How do I design the RF front end for a passive RFID reader at UHF frequencies?

Designing the RF front end for a passive RFID reader at UHF frequencies (860-960 MHz, the EPC Gen2/ISO 18000-63 band) creates the transmitter and receiver that communicates with passive RFID tags through backscatter modulation. The RF front end consists of: a transmitter (generates the continuous-wave RF carrier that powers the passive tags and carries the reader's command data; the transmitter modulates the carrier with reader commands using ASK or PR-ASK (phase-reversal ASK) modulation; output power: +20 to +36 dBm (100 mW to 4 W) depending on the regional regulation (FCC: 36 dBm EIRP, ETSI: 33 dBm ERP); the PA must be highly linear to minimize spectral regrowth into adjacent channels), a circulator/coupler for TX-RX isolation (since the reader transmits and receives on the same frequency simultaneously (full-duplex), TX-to-RX isolation is critical; a circulator provides 20-30 dB isolation; additional techniques: active TX leakage cancellation (30-50 dB additional isolation), and careful antenna return loss (better than 20 dB) to minimize reflected TX power into the RX), a receiver (detects the very weak backscattered signal from the tag while the transmitter is active; the tag's backscattered signal is approximately -60 to -80 dBm (60-80 dB below the TX power); the receiver must extract this signal from the TX leakage and noise; receiver architecture: direct conversion (homodyne): mixes the received signal with the TX frequency to demodulate the tag's baseband signal; works well but: susceptible to DC offset and flicker noise. Superheterodyne: down-converts to an IF; better dynamic range but more complex), and the antenna (a circularly polarized patch antenna or array; CP is used because the tag's orientation is unknown; gain: 6-9 dBic for a single patch; 12+ dBic for a 4-element array).