A common baseband digital signal is applied to a plurality of frequency modulators generating frequency modulated signals the carrier frequencies of which are equal to that of a common reference frequency oscillator, and the peak frequency deviations of the frequency modulated signals are made slightly different from each other.
In a system for frequency-diversity transmission, separate and distinct frequency carriers are obtained from the frequency modulation of an oscillator by a pure frequency with an index such that a certain number of Bessel lines thus obtained constitute a corresponding number of separate carriers. All the carriers are subsequently modulated by the information, then power-amplified in a single amplifier having a peak power equal to that of a single signal whose effective power would be the sum of those of the separate carriers. By means of this system, equipment for radio communication by tropospheric or ionospheric scatter propagation can be constructed in a simple manner and at low cost, with a high diversity order.
In this system in which the information signals to be transmitted are converted into the digital form, transmission is done via a frequency-division multiplexer which so transmit the bits of the information signals in parallel that the duration of the bits transmitted through the channels of the multiplexer is longer than double the difference in propagation time between two carriers obtained from the two transmitters which are nearest in the receiving zone, where the said carriers have levels which are near to one another. This solves the problem of overlap between the information signals. To control the problem of fading, three types of transmitters which have three carriers whose deviation is very small compared with the bandwith of a channel are used in the transmitter network.
A communication system is provided in which a transmitter performs modulation upon a number sequence to be transmitted. The modulation scheme includes embedding a sequence of numbers into a waveform such that the sequence is present in the waveform on multiple time scales. The transmitted waveform has a selected number of different frequency bands of successively doubling bandwidths. Each of the frequency bands includes the sequence of numbers, repeated therein at a certain rate. The rate is directly proportional to the bandwidth of the frequency band. The communication system further includes a receiver designed to average the value of the repeated sequence as received by the receiver. This scheme allows for accurate communication over noisy, uncertain, and/or hostile channels in both point to point and broadcast communication applications.
A diversity system utilizing a plurality of branches has been found for an angle modulated digital signal transmission. The received signal on each branch is modulated with a local signal which has the same period as the digital signal, and satisfies the orthogonal relationship with each other. The modulated signals are combined by simply summing them, and the combined signal is differentially detected. In the case of two branches, said orthogonal local signals are .sqroot.2/T.multidot. sin (2.pi./T)t, and .sqroot.2/T.multidot. cos (2.pi./T)t, where T is the bit duration of the digital signal, and said local modulation is the amplitude modulation. The present invention can provide the same diversity effect as the prior maximal ratio combining system, although the present invention does not utilize a complicated cophasing means between each branches.
Methods, systems, and apparatuses for down-converting an electromagnetic (EM) signal by aliasing the EM signal are described herein. Briefly stated, such methods, systems, and apparatuses operate by receiving an EM signal and an aliasing signal having an aliasing rate. The EM signal is aliased according to the aliasing signal to down-convert the EM signal. The term aliasing, as used herein, refers to both down-converting an EM signal by under-sampling the EM signal at an aliasing rate, and down-converting an EM signal by transferring energy from the EM signal at the aliasing rate. In an embodiment, the EM signal is down-converted to an intermediate frequency (IF) signal. In another embodiment, the EM signal is down-converted to a demodulated baseband information signal. In another embodiment, the EM signal is a frequency modulated (FM) signal, which is down-converted to a non-FM signal, such as a phase modulated (PM) signal or an amplitude modulated (AM) signal.
