Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1998-10-28
2003-08-05
Nguyen, Chau (Department: 2733)
Multiplex communications
Communication over free space
Repeater
C455S453000
Reexamination Certificate
active
06603745
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to load estimation and overload detection in a multiple access system.
II. Description of the Related Art
FIG. 1
is an exemplary embodiment of a terrestrial wireless communication system
10
.
FIG. 1
shows three remote units
12
,
13
, and
15
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
is shown as a mobile telephone unit installed in a car.
FIG. 1
also shows the fixed location remote unit
15
in a wireless local loop system and the portable computer remote unit
13
in a standard cellular system. In the most general embodiment, the remote units may be any type of communication unit. For example, the remote units may 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
,
13
and
15
and reverse link signal
19
from the remote units
12
,
13
and
15
to the base stations
14
.
In the discussion that follows, to aid in illustration, the invention is described with reference to a commonly known, wireless link industry standard. In fact, the generic principles of the invention can be directly applied to many multiple access communication systems. The discussion that follows assumes operation in accordance with the system described in TIA/EIA/IS-95-A published by the Telephone Industry Association entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” commonly referred to as IS-95.
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 systems using IS-95, 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 between 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 an 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 interference to other remote units. This excessive 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 large signal level.
In order to increase capacity, the power transmitted by a remote unit over the reverse link may be 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 though which communication is established measures the received signal to interference ratio 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 which receives its signal at the highest signal to interference ratio. 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 incorporated herein.
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 the terrestrial environment, multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. If an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. In general, the terrestrial channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. If an ideal impulse is transmitted over a time varying multipath channel, the received stream of pulses changes in time offset, attenuation, and phase as a function of the time at wh
Antonio Franklin P.
Hamdy Walid
Baker Kent
Greenhaus Bruce
Lee Andy
Nguyen Chau
Qualcomm Incorporated
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