1. Field of the Invention
This invention relates generally to multiplexed communication systems and, more particularly, to noise generated by rapid change in transmitted power during the interval reserved for the transmission of information. This noise is generally referred to as splatter.
2. Description of the Related Art
A typical communication system 10 using time division multiplex techniques is shown in FIG. 1A. An antenna 101 is coupled to a two position switch 102. When the switch 102 is coupled to a first terminal, the signal received by the antenna 101 is applied to low noise amplifier 103. The output signal from the low noise amplifier 103 is applied to mixer unit 104. The mixer unit 104 also receives a signal from the local oscillator unit 105. The signals from the low noise amplifier 103 and the local oscillator unit 105 are combined in mixer unit 104 and applied to filter unit 106. The filtered signal from filter unit 106 is applied to an IF (intermediate frequency) unit 107. The output signal from the IF unit 107 is applied to demodulation unit 108. The demodulation unit 108 recoveries the data (information) that has been encoded in the transmitted signal and applies the data to an output terminal. When the switch unit 102 is in the second position, the output signal from power amplifier is applied to the antenna for transmission of information. The power amplifier 109 receives signals from mixer unit 110. The mixer unit 110 combines signals from the local oscillator unit and from a unit (not shown) that has encoded an intermediate signal with data to be transmitted. Typically, the intermediate frequency is fixed. However, the local oscillator unit 105 has a controllable voltage and can be implemented, for example, by a voltage controlled oscillator.
Referring to FIG. 1B, the operation of the time division multiplex system 10, operating in a duplex mode of operation is illustrated. For purposes of the operation of the time division multiplex system, time can be divided into a series of equal intervals. During alternate intervals, the system is transmitting, i.e., switch 102 has coupled the antenna 101 to the power amplifier 109. During the alternate intervals when the system 10 is not transmitting, the switch 102 couples the antenna 101 to the low noise amplifier 103. During these alternate intervals, the system is in a receiving mode. Thus, the system alternately transmits data and receives data. This mode of operation is referred to as multiplexing.
One of the problems that arises with a multiplexing mode of operation is the noise introduced by the rapid switching from a power-off mode to a power-on mode. In the idealized square wave form illustrated in FIG. 1B, the rapid change in the transmitted signal introduces frequency harmonics. These harmonics represent noise in the transmitted signal. The noise interferes with the detection and demodulation of the transmitted signal.
Referring to FIG. 2A, a portion of the apparatus shown in FIG. 1A is reproduced. In addition, an enabling signal TXEN that controls the transmission/reception cycle is shown. The enabling signal TXEN controls both the position switch 102, activates and inactivates the power amplifier 109, and can be applied to the mixer unit 110. The enabling signal TXEN determines, by controlling switch 102, whether the communication system is in a transmission mode or in a reception mode. By turning activating the power amplifier only when the switch is in the transmission mode, the leakage signal into the reception portion of the communication system can be reduced. Finally the application of the signal to the mixer unit 110 symbolically illustrates the point that the encoded carrier signal is not transmitted continuously. Therefore, the availability of the encoded of the carrier signal must be synchronized with the activation of the power amplifier 109.
Referring to FIG. 3A and FIG. 3B, the problem of noise generated by rapid transitions from a power-off to power-on mode of the power amplifier is illustrated. In FIG. 3A, a recording of a measurement by a spectrum analyzer, taken at the antenna port of the communication system 10, is shown. The center frequency of the spectrum analyzer is set to the same frequency as the frequency of the signal transmitted by system 10 (in this measurement, that frequency was 902 MHz). The recording shows the envelope of the pulsed radio frequency energy. The envelope is not filtered. The abruptly rising leading edge of the transmitted signal is clearly visible. The result of not filtering the transmitted signal is shown in FIG. 3B. In the recording of FIG. 3B, the center frequency of the spectrum analyzer is set to 905 MHz, 3 MHz, above the center frequency of the transmitted signal. An energy spike is seen that is coincident with the rising edge of the transmitted signal. (As will be clear to those skilled in the art, a similar spike would be measured at 3 MHz below the center frequency of the transmitted signal.) These energy spikes result in interfering noise energy that degrades the performance of the communication system in the broadcast band for which the communication system 10 is intended.
A need has been felt for apparatus and an associated method that features a reduction in the noise associated with the abrupt transition in transmitted power found in multiplexed communication systems. The apparatus and associated method would further feature procedures for increasing the integrity of the data in the presence of the power transition.
The aforementioned and other features are accomplished, according to the present invention, by providing a ramped transmitted signal. The transmitted signal is provided with a ramped profile by using a ramped enabling signal to switch the power amplifier from an off state to an on state. The ramped transmitted signal reduces the noise resulting from abrupt leading edge of transmitted signal, but results in diminished amplitude of the transmitted signal during the initial enabling of the power amplifier. In order to avoid loss of information during the transition period, the transmitted data is provided with a preselected preamble that does not include encoded data.