# Multifunction RF Systems for Naval Platforms
## 3 MFRF System Design Considerations
### Ideal MFRF System Architecture
In an ideal architecture, each Tx channel has its own digital waveform generator (DWG), which can also access a DRFM to support coherent jamming for the EA function. A DWG performs the following steps: receive arbitrary waveform parameters from the signal/data processor of the MFRF system; digitally generate samples of the desired waveform with a _di_rect _di_gital synthesizer (DDS), or extract waveform samples from a DRFM; use a _di_gital-to-_a_n_alog converter (DAC) to convert the samples to the analog domain; and finally, translate the waveform to the required frequency band with analog modulation circuitry. The presence of a DWG in each Tx channel provides the ultimate flexibility when using the array for transmission, in the sense that from one point in time to the next, the array can be instantly repartitioned through software control into Tx subarrays of arbitrary size, with each subarray forming an independent Tx beam for a RF function. This high level of array reconfiguration capability facilitates optimal MFRF system performance under normal circumstances, and graceful performance degradation if parts of the array suffer failure. A second key advantage of including a DWG in each Tx channel is that TTD beamforming can be readily achieved by digitally introducing a time delay between identical waveforms generated for adjacent radiating elements in a subarray. This approach ensures that Tx beams employing wideband waveforms can enjoy maximum array gain when steered off boresight, without the need for analog TTD circuits elsewhere in the Tx channels. TTD beamforming is necessary primarily for the EA function, since the instantaneous bandwidth of waveforms transmitted by this function may be as high as 1 GHz. Finally, a DWG in each channel also provides a straightforward means to weight the waveform amplitude across the subarray elements for sidelobe control or adaptive placement of nulls in the Tx beam. The digital receiver in each Rx channel contains an ADC that digitizes the signal received from the radiating element. This element-level digitization permits receive beams to be formed entirely in the digital domain, that is, without the need for RF phase shifters or TTD circuits elsewhere in the Rx chain. This scheme has three key benefits. First, many simultaneous receive beams can be formed in different directions using the same set of array elements, thereby providing instantaneous coverage of a large volume. The maximum number of these independent simultaneous beams is limited only by available signal processing power. This is a particularly useful capability for the ES and radar functions which must detect and localize threats as quickly as possible within a large surveillance area. Secondly, TTD formation of receive beams can be easily realized in software by introducing time shifts between digitized signals from adjacent array elements before coherent summation across the elements. TTD beamforming on receive is important for the ES function, which must be capable of detecting and characterizing signals of instantaneous bandwidths up to 1 GHz. For such signals, conventional beamforming with phase shifters would result in reduced array gain at scanning angles off boresight, compared to TTD beamforming which provides the highest achievable array gain at all scan angles for signals of any bandwidth. Consequently, TTD beamforming optimizes detection for a MFRF system. Finally, element-level digitization, similar to the benefit provided by a DWG in each Tx channel, allows the maximum flexibility in allocation of array resources to receive operations associated with the different RF functions. In this ideal case, the element-level digitization is done directly at RF.