Pulse or digital communications – Spread spectrum
Reissue Patent
1997-09-02
2004-10-19
Cangialosi, Salvatore (Department: 2661)
Pulse or digital communications
Spread spectrum
C370S902000
Reissue Patent
active
RE038627
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to spread-spectrum communications and more particularly to a system and method for a high capacity spread-spectrum communications channel.
DESCRIPTION OF THE PRIOR ART
Referring to
FIG. 1
, message data, d(t), are processed by spread-spectrum data modulator
51
, using a message-chip-code signal, g
1
(t), to generate a spread-spectrum data signal. The data spread-spectrum signal is processed by transmitter
52
using a carrier signal at a carrier frequency, f
o
, and transmitted over communications channel
53
.
At a receiver, a spread-spectrum demodulator
54
despreads the received spread-spectrum signal, and the message data are recovered by synchronous data demodulator
60
as received data. The synchronous data demodulator
60
uses a reference signal for synchronously demodulating the despread spread-spectrum signal. The square-law device
55
, bandpass filter
56
and frequency divider
57
are well known in the art for generating a reference signal from a received modulated data signal. A Costas Loop or other reference signal generating circuit is also adequate.
The spread-spectrum system of
FIG. 1
is limited to a single communications channel, and would not work well for communicating high capacity information using spread-spectrum in a fading environment. Consider the T
1
, Ethernet, and T
3
networks, by way of example, and assume the data modulator
51
spread-spectrum processes the message data, d(t), with a chip rate of 25 megachips per second. For the T
1
network which communicates data at up to 1.544 megabits per second, the spread-spectrum system of
FIG. 1
has a processing gain of approximately 17. For the Ethernet, which communicates data at up to 10 megabits per second, the spread-spectrum system of
FIG. 1
has a processing gain of 2.5. For the T
3
network, which communicates data at up to 43 megabits per second, the spread-spectrum system of
FIG. 1
has a processing gain of approximately 0.6. Thus, for a particular spread-spectrum channel, data communicated at a higher rate results in a lower processing gain. The lower processing gain can result in channel degradation and loss of the advantages of spread-spectrum. For example: resistance to fading caused by multipath and inability to share the spectrum with other spread spectrum systems such as a PCN system.
Information may be transmitted through the spread-spectrum channel by using M-ary modulation schemes, such as quadrature phase-shift-keying (QPSK) modulation or 16-ary amplitude and phase modulation, for increasing the data rate through the channel. The M-ary modulation schemes are modulated with spread spectrum. Problems with M-ary modulation schemes include increased equipment complexity for the transmitter and receiver, and higher error rates because the decision regions between symbols, such as phase angles or amplitude levels, for example, become closer. Using M-ary signaling schemes, accordingly, can cause degradation in channel performance. Thus, a need exists for a system and method that provides a high capacity spread-spectrum channel, which does not have increased equipment complexity, a degradation in performance, or loss of the advantages of spread-spectrum modulation.
Today, high data rate transmission employs 64QAM without spread spectrum. These systems are extremely sensitive to noise since the separation of symbols is small. They therefore require a high signal-to-noise ratio. Hence, all interference must be small. This limits their capability to efficiently use the spectrum.
OBJECTS OF THE INVENTION
A general object of the invention is a synchronous, high-capacity, spread-spectrum-communications system.
Another object of the invention is a system and method for communicating information at a high rate through a spread-spectrum channel, while maintaining the advantages of spread-spectrum including high processing gain, resistance to fading and an ability to share the same spectrum with other users.
A further object of the invention is to synchronously demodulate a plurality of modulated-data signals embedded in a spread-spectrum-communications signal.
SUMMARY OF THE INVENTION
According to the present invention, as embodied and broadly described herein, a high capacity spread spectrum communications system for use over a communications channel is provided comprising a high-capacity-spread-spectrum transmitter and a high-capacity-spread-spectrum receiver. The high-capacity-spread-spectrum transmitter includes demultiplexer means, generic means, a plurality of message means, summer means, and transmitter means. The demultiplexer means demultiplexes the message data into a plurality of demultiplexed-data signals. The generic means generates a generic-chip-code signal. The plurality of message means generates a plurality of message-chip-code signals. Each of the plurality of demultiplexed-data signals and each of the plurality of message-chip-code signals are synchronized to each other and optionally to the generic-chip-code signal. The message-chip-code signals must be orthogonal or near orthogonal for the high-capacity-spread-spectrum system to work.
Each spreading means of the plurality of spreading means, spread-spectrum processes a demultiplexed-data signal with a respective message-chip-code signal to generate a spread-spectrum-processed signal. The summer means combines the plurality of spread-spectrum-processed signals, and if used, the generic-chip-code signal. The combined signal typically is a multi-level signal, with an instantaneous-combined voltage level equal to the sum of the voltage levels of the plurality of message-chip-code signals and, if used, the generic-chip-code signal. A multi-level signal is a signal with multiple voltage levels.
The transmitter means transmits the combined plurality of spread-spectrum-processed signals, and if used, the generic-chip-code signal, on a carrier signal over the communications channel as a spread-spectrum-communications signal. While the transmitter means may use a linear power amplifier for optimum performance, a nonlinear power amplifier also may be used without significant degradation or loss in performance. Thus, the sum of voltage levels need not be an exact linear sum, with only a little loss in performance.
The high-capacity-spread-spectrum receiver can be used for simultaneously receiving a plurality of spread-spectrum channels of a spread-spectrum-communications signal. The high-capacity-spread-spectrum receiver includes acquisition and tracking means, multiplexer means, and a plurality of spread-spectrum receivers, and optionally generic-spread-spectrum-processing means. Each spread-spectrum receiver includes message-spread-spectrum-processing means, detection means and bit-synchronization means.
The generic-spread-spectrum-processing means, if used, recovers a carrier signal from a spread-spectrum channel of a received spread-spectrum-communications signal, and generates a replica of the generic-chip-code signal. The acquisition and tracking means acquires and tracks the recovered carrier signal. The acquisition and tracking means also synchronizes the generic-spread-spectrum-processing means to the recovered carrier signal.
The generic-spread-spectrum-processing means is not required for the high-capacity-spread-spectrum receiver. The acquisition and tracking means may be coupled to an output of a message-bandpass filter, and appropriate circuitry, such as that shown in
FIG. 1
, can be used to recover the carrier signal.
The plurality of message-spread-spectrum-processing means despreads the received spread-spectrum-communications signal as a plurality of modulated-data signals. The plurality of message-spread-spectrum-processing means derives synchronization from one of the modulated-data signals, or a replica of the generic-chip-code signal provided by generic-spread-spectrum-processing means.
The plurality of detection means detects the plurality of modulated-data signals as a plurality of detected signals, respectively. The plurality of detection means may be
Cangialosi Salvatore
InterDigital Technology Corp.
Volpe and Koenig PC
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