Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-04-28
2001-11-06
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S343000
Reexamination Certificate
active
06314130
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to systems and methods for tracking the delays of channel paths in general and of jointly tracking the time delays of multiple paths (multipath components) in particular.
BACKGROUND OF THE INVENTION
In recent years, direct sequence (DS) code division multiple access (CDMA) spread spectrum communication systems and methods have experienced growing attention worldwide. The IS-95 cellular communication standard is one example of an application of DS-CDMA communications, as described in the article TIA/EIA/IS-95-A, “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, Feb. 27, 1996. Additional applications include third generation cellular systems, wireless multimedia systems, personal satellite mobile systems, and more.
In DS-CDMA communications, each user is assigned a distinct spreading code often referred to as pseudo noise (PN) sequence. The spreading code bits (called chips) are used to modulate the user data. The number of chips used to modulate one data symbol is known as the spreading factor of the system, and it is related to the spreading in bandwidth between the (unmodulated) user data and the CDMA signal. In this simplest form, the base-band equivalent of the transmitted CDMA signal, sampled at the chip rate 1/Tc, is
T
[
n
]
=
∑
i
=
1
κ
a
i
[
&LeftBracketingBar;
n
/
SF
&RightBracketingBar;
]
·
PN
i
[
n
]
,
where Tc is the chip duration, └x┘ denotes the integer part of x, SF is the spreading factor, &agr;
i
[└n/SF┘] and PN
i
[n] are the data symbol and spreading code of the i-th user, respectively, and K is the number of active users. Note that by the definition of └x┘, &agr;
i
[└n/SF┘] is fixed for SF consecutive chips, in accordance with the definition above that each data symbol is modulated by SF chips.
An important feature of DS-CDMA systems is that they provide the possibility of obtaining excellent immunity to multipath fading through resolving the individual, time separated multipath components and optimally combining them. The common approach for achieving this is to use a “rake” receiver as it is known in the art. Such a receiver assigns despreading correlators to each of the dominant multipath components and synchronizes them for maximum de-spread power. For each of the rake “fingers”, the phase and amplitude of the corresponding channel multipath component is estimated and used to apply amplitude weighting and phase alignment prior to combining. The weighted sum of the multipath components will experience considerably less fading than any of the individual components so that a diversity gain is obtained.
As is known in the art, a crucial requirement of the rake receiver is that its fingers are time aligned (synchronized) with the multipath components of the channel. This requires estimation of the multipath delays and is often achieved by a simple early-late time tracking mechanism. The early-late mechanism is, in fact, a delay-lock-loop that measures the energy prior (early) and after (late) the current sampling instances. These early and late energy measurements are used to lock on the sampling instance that maximizes the sampled signal energy. As it turns out, these maximal energy sampling instances leads, in many cases, to the desired synchronization of the rake fingers to the channel multipath components. However, some channels, for example those encountered in dense urban environments, consist of a large number of closely spaced multipath components. This leads to multipath clusters that are often spaced less than Tc apart. Conventional early-late time tracking mechanism are often incapable of tracking the delays associated with those closely spaced multipath clusters since their early and late measures are a superposition of the energies associated with several adjacent clusters. In such a situation, the rake fingers are not properly time aligned with the multipath clusters, leading to degradation in the receiver performance.
It would therefore be beneficial to have an improved time tracking mechanism that is more robust to the presence of closely spaced multipath components.
It would also be beneficial to have an improved criterion for finger assignment in closely spaced multipath environment.
In recent years several methods for combating closely spaced multipath components in DS-CDMA communication systems were derived. In U.S. Pat. No. 5,692,006 to Ross, U.S. Pat. No. 5,648,983 to Kostic et al. and U.S. Pat. No. 5,793,796 to Hulbert et al. it is suggested to avoid direct estimation of the path delays, Instead, a bank of closely spaced fingers is utilized to effectively cover a pre-specified delay window. Thus instead of actually estimating the multipath delays all possible delays in the window are examined and weighted according to some quality measure criterion. In U.S. Pat. No. 5,692,006 a conventional LMS algorithm is used to adaptively estimate optimal finger weighting, whereas in U.S. Pat. No. 5,648,983 a weighted least squares solution is used to assign the finger weights.
Other solutions can be found in:
EP Patent Publication 704 985 A2 to Hulbert;
U.S. Pat. No. 5,764,688 to Hulbert et al.;
I. Dumont, et al., “Super-resolution of Multipath Channels in a Spread Spectrum Location System”,
Electronics Letters,
Vol. 30, No. 19, Sep. 15, 1984; and
Makoto Takeuchi et al., “A Delay Lock Loop Using Delay Path Cancellation for Mobile Communications”,
Electronics and Communication in Japan,
Part 1, Vol. 79, No. 4, 1996.
SUMMARY OF THE INVENTION
The present invention provides an improved time tracking mechanism that is more robust in the presence of closely spaced multipath components.
The present invention also provides a criterion for finger assignment in a closely spaced multipath environment.
There is therefore provided in accordance with a preferred embodiment of the present invention a method used in a receiver having at least two fingers forming a finger block, the finger block tracking at least one path of a multipath channel. The method includes the steps of generating direction metrics of each of a set of possible directions of joint movement of the fingers of the finger block, selecting one of the direction metrics according to a predetermined criterion, and moving the fingers of the finger block in the directions indicated by the selected direction metric.
Moreover, in accordance with a preferred embodiment of the present invention, the selected metric is the maximal direction metric.
Furthermore, in accordance with a performed embodiment of the present invention, the step of moving adjusts the fingers of the finger block only if the selected direction metric is the maximal direction metric and exceeds a comparison direction metric by at least a predetermined threshold.
Additionally, in accordance with a preferred embodiment of the present invention, the method further includes the step of redefining finger blocks after the step of moving.
Moreover, in accordance with a preferred embodiment of the present invention, the finger block is formed of two fingers.
Furthermore, in accordance with a preferred embodiment of the present invention, the direction metrics are generated for five, six or nine different directions of joint movement.
Additionally, in accordance with a preferred embodiment of the present invention, the finger block is formed of two closely spaced fingers.
Moreover, in accordance with a preferred embodiment of the present invention, the closely spaced fingers are ⅞ chip apart.
Furthermore, in accordance with a preferred embodiment of the present invention, the finger block is formed of three fingers.
Additionally, in accordance with a preferred embodiment of the present invention, delays between fingers are set to be no smaller than ⅞ chip.
Moreover, in accordance with a preferred embodiment of the present invention, the step of generating includes the step of time averagi
Rainish Doron
Smolyar Lev
Yellin Danny
DSPC Technologies Ltd.
Eitan Pearl Latzer & Cohen-Zedek
Pham Chi
Tran Khai
LandOfFree
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