Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-09-30
2002-08-27
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S453000, C455S067150
Reexamination Certificate
active
06442398
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to load estimation in a multiple access system.
2. Description of the Related Art
FIG. 1
is an exemplifying embodiment of a terrestrial wireless communication system
10
.
FIG. 1
shows three remote units
12
A,
12
B and
12
C and two base stations
14
. In reality, typical wireless communication systems may have many more remote units and base stations. In
FIG. 1
, the remote unit
12
A is shown as a mobile telephone unit installed in a car.
FIG. 1
also shows the portable computer remote unit
12
B and the fixed location remote unit
12
C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, the remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system (PCS) units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment.
FIG. 1
shows a forward link signal
18
from the base stations
14
to the remote units
12
and a reverse link signal
20
from the remote units
12
to the base stations
14
.
In a typical wireless communication system, such as that illustrated in
FIG. 1
, some base stations have multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas as well as independent processing circuitry. The principles discussed herein apply equally to each sector of a multi-sectored base station and to a single sectored independent base station. For the remainder of this description, therefore, the term “base station” can be assumed to refer to either a sector of a multi-sectored base station or a single sectored base station.
In a code division multiple access (CDMA) system, remote units use a common frequency bandwidth for communication with all base stations in the system. Use of a common frequency bandwidth adds flexibility and provides many advantages to the system. For example, use of a common frequency bandwidth enables a remote unit to simultaneously receive communication signals from more than one base station, as well as transmit a single signal for reception by more than one base station. The remote unit discriminates the simultaneously received signals from the various base stations through the use of the spread spectrum CDMA waveform properties. Likewise, the base station can discriminate and separately receive signals from a plurality of remote units.
Various methods exist for transferring communication with the remote unit from one base station to another through a process known as handoff. Handoff may be necessary if a remote unit operating in the coverage area of an original base station moves into the coverage area of a target base station. One method of handoff used in CDMA systems is termed a “soft” handoff. Through the use of soft handoff, communication with the target base station is established before termination of communication with the original base station. When the remote unit is communicating with two base stations, both the remote unit and base stations create a single signal from the multiple received signals. Through the use of soft handoff, communication between the remote unit and the end user is uninterrupted by the eventual handoff from the original base station to the target base station. U.S. Pat. No. 5,267,261 entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,” assigned to the assignee of the present invention and incorporated herein by this reference, discloses a method and system for providing communication with a remote unit through more than one base station during the handoff process.
In a wireless system, maximizing the capacity of the system in terms of the number of simultaneous calls that can be handled is extremely important. System capacity in a spread spectrum system is increased if the power received at the base station from each remote unit is controlled such that each signal arrives at the base station receiver at the minimum level required to maintain the link. If a signal transmitted by a remote unit arrives at the base station receiver at a power level that is too low, the signal to interference ratio may be too low to permit high quality communication with the remote unit. If, on the other hand, the remote unit signal arrives at a power level that is too high, communication with this particular remote unit is acceptable, but the high power signal acts as unnecessary interference to other remote units. This unnecessary interference may adversely affect communications with other remote units. Thus, in general, a remote unit located near the base station transmits a relatively low signal power while a remote unit located at the edge of the coverage area transmits a relatively high signal power.
In standard CDMA systems, in order to increase capacity, the power transmitted by a remote unit over the reverse link is controlled by each base station through which active communication is established (i.e. each base station with which the remote unit is in soft handoff.) Each base station through which communication is established measures the received signal quality and compares it to a desired set point. Each base station, periodically, generates and sends a power adjustment command to the remote unit. The power adjustment commands puncture the user traffic data on the forward link traffic channel.
The power adjustment command orders the remote unit to either increase or decrease the power at which it is transmitting the reverse link signal. The remote unit increases its transmit power level only if every base station commands an increase. In this way, the transmit signal power of a remote unit in soft handoff is controlled mainly by the base station which receives its signal at the highest signal quality. A system for base station and remote unit power control is disclosed in U.S. Pat. Nos. 5,056,109, 5,265,119, 5,257,283 and 5,267,262 which are assigned to the assignee hereof.
In more advanced systems, in addition to controlling the power level at which the remote unit transmits on the reverse link, the data rate at which the remote unit transmits on the reverse link is also controlled. A remote unit located on the edge of a coverage area can reduce the data rate at which it transmits in order to increase the signal quality of the signal as received at the base station. By reducing the data rate, the time devoted to each bit may be increased, thus, increasing the energy devoted to each bit and increasing the performance of the link. A remote unit which transmits at less than the full rate generates less interference and consumes less system resources than a remote unit transmitting at full rate, thereby, freeing system resources for use by other remote units.
The power adjustment commands compensate for the time-varying path loss in the wireless channel. Path loss in the wireless channel is defined as degradation or loss suffered by a signal as it travels between the remote unit and the base station. Path loss is characterized by two separate phenomenon: average path loss and fading. In a typical wireless system, the forward link and reverse link operate on different frequencies. Nevertheless, because the forward and reverse links operate within the same frequency band, a significant correlation exists between the average path loss of the two links. On the other hand, fading is an independent phenomenon for the forward and reverse link and varies rapidly as a function of time, especially when the remote unit is in motion or is located near objects in motion.
In an exemplifying wireless system, each remote unit estimates the path loss of the forward link based on the total power at the input of the remote unit. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the remote unit. From the estimate of the average forward link path loss, the remote
Judson Bruce A.
Padovani Roberto
Baker Kent D.
Chow Charles
Hunter Daniel
Kalousek Pavel
Qualcomm Incorporated
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