Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-02-25
2001-05-08
Tse, Young T. (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S148000, C375S340000, C375S349000, C455S522000
Reexamination Certificate
active
06229841
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to energy estimation in a wireless receiver.
II. Description of the Related Art
FIG. 1
is an exemplifying embodiment of a terrestrial wireless communication system
10
and such a system can be generally discussed with reference thereto.
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 a 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, remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system 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, a single-sectored base station or a multi-sectored base station.
In a code division multiple access (CDMA) system, remote units use a common frequency band for communication with all base stations in the system. Use of a common frequency band adds flexibility and provides many advantages to the system. For example, the use of a common frequency band enables a remote unit to simultaneously receive communications from more than one base station as well as transmit a signal for reception by more than one base station. The remote unit can discriminate and separately receive 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. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein.
CDMA communication techniques offer many advantages over narrow band modulation techniques. In particular, the terrestrial channel poses special problems by the generation of multipath signals which can be overcome through the use of CDMA techniques. For example, at the base station receiver, separate multipath instances from a common remote unit signal can be discriminated and separately received using similar CDMA techniques as those used to discriminate between signals from the various remote units.
In the terrestrial channel, multipath is created by reflection of signals from obstacles in the environment, such as trees, buildings, cars and people. In general, the terrestrial channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over a multipath channel, a stream of pulses is received. In a time varying multipath channel, the received stream of pulses changes in time location, amplitude and phase as a function of the time at which the ideal impulse is transmitted.
FIG. 2
shows an exemplifying set of signal instances from a single remote unit as they appear upon arrival at the base station. The vertical axis represents the power received on a dB scale. The horizontal axis represents the delay in arrival of the instances at the base station due to transmission path delays. An axis (not shown) going into the page represents a segment of time. Each signal instance in the common plane of the page has arrived at a common time but was transmitted by the remote unit at a different time. In a common plane, peaks to the right represent signal instances which were transmitted at an earlier time by the remote unit than peaks to the left. For example, the left-most peak
20
corresponds to the most recently transmitted signal instance. Each signal peak
20
-
30
corresponds to a signal which has traveled a different path and, therefore, exhibits a different time delay and a different phase and amplitude response.
The six different signal spikes represented by peaks
20
-
30
are representative of a severe multipath environment. Typical urban environments produce fewer usable instances. The noise floor of the system is represented by those peaks and dips having lower energy levels.
Note that each of the multipath peaks varies in amplitude as a function of time as shown by the uneven ridge of each multipath peak
20
-
30
. In the limited time shown, there are no major changes in the amplitude of the multipath peaks
20
-
30
. However, over a more extended time range, multipath peaks diminish in amplitude and new paths are created as time progresses. The peaks can also slide to earlier or later time offsets as path distances change due to movement of objects in the coverage area of the base station.
In addition to the terrestrial environment, multiple signal instances can also result from the use of satellite systems. For example, in a GlobalStar system, remote units communicate through a series of satellites rather than terrestrial base stations. The satellites each orbit the earth in approximately 2 hours. The movement of the satellite through its orbit causes the path distance between the remote unit and the satellite to change over time. In addition, as a satellite moves out of range of the remote unit, a soft hand-off from one satellite to another satellite is performed. During the soft hand-off, the remote unit demodulates signals from more than one satellite. These multiple signal instances can be combined in the same manner as the multipath signal instances in a terrestrial system. One difference, however, is that the signal instances tend to be offset from one another by approximately 1-50 milliseconds in the terrestrial environment while the signal instances received through two satellites tend to be offset from one another the order of 0-20 milliseconds.
FIG. 3
is a block diagram of a prior art receiver which can be used in a terrestrial multipath environment or a satellite environment which incorporates soft hand-off capability. The diversity receiver shown in
FIG. 3
is often called a “rake” receiver. Typically, a rake receiver comprises a demodulator which in turn comprises a series of demodulation elements, each one of which represents one finger in the rake receiver. Each demodulation element can be assigned to demodulate a unique signal instance.
Typically, in a rake receiver, the demodulation elements are assigned to signal instances which have been detected by a searcher element. The searcher element continues to search for newly developing signal instances by continually correlating the incoming signal samples at a variety of time offsets. The output of the searcher element is provided to a system controller. Based upon the output of the searc
Levin Jeffrey A.
Riddle Christopher C.
Sherman Thomas B.
Brown Charles
Greenhaus Bruce
Qualcomm Incorporated
Tse Young T.
Wadsworth Philips
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