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Analogue vs. Digital Beamforming

Analog Summation

Figure 1: Analogue beamforming of the antenna pattern

Analog Summation

Figure 1: Analogue beamforming of the antenna pattern

Analogue vs. Digital Beamforming

Analogue Beamforming

Analogue Beamforming (ABF) means, that the received echo signals from each element of the phased array antenna, are combined at the RF carrier frequency level. This analog beamformer feeds up to four centralized receiver channels, down-converting the signal to the basic band (or to an intermediate frequency, IF). The following Analogue-to-Digital Converter (ADC) digitizes the IF or the video signals.

Digital Summation

Figure 2: Digital beamforming of the antenna pattern

Digital Summation

Figure 2: Digital beamforming of the antenna pattern

Digital Beamforming

Digital beamforming (DBF) can be realized at the antenna element level or at the sub-array level. In DBF architecture, there are many digital receivers, one own receiver at each of the radiating elements of the antenna. The down-converting to IF-frequency and digitizing the signals is realized at each individual antenna element (or small groups of them). Noise and signal distortion in each receiver is decorrelated among all receivers.

Multiple independent beams steered in all directions can be formed in the digital beamforming processor. The benefits of digital beamforming include:

Figure 2: Digital beamforming of the antenna pattern

(Courtesy of EADS Defence & Security Ulm)

Figure3: Standardised Modular Transmit-/Receive-Module
(SMTRM) (Courtesy of Cassidian Ulm,
former EADS Defence & Security)

Standardised Modular Transmit-/Receive-Module (SMTR®)

The Transmit-/Receive-Modules are the key components based on AESA technology digital beamforming. The modules contain a part of transmitters’ power amplifier and parts of the receiver and can be mass-produced to achieve a better cost/performance ratio. These modules may be used in various programs with only slight individual adaptations being required. By the compact design, the line losses are low during signal processing. For example, the standardized modular transmit-/receive-modules are used in the X-band multi-function fire control radar of the Medium Extended Air Defence System (MEADS), in the German Armed Forces ground surveillance radar BÜR, TerraSAR-X space radar, and in the Eurofighter’s E-Captor radar.

The SMTRM is a hermetically covered circuit board with the dimensions of 64,5 mm length, a width of 13,5 mm a height of and 4,5 mm. This board contains a power amplifier being in the shape of two monolithically integrated circuits fed with different phaseshift, a ferrite circulator for connection of a transmitter/receiver to the transmit/receive path, a monolithically integrated limiter, and a low noise pre-amplifier. The received signal will be down-converted into an intermediate frequency. All circuits are performed in Gallium-Arsenide (GaAs) semiconductor technology.

Beamforming Processor

This ability to organize various antenna patterns at the same time has only become possible with the technology of a digital receiver because only digital signals can be copied any number of times without loss. In practice, the received signal is converted into an intermediate frequency and then digitized at once. At an IF of 75 MHz, the analog to digital converter requires a sampling frequency of 100 MHz.

Figure 4: Block diagram of a beamforming processor

Figure 4: Block diagram of a beamforming processor

Figure 4: Block diagram of a beamforming processor

Figure 4 shows a block diagram for a typical beamforming processor. Every single antenna of the phased array antenna has got its own receiver channel and is followed by its own analog-to-digital converter and a digital down converter (DDC). For a correct summation, there is a special transversal filter that equalizes the frequency response and corrects the individual propagation delay in this one receiving channel. This transversal filter is also referred to as a finite impulse response filter (FIR). It is tuned in to a special automatic calibration routine. For this calibration, a known RF- test signal will be fed into the receiver channel, which is either linear frequency modulated over the entire bandwidth) or it is a white noise with a known magnitude. Required weights for suppression of sidelobes are also made ​​in this filter. The data of all the analog-to-digital converters of the receive channels are fed as a complex (I&Q) signal via a phase shifter stage in any summation stages. The number of summation stages determines the number of possible simultaneously received antenna beams. In the figure, this number of summation stages is assumed to be 100.

noise level

Figure 5: Green channel: signal-to-noise ratio of a single receive channel; red channel: sum of two in-phase receive channels.

noise level

Figure 5: Green channel: signal-to-noise ratio of a single receive channel; red channel: sum of two in-phase receive channels.

Improved sensitivity

Each individual receive channel has a signal-to-noise ratio (SNR) comparable to the receive path in the radar receiver using analog beamforming. However, noise is a chaotic process and must differ in two receive channels. It is therefore unlikely that individual noise peaks will be the same in two different receivers, although this cannot be ruled out entirely.

Thus, if two receive channels are summed in phase, then the echo signal sums to a larger pulse, but the noise is decorrelated, the phases and amplitudes do not match. Therefore, the noise cannot sum to the same extent as the echo signal. This improves the signal-to-noise ratio by about 2 dB even with a sum of two receive channels, resulting in higher sensitivity of radar using digital beamforming. In a configuration of 37 receive modules (as in Raytheon’s scalable Air and Missile Defense Radar, AMDR), this results in an improvement in sensitivity of about 15 dB in practice.