Similarly, during the receive operation, the RF signal is routed and amplified, phase-shifted, combined, and delivered to an external digital-signal-processing unit. The phase-shifting function, if repeated for multiple input antennas with fixed increments and properly weighted and combined, will result in spatial beam forming that can be applied for target search and tracking applications.
The RF unit consists of a distributed low-noise amplifier (DLNA), a CMOSbased low-power controller, analog switches to select the Rx or Tx mode, and delay-line routing or a phase-shifter unit (Fig. 4, again). The power-amplifier (PA) function is replaced with the output of a low-power DLNA, so ample signal energy is transmitted without resorting to a high-power PA.
The controller function addresses power-management based on a peak-detection mechanism, DLNA gain control for optimum and constant SNR performance, and a phase shifter (delay-line array) that's based on at least a 2-bit digitally-controlled shifter. The RF unit's communication with an external electronic unit (a baseband processor and media-accesscontroller functions) is based on a series of received pulse blocks that contain information to address.
This includes initialization, element selection in case of array-antenna-deployment (row and/or column selection) capability for addressing a tile array,3 selecting the amount of the phase shift/delay, an optional field to address LNA gain control, Tx or Rx switch selection, and power management to address sleep mode of operation.
The DLNA and integrated amplifier section can be a multistage bipolar-or CMOS-based design using less than 100 mW to provide a 10-dB gain in active mode. The phase shifter provides at least 2 bits of controllable phase shifting with a maximum variation of 10° rms. This translates to an accuracy of 0.6 ps rms at 4.15-ps resolution operating at 60 GHz. The RF switches, DLNA, phase shifter, and controller are all included in a die that fits underneath the antenna plate as an element within the whole wafer. Proper shielding and isolation are provided to eliminate electromagnetic coupling to active devices.
For data communications, stringent limitations are imposed on the unit's design. To attain a 70% eye opening (the capture window time slot) for signal detection, the maximum tolerable total jitter should be less than 1.2 ps. As a result, the total jitter limitation—including rise time and fall time on precise location of the beam, the SNR of the RF data path, and the active circuitry—should be less than 2 ps at 60 GHz.
This requirement clearly is beyond the capability of current technology for low-power operation. As a reference, the total jitter for OC-192 (10 Gbits/s) and OC-768 (40 Gbits/s) is less than 2 ps.4 This kind of limitation on a Tx/Rx millimeter-wave signal at V-band makes the design of a UWB impulse radio very attractive. A pulse-position modulation (PPM) scheme appears to be the most attractive solution.5
ANTENNA ARRAY GAIN AND BEAM WIDTH
The isolated antenna element provides a bandwidth of 7% for a 2:1 voltage standing wave ratio (VSWR) at a gain of better than 7 dB. The element is excited through a metal rod connected to a via that's filled with a deposited metal layer. That layer is connected to the output of an analog RF switch powered by a PA, or an equivalent DLNA after proper phase shifting.
Similarly, the element can deliver received radiation to the input of the RF switch feeding the DLNA and then perform phase shifting.
Figure 5 shows the simulation results for an array of antenna dipole loops built on a silicon substrate. This simulation was based on a honeycomb substrate with relative permittivity of 2.2. At 60 GHz, 32 elements per row provide a beam width of less than 2°, yielding a 1-m resolution at a 150-m far field. A 2° beam with a side lobe of less than 5 dB has been obtained for a 32-by-2 tile array (two rows of 32 antenna loops). The 32-by-2 antenna array's gain is about 21 dBi. A matrix of 32 by 32 (1024 elements) provides an antenna gain of better than 37 dBi.
With proper phase and amplitude management, the array's beam can be adjusted to address a variety of adaptive functions. These can include high-resolution scanning of far and near targets, dynamically controlled distant tracking, and power management to reduce RF interference on mass-market guidance systems, such as automotive collision-avoidance radars.
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