Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-12-22
2001-10-02
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Angle modulation
C375S271000, C329S345000
Reexamination Certificate
active
06298099
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to general wireless communication systems, and in particular to continuous phase modulation methods and systems.
BACKGROUND
When transmitting a collection of symbols across a noisy channel, there exists an inherent tradeoff between the power used to transmit each symbol and the length of time or the bandwidth required to transmit the symbol. This tradeoff is shown graphically in the communication efficiency plane of
FIG. 1
, in which the horizontal axis is the ratio of energy-per-bit to noise power spectral density, E
b
/N
o
, and the vertical axis is the ratio of the communication rate to bandwidth (bits/sec/Hz). The Shannon limit represents the theoretical maximum capacity of the channel to transfer data as E
b
/N
o
varies. It is apparent from inspection of
FIG. 1
that as more power is used to transmit information, information can be communicated across a channel more rapidly.
The somewhat abstract tradeoff illustrated in
FIG. 1
can immediately be understood by considering the case of a speaker attempting to communicate information, for example a telephone number, to a listener across a noisy room. One approach in such a case is for the speaker to shout, thereby rendering the numbers clearly audible to the listener over the noise in the room. Using this method, the speaker can communicate the entire telephone number in a very short time.
Another, far more subtle, approach is for the speaker to speak in a whisper but to elongate the delivery of each syllable. When the speaker transmits the message using this method, the listener can generally pick out the underlying drone of the speaker's message from the background noise, in effect, filtering out the speaker's message Integrating the received signal and noise over a sufficiently long integration interval. The disadvantage of this second approach is that it requires considerably more time to transmit the entire telephone number across the room.
In the context of
FIG. 1
, the speaker who shouts the phone number across the room operates toward the right-hand side of the communication efficiency plane. As a result, that speaker will be able to operate at a high baud rate and to therefore transmit the entire number very quickly. In contrast, the speaker who whispers the phone number operates toward the left-hand side of the communication efficiency plane and is therefore constrained by the Shannon limit to operate at a low baud rate and to therefore transmit the entire number slowly.
Every communication system can be characterized by an operating point in the communication efficiency plane of FIG.
1
. One goal of communication systems design is to place that operating point as close as possible to the Shannon limit. Where bandwidth is a limited resource, one can approach the Shannon limit using a high power system. Such a system does not require large bandwidth for near-error free transmission. Conversely, where power is a limited resource, one can approach the Shannon limit by using a low power system operating over a large bandwidth.
In a wireless communication system, the availability of power directed toward the receiver is limited by the transmitter's power rating and antenna gain. The availability of bandwidth is constrained by the desirability of sharing the available spectrum with as many channels as possible and by FCC governmental and international regulations. Consequently, in a wireless communication system, both power and bandwidth are limited resources.
In addition, a communication system in which different symbols are modulated onto the carrier as different power levels will have certain symbols represented by the lowest power level. Because these symbols are transmitted at lower than average power levels, they will inevitably be more prone to corruption by noise than symbols represented by higher than average power levels. As a result, the error rate associated with the transmission of a message will depend on the content of that message. A higher average power will be required for these symbols to reduce the error rate to a desired level.
Constant power signals are preferred for many wireless systems such as satellite communication systems which typically use more efficient class C operating amplifiers. Because these are non-linear amplifiers that operate at or beyond saturation, a waveform having other than constant amplitude can experience profound distortion as it passes through such an amplifier.
Constant power transmission can be achieved by modulating either the frequency or the phase of a carrier wave. Of these two modulation alternatives, phase modulation is far preferable for satellite communication systems because of its greater bandwidth efficiency. The reason for this greater bandwidth efficiency can be readily understood by considering the operation of a frequency modulation system.
In a frequency modulation system such as frequency shift keying (FSK), each symbol corresponds to a particular frequency. When two symbols correspond to two frequencies that are very close together, the probability of a demodulation error due to noise in the communication channel is high. In order to reduce the probability of such an error, the difference of the two frequencies must exceed a certain fixed amount that is proportional to the channel bandwidth. An increase in the signaling rate entails an increase in the number of frequencies employed. This, of course, consumes bandwidth.
In a conventional phase modulation system, each symbol corresponds to a particular phase angle associated with a carrier having a single frequency. As a result, a large number of symbols can be transmitted without requiring multiple frequencies. Phase modulation systems are thus particularly desirable in applications such as satellite communication systems in which bandwidth is at a premium.
In its simplest form, a phase modulation system operates by transmitting a carrier having a fixed phase angle which is representative of a first symbol during a first time interval. In the context of this application, the “phase angle” of a carrier refers to the principal branch of the arc tangent of the ratio of the imaginary part of the carrier to the real part of the carrier. During a second time interval, the system transmits the carrier but with a fixed phase angle corresponding to a second symbol (not necessarily different from the first symbol). The system then operates at the second phase angle for the duration of the second time interval. The system continues to operate in the foregoing manner until all the symbols that make up the message have been sent. The phase considered as a function of time (hereinafter referred to as the “phaseform”) for this system thus traces a discontinuous path in time as shown in the phase cylinder in FIG.
2
.
A disadvantage of the foregoing method of operating a phase modulation system is that, it is not possible to efficiently shape the phaseform so that spectral energy will remain concentrated within the allocated bandwidth while maintaining signaling rate. For systems that employ non-overlapping phase symbols, the symbols arc typically phases that are held constant during each symbol transmission interval. For instance, PSK employs constant phase symbols.
Unfortunately, phaseform symbols that have constant values introduce discontinuities whenever a symbol is followed by a different one, and the discontinuity causes energy to spill into adjacent channels, causing adjacent channel interference If the duration of each of the successive non-overlapping symbols is increased to allow shaping for improved energy concentration and decreased adjacent channel interference, then the signaling rate is decreased.
Another approach to overcoming the foregoing disadvantage is to increase the time interval required to transmit a particular symbol but to also allow portions of two or more symbols to be transmitted during each time interval. This type of phase modulation is best understood with reference to
FIG. 3
, which shows the transmission of
Resnikoff Howard L.
Tigerman Mark
Foley Hoag & Eliot LLP
FutureWave, Inc.
Liepmann W. Hugo
Oliver Kevin A.
Pham Chi
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