Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1997-07-31
2002-02-05
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S347000
Reexamination Certificate
active
06345078
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to mobile communication systems in general and, more particularly, to a scheme for assigning receiver fingers of a multiple finger receiver to different offsets corresponding to multipath components.
2. Description of the Related Art
A mobile communications channel can rarely by modeled as purely line-of-site. Therefore, one must consider the many independent paths that are the result of scattering and reflection of a signal between the many objects that lie between and around the mobile station and the base station. The scattering and reflection of the signal creates many different “copies” of the transmitted signal (“multipath signals”) arriving at the receiving station with various amounts of delay, phase shift and attenuation. As a result, the signal received at the mobile station from the base station (and at the base station from the mobile station) is made up of the sum of many signals, each traveling over a separate path. Since these path lengths are not equal, the information carried over the radio link will experience a spread in delay as it travels between the base station and the mobile station. The amount of time dispersion between the earliest received copy of the transmitted signal and the latest arriving copy having a signal strength above a certain level is often referred to as delay spread. Delay spread can cause intersymbol interference (ISI). In addition to delay spread, the same multipath environment causes severe local variations in the received signal strength as the multipath signals are added constructively and destructively at the receiving antenna. A multipath component is the combination of multipath signals arriving at the receiver at nearly the same delay. These variations in the amplitude of the multipath components is generally referred to as Rayleigh fading, which can cause large blocks of information to be lost.
Resistance to multipath fading is a reason for using spread spectrum systems for wireless communications. Spread spectrum signals are pseudorandom and have noise-like properties when compared with the digital information data. Certain spread spectrum systems, such as code-division multiple access (CDMA) systems, spread the baseband data by directly multiplying the baseband data pulses with a pseudo-noise (PN) code, which is a binary sequence that appears random but can be reproduced by the intended receiving station. The PN code has a much higher pulse rate than the data pulse rate, and a single pulse of the PN code is called a chip. Spread spectrum signals are demodulated in part at the receiving station through cross-correlation with a locally-generated version of the PN code. Cross-correlation with the correct PN code de-spreads the spread spectrum signal and restores the modulated message to the narrower band of the original data, while cross-correlating the signal from an undesired user with the PN code results in a small amount of noise. Because spread spectrum signals are spread over a large bandwidth, only a small portion of the spectrum experiences fading at any given time. The resistance of spread spectrum systems to multipath fading can also be explained from the fact that delayed versions of the transmitted signal should be generally uncorrelated with the original PN code and will simply appear as noise from another uncorrelated user.
Spread spectrum systems, such as CDMA systems, however, can advantageously use the delayed versions of the transmitted signal. Spread spectrum systems exploit the multipath environment by combining the information obtained from several resolvable multipath components. In CDMA systems, the effects of multipath are combated and advantageously exploited by using a multiple-branch (RAKE) receiver.
FIG. 1
shows a RAKE receiver
10
with four “fingers”
12
a-d
. The RAKE receiver
10
can be implemented using a CDMA Cell Site Modem ASIC provided by Qualcomm of San Diego, Calif. as well as the control thereof. The RAKE receiver
10
attempts to collect the delayed or offset versions of the original signal by providing parallel demodulators or fingers
12
a-d
. Each demodulator
12
a-d
uses a different amount of delay or offset corresponding to a multipath component of the signal from a particular antenna
14
. Initially, processing circuitry
18
assigns a delay or offset corresponding to a multipath component to each demodulator
12
a-d
. Afterward, tracking loops
20
a-d
make adjustments to the assigned delay or offset for the demodulators
12
a-d
. In a current CDMA RAKE receiver, finger tracking loops
20
a-d
perform ⅛ PN chip adjustments to the assigned offsets or delays of the demodulators
12
a-d
. Searcher circuitry
19
performs a search to find the strongest multipath components within a range of offsets or delays. The results from the searcher
19
are used for the initial finger assignments and/or for any finger re-assignments after a finger
12
a-d
is disabled. A combiner
22
combines the outputs from the demodulators
12
a-d
and outputs the combined signal to the remainder of the receiver
10
. The receiver
10
includes other aspects which are not discussed. For example, the combined signal is subsequently decoded. Furthermore, the signal received at the antenna
14
which is demodulated as generally described above can undergo additional processing depending on the particular implementation. For example, base stations typically use non-coherent demodulation, and mobile stations typically use coherent demodulation.
Each demodulator
12
a-d
de-spreads the incoming signal using the PN code and the delay or offset assigned to the demodulator
1212
a-d
. As such, the demodulators
12
a-d
extract multipath components of the original signal. The use of the parallel demodulators
12
a-d
improves the signal-to-noise ratio (SNR) of the received signal for the given user and provides a statistical and power diversity gain because uncorrelated multipath components will fade independently. Ideally, multipath components are uncorrelated when the components are more than 1 PN chip (approximately 0.8138 microseconds in IS-95 CDMA) from each other. The finger tracking loop
20
a-d
for each demodulator
12
a-d
of the RAKE receiver
10
is designed to keep the assigned finger delay or offset synchronized with the delay or offset yielding the strongest finger energy for the multipath component being tracked. Typically, an early-late gate tracking mechanism adjusts the assigned delay or offset based on the difference in finger energy between an early hypothesis (less delay) and a late hypothesis (more delay). As such, each tracking loop
20
a-d
adjusts the delay or offset for its finger
12
a-d
toward the local maximum of the correlation between the PN code and the received spread signal. A multipath component of a finger
12
a-d
having a particular offset will be partially correlated with a multipath component of another finger
12
a-d
having a difference in offset of less than 1 PN chip. Due to the partial correlation between the multipath components, the fingers
12
a-d
could end up tracking the same multipath component. Because of the early-late gate tracking mechanism, the tracker
20
could potentially be affected by multipath components having a difference in offset of more than 1 PN chip. For example, if the tracker uses +/−¼ chip early/late correlation hypothesis, the tracker
20
a-d
could be potentially influenced by a multipath component that is 1¼ chips away. Thus, even demodulators
12
a-d
assigned to offsets or delays greater than 1 PN chip in difference can still end up tracking the same multipath component.
For ease of explanation,
FIGS. 2
a-c
represent the finger strength depending on the PN chip offset (delay) for several simplified situations involving two multipath components A and B.
FIG. 2
a
shows the output
28
representing the correlation between PN de-spreading codes and the received signal for two, unfaded multipath components A and B sepa
Bayard Emmanuel
Lucent Technologies - Inc.
Pham Chi
LandOfFree
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