Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-02-19
2001-12-04
Maung, Nay (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S012100, C455S427000, C455S456500
Reexamination Certificate
active
06327471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to handoffs and dropoffs in a cellular communication system. More particularly, the present invention relates to establishing whether a cellular radiotelephone with a weak signal should be handed off or dropped off.
2. Description of Related Art
A radiotelephone system consists of many base stations, also known as cell sites, each coupled to a mobile telephone switching office (MTSO) connected to the public switched telephone network (PSTN). Each base station is equipped with an antenna that receives and radiates radiotelephone signals in order to communicate with cellular radiotelephones and switch the signals from the radiotelephones to the mobile telephone switching office. The base station also provides the proper channels for the radiotelephones, thus enabling communication with the base station. The mobile telephone switching office switches the signals from the base station to the PSTN and vice versa.
Code Division Multiple Access (CDMA), and spread spectrum systems in general, were previously dismissed as unworkable in the mobile radio environment because of the near-far problem. Near-far problem occurs in cellular communication systems where different transmitters interfere with the base station transmitter. In the mobile radiotelephone environment, some users may be located near the base station, and others may be located far away. The propagation path loss difference between those extreme users can be many tens of dB. This reduces overall, average link capacity. The anti-jamming capacity of spread spectrum systems, such as CDMA systems, aids in such situations, but unfortunately not enough to overcome the disparity between a strong nearby interfering signal and an attenuated remote signal. CDMA systems use codes to separate one signal from another. If there are enough different codes available, the limit on the number of users is determined by a system jamming margin, because a receiver will work as long as the composite power of all unwanted signals does not exceed the power of the desired signal by more than a jamming margin.
It had been assumed that all mobile radiotelephones in a cell had to transmit constant power. If, however, rather than using constant power the transmitters can be controlled in such a way that their power level can be changed so that the received power levels from all the users are roughly equal, the benefits of the spread spectrum system are realized. In systems where the received power is controlled, the subscribers can occupy the same spectrum, and the hoped-for benefits of interference averaging accrue. However, the systems that employ power control for intracell solution of the near-far problem suffer from significant loss in receiver sensitivity because all signals must arrive at the receiver (base station) at equal power levels, to maximize the number of simultaneous users. Power control may be accomplished in each radiotelephone by verifying its distance from the base station and adjusting its output power accordingly, so that the base station receives the signal at the predetermined signal power level. Otherwise, the weakest mobile transmitter, especially near the edge of a cell, dictates performance of the other cell users, which than have to change their transmitting power to accommodate the weak user.
In order to prevent increase of transmitting signal power of a radiotelephone at the edge of a cell in order to improve transmission and thus cause jamming of the base station by requesting all units in the new cell to increase their transmitting power to that level, handoff to another cell base station is performed by the current cell base station. However, if a user's signal is very weak, the radiotelephone has to be dropped off from the cell by the base station before it wipes out the entire network. Very weak users are not handed over to another cell base station because their performance will be poor near the edge of the adjacent cell after the handoff.
In CDMA systems with power control, if adjacent cells have a base station that can be heard by several base stations, a radiotelephone in that cell has to be in communication with all those base stations in order to accomplish a soft handoff. Soft handoff, also called make-before-break, is performed in CDMA systems where all cells use the same frequency (as defined by the IS-95A standard), and it is possible to make connection to the adjacent cell before leaving the current cell. Soft handoffs require less power, which reduces interference and increases capacity. In a soft handoff, control is handed over to the most convenient base station to service the user, while all other base stations stay in full communication with the user. When the radiotelephone reaches the edge of the cell, the controlling base station assists in its handoff to transfer control to another cell. The base station, however, does not know the location of the radiotelephone and it has to page all radiotelephones in the cell and wait for the arrival of their response, which takes time, and determine the candidate for dropoff or handoff from the response.
In a CDMA system with power control, each participating cell in a soft handoff transmits the same traffic stream to the mobile radiotelephone, bit-for-bit. They do so on any available code channel. Each base station chooses a code channel simply on the basis of availability. The radiotelephone must implement, in its rake receiver, multiple fingers that are capable of “tuning” to any of the, typically 63, available code channels. Embedded in the forward CDMA channel are the reverse power control bits. Each power control bit is interpreted as a command to raise or lower power by an increment of approximately {fraction (3/4 )}dB. Each base station makes power control decisions independently. The mobile station is responsible for demodulating the power control bits and raising or lowering its power accordingly. The goal of the power control is to maintain the reverse link transmit power at the lowest possible level commensurate with adequate error performance. The mobile radiotelephone is thus required to interpret the power control bits, which will often disagree, as requiring an increase in power only if all base stations in the handoff say “up”. If any participating base station says “down”, then the mobile radiotelephone is required to reduce power. This rule is sometimes called “OR of the downs”.
CDMA systems uses Mobile Assisted Handoff (MAHO). In practice, this means that the mobile radiotelephone continuously searches for a pilot code using a PN correlator specifically designated for this purpose. All base stations use the same code. If the mobile radiotelephone already has a notion of CDMA system time, as it does if it is already involved in a call, then it can report the relative timing of a newly detected pilot. What distinguishes base stations from one another is the phase of their pilots. The period of each pilot is 26.667 ms. They are separated by a minimum of 64 chips, which is about 52 ms or about 15 km at the speed of light. The mobile radiotelephone timing will normally be sufficiently good that a reported pilot offset unambiguously identifies the base station it has detected.
The mobile radiotelephone reports pilots on the basis of their pilot-to-interference ratio (PIR). The PIR is compared to an absolute threshold to determine when it should be reported as a handoff candidate. This absolute or first threshold is a parameter that the mobile radiotelephone obtains from the overhead messages broadcast by the base stations. When a pilot crosses the first threshold, its presence is reported, via a message, to the network. The network adds that base station to the so-called active set, which is the set of base stations that are participants in the soft handoff of the mobile radiotelephone in question. A second threshold which is not absolute but relative is compared to the difference between the largest PIR in the active set and the PIRs of all other members.
Conexant Systems Inc.
Maung Nay
Thomas Kayden Horstemeyer & Risley LLP
Vuong Quochien B.
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