# Reconfigurable Multi-Beam Receiver Based on Microwave Photonics Technology
## 2 System Architecture  
The conventional multi-beam receiver based on the true-time delay line is described in Figure 1. The signal received by every antenna element (AE) is modulated on the optical carrier with different and independent wavelengths; the wavelength spacings are followed with ITU G.692 standard [17] to avoid interference. The optical carrier with the signal is next sent to the splitter and divided into multiple duplications with an equal number to the number of the system-supporting received beams. After splitting, each duplication passes the individual optical delay line (ODL) and is enabled with delay weighting. The multi-wavelength signals are sent to the WDM multiplexer to couple together as a group, each containing different wavelengths. Finally, the photoelectric conversion is completed at the photonic detector, and the signal in the electronic domain is obtained. The ODL in the structure was demonstrated earlier in [6]. The true-time delay value offered by the ODL is independent of the optical carrier's wavelength and has a long range. The number of the received beam is fixed. Let us assume that the numbers of the AEs and the received beam in the analyzed system are \(N\) and \(M\), respectively. In the conventional architecture, the signal received by one AE is passively split into \(M\) parts, each corresponding to one beam. When the system receives only one beam, the optical power in the remaining unused ports will be wasted. Meanwhile, the energy in the current used port remains at a low level. To surmount the above demerit, we propose introducing the switch before the splitter part to realize a reconfigurable multi-beamformer, as shown in Figure 2.  
Two reconfigurable parts of the switch are added to the conventional multi-beam structure. The splitter is expanded, and each one has different output ports. But this new structure still has the same number of lasers, modulators, ODL, MUX, and PD. The function of the switch before the splitter is to choose the suitable splitter and reconfigure the system's working mode; we named it the mode-switch. The proposed structure only supports the working mode when the number of the beam is any exponential power multiple of 2, and the working mode can be denoted with the sequence number \(m\), \(m=1\), 2, 4,..., 2\({}^{n}\),..., \(M\), \(n\)\(\in\)\(Z\). When \(m=1\), the system works on a single-beam mode; similarly, when \(m=2\), the system works on dual-beam mode, and so on and so forth. The switch was also applied before true-time delay units instead of the combiner to decrease the insertion loss, named the combiner-switch. The precedence of beam ports is allotted in descending numerical order in Figure 2. Output 1 works for the single-beam mode, dual-beam mode, and up until the \(M\)-beam mode, while the total number of the mode is \(\log_{2}(M)+1\). Output 2 works for the dual-beam mode to the \(M\)-beam mode. The number of modes supported in output 2 is less 1 than output 1. As for the \(n\)-th output, when \(  
output \(n\) only works for the \(M\)-beam mode. Inside, \(\left\lfloor\cdot\right\rfloor\) denotes a round-up to an integer. For convenience, as described in the following, the structure in Figure 2 is named the structure descending-precedence (structure DP). The asymmetrical switch will occur in the structure DP when we build the odd-ports multi-stage switch in the fundamental block (\(1\times 2\) switch). The asymmetrical switch may cause inconsistency in amplitude and phase. The optimized structure, which is designed to keep the consistency, is presented in Figure 3. In the optimal structure, output \(1\) is used for the single-beam mode or the \(M\)-beam receive mode, and outputs \(2\) and \(3\) are used for the dual-beam or \(M\)-beam receive mode. Thus, any output \(n\), except for the \(M\)-th port, only works for two modes. One is the \(M\)-beam mode, and the other is the \(2^{\text{[log2($n$)]}}\) mode. \(\left\lceil\cdot\right\rceil\) denotes the round-down to an integer.