Multi-channel beamforming is a signal processing technique for directional signal transmission or reception and can, for example, be used in radar systems, such as for example automotive radar systems, for example installed in a car or other vehicle. This technique exploits that signals at particular angles experience constructive interference while others experience destructive interference.
Beamforming may be applied in the digital domain or in the analog domain, where a phased-array or smart antenna is an electronically steerable directional antenna. It can be used, for example, in radar or in wireless communication systems. A phased-array receiver for analog beamforming contains a group or matrix of antenna elements and associated receive channel circuits, i.e. receiver circuits, in which the relative phases of received signals are varied in such a way that the effective reception pattern of the array is directed in a desired direction and suppressed in undesired directions. Depending on the operation frequency range, overall power consumption and receiver circuit size, the group of receiver circuits may, for example, be provided on the same integrated circuit die or as separate devices. A phased-array receiver employing beamforming controls the phase and relative amplitude of the incoming signal received by each antenna element and combines the output signals delivered by receive channels associated with each antenna element in such a way that a particular radiation pattern can preferentially be observed, i.e., a phased-array receiver uses the phase difference of an incoming signal received at different antenna elements of the antenna matrix to determine the angular position of a target.
A signal may be a time varying physical quantity carrying information, e.g. a varying voltage level, for example occurring at an antenna element or receiver input when receiving an electromagnetic wave.
For a phased-array receiver, a receiver circuit can be implemented as an I-Q receiver circuit, wherein the received signal is split and applied to two signal paths, then mixed with a pair of mixing frequency signals, for example, generated by a local oscillator circuit, with a relative 90° phase shift. The result of the frequency mixing is an in-phase signal or I-signal, and a quadrature signal, or Q-signal. As shown in FIG. 1, a prior art receiver circuit 10 commonly contains an input balun circuit 12 that is arranged to receive incoming signals, e.g., RF signals, i.e., radio frequency signals, for example, by connecting its input 14 to an antenna element. Just to give an example, this frequency range includes frequencies of radar signals, such as for example 77 GHz radar signals often used in automotive radar applications. A balun circuit is a type of electrical transformer circuit that may, for example, convert an electrical signal that is referred to ground, i.e., single-ended, to signals that are balanced, i.e., differential signals, by introducing a 180° phase-shift between them. The shown input balun circuit 12 contains a balanced output, i.e. an output with two terminals for differential signal output, wherein one terminal is arranged to provide a signal corresponding to the original input signal and a second output is arranged to provide the signal with a relative phase-shift of, for example, 180°. The balanced output of the shown input balun circuit 12 is connected to a balanced input of an RF input stage, i.e., an input RF amplification circuit 16.
The input RF amplification circuit 16 contains a balanced output connected to a first or in-phase double balanced mixing circuit 18 arranged to receive at a balanced RF input the differential amplified RF signal and to receive at a balanced mixing frequency input 20 a mixing frequency signal generated by a local oscillator (LO). The balanced output of the input RF amplification circuit 16 is also connected to a second or quadrature double balanced mixing circuit 22 arranged to receive at a balanced RF input the differential amplified RF signal and to receive at a balanced mixing frequency input 24 a mixing frequency signal with a relative 90° phase-shift against the mixing frequency signal applied to the in-phase double balanced mixing circuit 18. The in-phase double balanced mixing circuit 18 contains a balanced intermediate frequency (IF) output 26 arranged to provide a differential in-phase intermediate frequency signal or IF,I-signal. The quadrature double balanced mixing circuit 22 contains a balanced intermediate frequency (IF) output 28 arranged to provide a differential in-phase intermediate frequency signal or IF,Q-signal.
The double balanced mixing circuits 18, 22 may be implemented as standard Gilbert cells. A Gilbert cell is a transistor circuit used as an analog frequency mixer, wherein the output current is a multiplication of the differential input currents, wherein unwanted mixing products are at least partly suppressed.
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