communication quality is monitored and connections can be handed over from one channel or base station to another based on monitored quality.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
In cellular systems, the capability is typically provided to transfer handling of a connection between, for example, a mobile station and a serving base station to another, neighboring base station, as the mobile station changes its position and so moves out of the coverage area of one base station and into the coverage area of another base station. This type of handoff is commonly referred to as an “intercell” handoff as the coverage areas associated with base stations are commonly referred to as “cells”. Depending upon the quality of the current channel, it may also be desirable to transfer a connection from one channel of the base station to another channel supported by the same base station, which handoffs are commonly referred to as “intracell” handoffs.
To smoothly complete a handoff, the network controlling the base stations first determines, for each mobile station, whether the need for handoff from its currently serving base station is imminent and secondly determines to which new base station (or channel) handoff should be effected. In making the former decision, the radiocommunication system monitors the quality of the mobile station's current connection. In making the latter decision it is desirable that the network controller know either how well each neighboring base station can receive signals from a mobile station in question, how well the mobile station in question can receive signals from each neighboring base station, or both. Monitoring signalling associated with a serving base station and neighboring base stations typically involves making signal strength measurements, which measurements can be made in a variety of ways depending, for example, on the type of access methodology employed by the radiocommunication system.
Many existing radiocommunication systems are based on an access method known as Frequency Division Multiple Access (FDMA), in which each mobile station transmits on a unique frequency within its current base station area. The mobile station is thus unaware of signals on other frequencies from surrounding base stations. In FDMA systems it is typically considered too costly to equip mobile stations with an extra receiver that could be used to scan other base station frequencies to determine their received signal strength. Instead, it is common for base stations to be equipped with a scanning receiver that searches for the signals of approaching mobile stations. When the base stations scan for the signal strength of mobile stations, the method could be termed base assisted handover (BAHO). The network then hands over a mobile station from a base station covering an area that the mobile is leaving to the base station that reports the best reception of the mobile station's signal.
More recent cellular telephone standards employ Time Division Multiple Access (TDMA) in which a fixed time period (e.g., 20 mS) on each pair of radio frequency links (one used in the direction from mobile station to base station (uplink), the other used in the direction from base station to mobile station (downlink)) is divided into a number (e.g., 3) of short timeslots (e.g., 6.6 mS) that are cyclically used by different mobile stations. Thus, a first mobile station transmits in the first timeslot in each period, a second mobile station transmits in the second timeslot in each period and so on. Likewise the base station transmits to one mobile station in the first timeslot, another mobile station in the second slot and so on. By offsetting the allocation of timeslots in the two communication directions, base to mobile (the downlink) and mobile to base (the uplink), it can be arranged that a first mobile transmits in the first timeslot and receives in the second timeslot; a second mobile transmits in the second timeslot and receives in the third, while a third mobile transmits in the third timeslot and receives in the first timeslot. An advantage of this arrangement is that a mobile station does not need to transmit and receive simultaneously, which reduces disturbance.
In the above three-timeslot example, each mobile station is active to transmit or receive in two of the three timeslots and is idle in the remaining timeslot. Therefore it is possible for TDMA mobile stations to use this idle time to receive signals from other base stations and measure their signal strength. Typically, such measurements are made on the control channels (or some type of other measurement channel) being broadcast by these base stations, rather than the traffic channels used to support circuit-switched or packet-switched connections.
By reporting these signal strength measurements to the base station using a slow speed data channel multiplexed with the traffic (e.g., voice), the network is informed about the base stations each mobile station can receive. The network can use this information to effect handoff to the preferred base station, and such a method is termed mobile assisted handover (MAHO). For example, the system can compare the measured signal strengths associated with neighboring base stations to the signal strength at which the mobile station is receiving transmissions from its serving base station. If a neighboring base station is received at a sufficiently higher signal strength (e.g., employing a hysteresis value to avoid a “ping-pong” handoff effect at cell borders), than the serving base station, then a handoff is performed. Systems providing MAHO can also have access to the base station measurements, and so are able to effect smoother and more reliable handovers because both uplink and downlink signal strengths are taken into account, instead of just uplink strengths in the case of BAHO.
In yet another access technique, Code Division Multiple Access (CDMA), mobile stations can share the same frequency band but communications are distinguishable by virtue of unique spreading codes. Even in CDMA systems it is possible to measure a signal strength of pilot channels associated with a particular base station. The base station and/or mobile station can use this information to determine when a handover to another code, or another frequency band in multicarrier CDMA, is desirable.
As can be seen from the foregoing, a common element in determining when a handoff is desirable involves the measurement and comparison of received signal strengths, and in particular measurements made on control channels of various neighboring base stations. However, there are several complicating factors relating to the output transmit powers of the transceivers within base stations which reduce the accuracy of conventional handoff algorithms.
The actual output transmit power of a particular transceiver can vary from transceiver to transceiver within a base station for a number of different reasons. For example, problems or changes to components within a transceiver may cause its actual output transmit power to vary from an intended value. Other factors which may cause variances in actual output transmit power between transceivers involve different power settings which are intended to permit, for example, a network operator to have greater control over system operation.
For example, control channels and traffic channels are not always transmitted by base stations using the same transmit power. In some systems it is po
Appiah Charles N.
Burns Doane Swecker & Mathis L.L.P.
Chin Vivian
Telefonaktiebolaget L M Ericsson (publ)
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