The Fast Beam Condition Monitor as standalone luminometer of the CMS experiment at the HL-LHC

For the Phase-2 CMS upgrade for the High-Luminosity LHC, a luminosity uncertainty of 1% is targeted. To achieve this goal, measurements from multiple luminometers with orthogonal systematics are required. A standalone luminometer, the Fast Beam Condition Monitor (FBCM) is being designed for the online bunch-by-bunch luminosity measurement. Its fast timing properties also enable the measurement of beam-induced backgrounds. In this paper, the hardware architecture and the readout protocol of the FBCM is described. The expected performance with a simple behavioral model of the front-end comprising a constant fraction discriminator is discussed, although the final implementation in the ASIC is still under discussion. Simulation results show that the FBCM will provide the required statistical uncertainty and deviation from linearity by employing 336 silicon-pad sensors, each with an area of about 3 mm$^{2}$. In addition, the front-end with sensitivity to at least 6000 electrons will satisfy the longevity constraints for an exposure of 1 MeV neutron equivalent fluence of $3.5\times10^{15}$ per cm$^{2}$.