Pulse or digital communications – Spread spectrum
Reexamination Certificate
2000-11-15
2004-04-20
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S141000, C708S252000
Reexamination Certificate
active
06724805
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to communication and/or measurement systems and, more particularly, to systems that use spread spectrum modulation to convert relatively narrow-band information, or message, signals to wide-band signals for transmission.
BACKGROUND
Demand for bandwidth resources as well as consumer telecommunications applications will drive the development of low-cost, low-power short-range transceivers. More efficient use of bandwidth can be made by tiling a cellular wireless system using a greater number of shorter range transceivers. This approach is called Space Division Multiple Access (SDMA) to draw a parallel to Time and Code Division Multiple Access (TDMA and CDMA). In a cellular or multi-hop peer-to-peer network, messages hop from one transceiver to another across a field of transceivers. The basic idea behind SDMA is to simultaneously increase the density and decrease the range of transceivers, the result is an increase in maximum number of message packets that may be transmitted across the network at any given instant because each message occupies less physical space at any given instant in time. To avoid frequent packet collisions, however, each transceiver needs to be able to share the available spectrum.
In addition to their use in wireless networks, short range transceivers will soon find consumer application in the home, automobile, and likely in wearable or “personal area” networks (PANs). If the internet is to become portable, then short range transceivers will become increasingly important for transmitting information over the last few meters to the user. Just as with present day cell phones, many of theses devices will need to be able to communication on a shared spectrum to a single base station. Therefore these transceivers require a low-cost, low-power, low-complexity scheme for channel sharing.
Direct Sequence Spread Spectrum (DS SS) offers an attractive solution to the channel sharing problem. The advantages of spread spectrum in general and DS SS in particular are manifold. Virtually all high performance commercial wireless systems employ DS SS. In addition to providing a simple scheme for channel code allocation, DS SS offers low peak power, excellent resistance to interference including interference from echoes known as multipath interference, easily scalable processing gain, excellent bandwidth efficiency when use in a micro-cell system, and graceful rather than catastrophic bit error rate (BER) degradation as more transceivers share the channel. In addition, improvements for DS SS systems are of interest for their applications in making high resolution timing measurements.
FIG. 1
illustrates a generalized functional block diagram of a spread spectrum communication system
10
. The transmitter
20
includes a pseudo-random number (PN) generator
24
that generates a string of pseudo random bits known as a PN sequence, x(t). This PN sequence is a string of bits that appear random by statistical standards, but which are actually generated by a deterministic algorithm. The PN sequence is effectively the carrier signal. The modulator
28
modulates the PN sequence by the message signal, m(t), thereby producing the transmit signal, T(t). The transmit signal is a bit stream that behaves like white noise, but has data hidden in it.
A received signal, S(t), is received by a receiver
30
. The received signal may be the transmit signal itself or it may be an attenuated and/or noisy version of the transmit signal due to interactions that occur over the transmission distance
14
. A receiver, such as receiver
30
in
FIG. 1
, must have a PN synchronizer
34
to recover the message signal from the received signal. The PN synchronizer is a PN generator that produces the same PN sequence as that created by the PN generator
24
in the transmitter
20
and that synchronizes its own PN sequence with that of the received signal. The receiver uses the synchronized PN sequence to demodulate the received signal and recover the message signal. In the process, the receiver ignores all other data signals modulated by other PN sequences, which is how DS SS helps solve the channel sharing problem.
The communication system illustrated in
FIG. 1
appears simple but is actually difficult to implement. The difficulty lies in getting the PN synchronizer
34
in the receiver
30
to generate a PN sequence that is synchronized with the PN sequence generated by the PN generator
24
in the transmitter
20
. Achieving this synchronization is called code acquisition. Maintaining synchronization is called tracking.
State-of-the-art acquisition systems can have long and unpredictable acquisition times which make them unacceptable for multi-hop peer-to-peer networks which make and break connections often as they route traffic. Acquisition times much less than the time it takes to transmit a packet are desirable for a short range transceiver system. Present day acquisition systems are also power-hungry, because they use high-speed Digital Signal Processing (DSP) components. The energy consumption of these components scales linearly (at best) with the frequency of the operations they perform. State-of-the-art acquisition systems are also relatively expensive to manufacture because it is essentially impossible to integrate the high speed 3,5 semiconductor components they require with a baseband system for which standard silicon is adequate.
A typical spread spectrum communication system will include an Linear Feedback Shift Register (LFSR) as the PN generator
24
in the transmitter
20
of FIG.
1
. The general equation for an LFSR follows:
x
n
=
∑
i
=
1
N
a
i
x
n
-
1
(
mod
2
)
From the theory of LFSR's, &agr;
N
=1 always whereas &agr;
i
=1 sets a tap of a given delay n and &agr;
i
=0 sets no tap. The equation describes a delay line of length N, with taps at delays for which &agr;
i
=1. The outputs of the taps are summed mod 2 and then shifted into the register. All x
n
always takes a discrete value of either 0 or 1.
FIG. 2
illustrates a functional block diagram of a four-bin, two-tap LFSR that implements the following recursion relation:
x
(
t
+&tgr;)=
x
(
t
−&tgr;)+
x
(
t
−4&tgr;) mod 2
FIG. 2
shows four bins in series with the first and fourth bin tapped. A tap communicates the value of the tapped bin to the mod 2 addition functional element. The mod 2 addition functional element then communicates the result to the first bin. The LFSR operates recursively going through a complete PN sequence and then repeating it. A bin is essentially a delay element that holds the value that the previous bin had before it updated.
FIG. 3
is a functional block diagram of an analog PN generator, known as the Cosine Analog Feedback Shift Register (Cosine AFSR) and proposed by Grinstein et al. in U.S. Pat. No. 5,737,360. The Cosine AFSR represents the first attempt at an analog device that produces a PN sequence for use in a spread spectrum communication system. A spread spectrum communication system may include a Cosine AFSR as the PN generator
24
in the transmitter
20
of FIG.
1
. Such a Cosine AFSR would directly feed back the output of the cosine function to the first bin. A spread spectrum communication system may also or alternatively include a Cosine AFSR as the PN sequencer
34
in the receiver
30
of FIG.
1
. Such a Cosine AFSR would use the adder to superpose the received signal onto the output of the cosine finction.
FIG. 3
specifically illustrates a Cosine AFSR that implements the following general equation:
x
n
=
1
2
[
1
-
cos
(
π
∑
i
=
1
N
a
i
x
n
-
1
)
]
A flaw in the Grinstein et al. discrete time Cosine AFSR is revealed when one attempts to use it to synchronize with a noisy PN sequence. One thousand such trials were run. In the trials, successful acquisition was defined as having produced 2N+1 error-free chips identical to
Massachusetts Institute of Technology
Vo Don N.
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