Transmitter Power Control Circuit

This invention relates to radio communication systems and, more particularly, to arrangements for controlling the transmitter output power.

The output power of a transmitter in a radio communication system is based upon the transmission objectives set forth for the system. These are derived from grade-of-service analyses of the results of subjective and objective tests. In digital radio systems, for instance, the problem is that in order to meet system objectives under adverse propagation conditions the power level required is considerably greater than the level needed during more normal conditions. In particular, it is only during the short periods of deep fading that high transmitter output power is required. This characteristic is peculiar to digital radio systems which can satisfactorily operate (at low error rates) very close to their threshold of operation where the error rate exceeds a certain limit. In contrast, analog FM and AM radio systems require high transmitter power at all times because of the cumulative nature of its thermal noise contribution.

It is, accordingly, the broad objective of the invention to control the transmitter output power in digital radio communication systems.

In accordance with the present invention, the received signal power of the digital signal is monitored at the receiver and a control signal is transmitted back to the transmitter to adjust the transmitter output power, as required.

The advantages of transmitting at reduced power levels are many in digital radio systems. First, the lower transmitted powers under normal propagating conditions allow operation of the repeater circuits in a highly linear mode. This is very important for the high level (i.e., 64QAM) modulation schemes used today or contemplated in the future. Such modulation methods contain a high degree of amplitude modulation which could be adversely affected by nonlinear repeater circuits. The two degradations of importance are eye degradation and spectrum spreading. The former affects bit error rate. The latter affects interference into adjacent channels. During the rare events of very deep fades the transmitter power will be maximum and the nonlinear distortion will increase. Fortunately at this extreme point, where the error rate reaches a very high value (10.sup.-3), the nonlinear distortions are considerably less destructive.

A second benefit of transmitter power control is that cochannel and adjacent channel interferences are significantly reduced. This will permit the coexistence of digital and analog systems sharing the same frequency band without resorting to such measures as space diversity antenna systems.

A third advantage of transmitter power control is the possibility of operating digital radio systems without using regeneration in every repeater station. The low nonlinear distortions, which approach single sideband conditions, may make regeneration only necessary in radio switching stations, a substantial operational benefit. Finally, there may be the simple power saving that results from operating at lower power levels.

FIG. 1 shows a radio system in accordance with the present invention; and

FIG. 2 shows an illustrative power control characteristic.

Referring to the drawings, FIG. 1 shows the relevant components of a two-way radio communication system. Included are portions of a transmitter 10 and receiver 11 of a west-to-east communication link and portions of a transmitter 12 and receiver 13 of an east-to-west link. Each of the transmitter portions 10, 12 shown comprises an i.f. amplifier with AGC 14, 17; a frequency up-converter 15, 18; an r.f. power amplifier 16, 19; and antennae 35, 36. Receiver portions 11, 13 shown comprise antennae 37, 38; low noise r.f amplifiers 20, 24; frequency down-converters 21, 25; and i.f. amplifiers with AGC 22, 26.

In accordance with this embodiment of the invention the two links are interconnected by means of feedback circuits 30 and 40 which connect the AGC circuits of the i.f. amplifiers 22, 26 of receivers 11 and 13, to the AGC circuits of the i.f. amplifiers 17, 14 respectively, of transmitters 12 and 10.

In the absence of feedback circuits 30 and 40, each transmitter is designed to have a constant power output which is sufficiently high to provide a certain level of overall system performance during periods of deep fades. However, as noted above, this high power level is not required most of the time. Accordingly, the system is modified to transmit at a level that is a function of the existing propagation conditions, as determined by the carrier signal level at the receiver. Thus, with respect to transmission in the west-to-east direction, the AGC voltage developed in i.f. amplifier 22 in receiver 11, which is essentially a d.c. voltage, and is a measure of the received signal level, is applied to FM deviator 31 whose output is an a.c. signal whose frequency is a function of the applied AGC voltage. This a.c. signal is coupled to the AGC circuit of i.f. amplifier 17 of transmitter 12 and serves to superimpose a modulation upon the amplitude of the east-to-west transmitted carrier signal. This modulation, in turn, causes the AGC voltage of i.f. amplifier 26 in receiver 13 to have an a.c. component, provided that the modulation frequency is within the bandwidth of the AGC control loop. This AGC component is then applied to the FM discriminator 41 in feedback circuit 40. The output of discriminator 41 is a d.c. signal whose magnitude is a function of the frequency of the a.c. signal applied thereto. Thus, the d.c. voltage forming the AGC signal of i.f. amplifier 14 in transmitter 10 is a function of the magnitude of the AGC signal in i.f. amplifier 22. If the latter is large, indicating a strong received carrier signal at receiver 11, a correspondingly large AGC voltage is applied to amplifier 14 in transmitter 10, thus reducing the gain of the amplifier and the magnitude of the signal applied to r.f. power amplifier 16. If, on the other hand, a signal fade develops, the AGC voltage in amplifier 22 decreases, thereby decreasing the AGC signal in amplifier 14. This increases the gain of amplifier 14, thereby increasing the output power transmitted by r.f. amplifier 16. Thus, an adaptive transmitter power control is provided with the addition of only an FM deviator at the receiver and an FM discriminator at the transmitter.

As is evident, a similar power control can be provided for the east-to-west channel by the addition of an FM deviator 42 (shown in broken outline) which converts the d.c. component of the AGC signal developed in amplifier 26 to an a.c. signal which modulates the west-to-east carrier, and an FM discriminator 32 (shown in broken outline) which detects the a.c. component of the AGC signal developed in amplifier 22 and converts it to a d.c. signal which controls the gain of amplifier 17. Thus, with a minimum amount of additional circuitry, two-way adaptive transmitter power control is achieved.

An important feature of the invention is the suppression of the a.c. components on the radio signal by the AGC circuits in i.f. amplifiers 22 and 26. This assures that no interference is being caused by these signals in the adjacent radio hops.

The overall power control characteristic is a function of the FM deviator, the FM discriminator, and the gain characteristics of the respective amplifiers. By varying any or all of these parameters any control characteristic can be realized. For example, in a control circuit constructed to test the concept, it was decided to maintain a constant output power for downfades up to 20 dB and upfades up to 5 dB. This was designated as Region I. As the fade increases beyond 20 db, the transmitter power is increased correspondingly. This second interval was designated Region II. Upfades are relatively frequent occurrences which can lead to overdrive conditions in the input stages of receivers 11 and 13. In order to prevent such conditions the transmitter output power is further reduced for upfades higher than 5 dB. FIG. 2 shows the relationship between transmitter power and fade depth for the example just described. Note that during normal and modestly faded conditions, the control system is in effect disconnected and a constant (relatively low) output power is maintained. This contributes to the high reliability of the power control system. The characteristic shown in FIG. 2 can be obtained in various ways by nonlinear shaping of the FM deviator, the FM discriminator and gain and AGC characteristics of the power control loop as is well-known in the art. Such non-linear shaping means is shown in FIG. 1 within dashed lines designated 33 and 43 at the two transceivers. As stated above, however, it should be understood that such non-linear shaping means 33 and 43 actually can be part of the F.M. deviator 31, 42, the FM discriminator 32, 41, and/or the AGC arrangements in elements 22,17 or 26,14, respectively, and is not necessarily a separate circuit.

In the above-described embodiment of the invention, the power control signals are shown as being transmitted over an FM-AM link, piggy-backed on the broadband digital signal traveling in the opposite direction. It will be recognized, however, that this signal can just as readily be transmitted over a separate service (auxiliary, telemetry) channel, if available. Stability of the overall control loop can be achieved by using known techniques of control system design.