Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-01-28
2003-09-02
Bayard, Emmanuel P. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06614836
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to wireless communication systems, such as but not limited to wireless local area networks (WLANs), and is particularly directed to a new and improved channel-matched correlation receiver, or RAKE receiver, that employs a direct sequence spread spectrum codeword correlation metric, in which unequal energies in respectively different codewords are corrected, so as to increase the receiver's tolerance to the effects of multipath distortion, without losing robustness to thermal noise.
BACKGROUND OF THE INVENTION
The ongoing demand for faster (higher data rate) wireless communication products is currently the subject of a number of proposals before the IEEE 802.11 committee, that involve the use of a new standard for the 2.4 GHz portion of the spectrum, which FCC Part 15.247 requires be implemented using spread spectrum techniques that enable data rates to exceed 10 megabits per second (Mbps) Ethernet speeds. The 802.11 standard presently covers only one and two Mbps data rates using either frequency hopping (FH) or direct sequence (DS) spread spectrum (SS) techniques. The FCC requirement for the use of spread spectrum signaling takes advantage of inherent SS properties that make the signals less likely to cause inadvertent interference by lowering the average transmit power spectral density, and more robust to interference through receiver techniques which exploit spectral redundancy.
One type of self-interference which can be reduced by SS receiver techniques is multipath distortion. As shown in
FIG. 1
, the power delay profile (PDF)
10
of a transmitted signal due to multipath within an indoor WLAN system, such as the reduced complexity example illustrated in
FIG. 2
, typically exhibits an exponentially-decayed Rayleigh fading characteristic. Physical aspects of the indoor transmission environment driving this behavior are the relatively large number of reflectors (e.g., walls) within the building, such as shown at nodes
12
and
13
, between a transmitter site
14
and a receiver site
15
, and the propagation loss associated with the longer propagation paths t
1
, t
2
and t
3
, which contain logarithmically weaker energies.
The power delay profile of the signal is the mean signal power with respect to time of arrival. When each time of arrival obeys a Rayleigh distribution, the mean power level of the signal establishes the variance of its corresponding Rayleigh components. A logical explanation of the exponentially decayed multipath effect is due to the fact that a signal's propagation delay t
i
is proportional to the total distance traveled. On-average, therefore, the strongest (those encountering the minimum number of obstructions), are the minimal obstruction transmission paths whose signals arrive earliest at the receiver.
In terms of a practical application, the root mean squared (RMS) of the delay spread for a multipath channel may range from 20-50 nsec for small office and home office (SOHO) environments, 50-100 nsec for commercial environments, and 100-200 nsec for factory environments. For exponentially faded channels, the (exponential) decay constant is equal to the RMS delay spread.
The presence of multipath generates interference for communications systems. This interference is the result of multiple copies of the same signal arriving at the receiver with different temporal relationships, different amplitudes, and different carrier phases. When the majority of the multipath delays are less than the inverse signal bandwidth, the majority of the interference is due to different amplitude and carrier phases rather than different signal temporal properties. This type of multipath interference is referred to as “flat” fading because all frequencies in the signal undergo the same multipath effects. Because the path delays are less than the symbol duration, the interference is confined to one symbol or is primarily intra-symbol. Frequency-selective fading in contrast occurs when paths with significant energy have relative delays greater than the inverse signal bandwidth. In this case, the interference is primarily due to different temporal relationships between the information symbols or what is commonly called intersymbol interference. The frequencies present in the signal undergo different multipath effects due the intersymbol interference and this type of interference is also called frequency-selective fading.
Interference from flat fading is seen at the receiver as a reduction in the signal-to-noise ratio and is generally impossible to combat unless diversity reception is available. There are, however, several receiver techniques available for reducing the impact of frequency selective fading. Because there are more options available for frequency-selective fading environments, many systems are designed so that the basic symbol duration is much shorter than necessary to support the information rate. In the frequency domain, the short symbol duration results in a larger bandwidth than required to support the information rate. In other words, the information bandwidth has been spread and hence this is referred to as spread spectrum. In actuality, this results in frequency diversity and consequently can be thought of as providing diversity for what was a flat fading environment.
Increasing the bandwidth of the signal or spreading the signal can be accomplished in a number of ways and the design of spreading codes for communications systems has been the topic of research and development for many years. Direct sequence (DS) techniques are one common set of methods. A direct sequence system uses many sub-symbols or “chips” to represent a single information symbol. To decode the transmitted data, the optimal DS receiver finds the candidate information symbol that is “closest” to the received data in a Euclidean distance sense. In other words, the receiver finds the symbol with the symbol with the minimum distance to the received sequence. In the absence of multipath, the minimum distance receiver is implemented with a correlation receiver since correlation is equivalent to distance when all sequences have the same energy. In the presence of multipath, the correlation receiver must take into account the distortion due to the channel. To account for the multipath channel, the correlation receiver is modified to include matching to the channel as well as to the possible symbol sequences. For DS systems, the spreading sequence can be selected to have nearly impulsive auto-correlation and low cross-correlation properties. When such sequences are used in a channel matched correlation receiver, the individual paths comprising the multipath are coherently combined and the detrimental effects of multipath are reduced because the receiver is taking advantage of the frequency diversity. The use of a channel matched correlation receiver is typically called a Rake receiver.
As diagrammatically illustrated in
FIG. 3
, in a channel-matched correlation or RAKE receiver, the received (spread) signal is coupled to a codeword correlator
31
, the output of which (shown as a sequence of time-of-arrival impulses
32
-
1
,
32
-
2
,
32
-
3
) is applied to a coherent multipath combiner
33
. The codeword correlator
31
contains a plurality of correlators each of which is configured to detect a respectively different one of the codewords of the multi-codeword set. As a non-limiting example, the coherent multipath combiner may be readily implemented as a channel matched filter (whose filter taps have been established by means of a training preamble prior to commencement of a data transmission session). The output of the coherent multipath combiner
33
may be coupled to a peak or largest value detector
35
, which selects-the largest magnitude output produced by the coherent multipath combiner as the transmitted codeword. Since the RAKE receiver is a linear system, the order of the operations carried out by the channel matched filter (coherent multipath combiner)
33
and codeword correlator
31
may be reversed, a
Halford Steven D.
Nelson George R.
Webster Mark A.
Bayard Emmanuel P.
Intersil America's Inc.
Stanford Gary R.
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