Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-10-02
2004-06-22
Tse, Young T. (Department: 2682)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S144000
Reexamination Certificate
active
06754252
ABSTRACT:
TECHNICAL FIELD
The present claimed invention relates to the field of digital communication. Specifically, the present claimed invention relates to an apparatus and a method for managing and locking fingers used to receive multipath signals.
BACKGROUND ART
Wireless telephony, e.g. cellular phone use, is a widely-used mode of communication today. Variable rate communication systems, such as Code Division Multiple Access (CDMA) spread spectrum systems, are among the most commonly deployed wireless technologies. Because of increasing demand and limited resources, a need arises to improve their capacity, fidelity, and performance.
Referring to prior art
FIG. 1A
, an illustration of multipath signal propagation between a conventional base station and a cell phone is shown. A conventional base station
104
transmits a signal to a mobile station, e.g., phone,
102
. Typically, the signal contains pilot information, that identifies the base station, and data information, such as voice content. A signal that can be transmitted directly to mobile phone
102
without interference, such as first signal
106
a
, provides the strongest signal. However, given the power limitations at which base station
104
can transmit the signal, and given the noise a signal may pick up, a need arises to improve the power and the SNR of the signal received at mobile phone.
Conventional methods will combine the portions of the transmitted signal that travel different paths to mobile unit
102
. The multiple paths arise because of natural and man-made obstructions, such as building
108
, hill
110
, and surface
112
, that deflect the original signal. Because of the paths over which these other signals travel, a time delay and performance deterioration intrinsically arises in the synchronization-sensitive and noise-sensitive data transmitted from base station
104
to mobile phone
102
. However, to provide the strongest possible signal to a mobile phone, two or more of the signals from these multiple paths, e.g. path
106
a
-
106
d
, may be combined. However, to efficiently combine and demodulate multipath signals, a need arises for a method to select the most worthwhile candidates from all the different multipaths received in mobile phone.
Corruption of a transmitted signal falls into two general categories: slowly-varying channel impairment and fast fading variation. Slowly-varying channel impairment arises from factors such as log-normal fading, or shadowing caused by movement or blocking as exemplified in prior art
FIG. 1A
, or slow fading. Slower variations, e.g., sub Hz, determine in effect, the “availability” of the channel. In contrast, only the fast fading variation affects the details of the received waveform structure and the interrelationships of errors within a message. Hence, a need arises for a method that effectively choose the properties of the signal that influence its condition for demodulation.
Referring now to prior art
FIG. 1B
, a graph of two conventional multipath signal strengths over time is shown. Graph
100
b
has an abscissa
122
of time and an ordinate of signal-to-noise ratio (SNR)
120
, e.g. pilot Ec/Io ratio. Third multipath signal
106
c
and fourth multipath signal
106
d
are shown as exemplary multipath signals received at mobile phone
102
. Conventional methods typically select for combining, the multipath signals with the highest SNR. Thus, at time span A
124
a
, the dark line representing fourth multipath signal
106
d
has a higher SNR level than third multipath signal
106
c
, assuming both signals have the same noise level. However, at time span B
124
b
, the dashed line representing third multipath signal
106
c
has a higher SNR level. Given the closeness of the SNR, or of the signal to noise ratio, of these two multipath signals, the choice as to which signal will be chosen for the demodulation finger can oscillate back and forth.
This oscillation is a condition known as “thrashing.” The drawback with thrashing is that it consumes a significant amount of system resources, such as processor operations. During thrashing, the processor can be overloaded with operations that constantly assign and deassign the multiple fingers to different multipath signals. Furthermore, thrashing may degrade the quality of the mobile phone
102
output signal, as the switching may cause an audible interference or it may introduce latency effects. Consequently, a need arises for a method to select the best multipath signal for combining while avoiding the effect of thrashing.
Furthermore, referring again to prior art
FIG. 1A
, conventional methods combine transmitted signals that travel different paths to mobile unit
102
. The multiple paths arise because of natural and man-made obstructions, such as building
108
, hill
110
, and surface
112
, that deflect the original signal. Because of the paths over which these other signals travel, a time delay and performance deterioration intrinsically arises in the synchronization-sensitive and noise-sensitive data that is transmitted from base station
104
to mobile unit
102
. To provide the strongest possible signal to a mobile unit, two or more of the signals from these multiple paths, e.g. path
106
a
-
106
d
, may be combined.
Corruption of a transmitted signal falls into two general categories: slowly-varying channel impairment and fast fading variation. Slowly-varying channel impairment arises from factors such as log-normal fading, or shadowing caused by movement or blocking from objects, as shown in prior art
FIG. 1A
, or from slow fading. Slower variations, e.g., sub Hz, determine in effect, the “availability” of the channel. In contrast, only the fast fading variation affects the details of the received waveform structure and the interrelationships of errors within a message. Interference on a signal can be caused by moving objects that temporarily block the signal, such as moving object
113
that interferes with signal
106
b
of prior art FIG.
1
A. Based upon the characteristic differences of these signals, a need arises for a method of capturing a signal while avoiding the detrimental characteristics of fast fading or short fading variation encountered at the receiving unit.
Referring now to
FIG. 1C
, a flowchart of a conventional process used for implementing fingers in a communication device is shown. Flowchart
100
c
begins with step
1002
. In step
1002
, an inquiry determines whether an assigned signal fails to meet a threshold for combining. If an assigned signal does fail to the single threshold, then flowchart
100
c
ends. If the assigned signal satisfies the threshold, then flowchart
100
c
ends. In step
1004
, the finger assignment is immediately deassigned, e.g. because it failed to meet the threshold. Following step
1004
, flowchart proceeds to step
1006
. In step
1006
, the communication device waits for the searcher to assign a new finger.
Prior art
FIG. 1C
presents several problems associated with the conventional management of assigned fingers. The first problem deals with thrashing. The second problem deals with unnecessary latency. In step
1002
, the only criteria by which fingers are deassigned is a single threshold for combining the signal. This single threshold is shown in prior art
FIG. 1B
as threshold
126
. By using only a single threshold, third multipath signal
106
c
is immediately deassigned, per step
1004
, as soon as it fails threshold
126
, e.g., at time
122
a
. Because of this limitation, one of the demodulating fingers must now wait for the searcher to identify a new multipath signal to be assigned, e.g., per step
1006
. This latency is shown as the delay
128
between time
122
a
and
122
b
, where third pilot
106
c
is deassigned and second multipath signal
106
b
is assigned.
In a different scenario, if no other multipath signals are available for demodulation, and a demodulating finger is available, then second multipath signal
106
b
may be constantly assigned and deassigned from the given demodulating finger based on its performance. That is, seco
Tse Young T.
Zawilski Peter
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