Phase-shifting techniques for wireless multiple-antenna transmitter applications

The wireless communications industry is experiencing explosive growth owing to the emergence of sub-micron CMOS technology in commercial analog and radio frequency integrated circuits. As the demand for data rates continue to increase, the multiple-antenna transceiver (MIMO) has become a technology of importance. By deploying antenna arrays at both the receiving and the transmitting ends, major impairments caused by the wireless channel can be mitigated, and signal-to-noise ratio and system capacity can be significantly increased.; One flavor of MIMO is the beamformer, which can optimize certain performance parameters (e.g., receiver SNR) to increase overall network capacity by dynamically adjusting receiver and/or transmitter array patterns. At the receiver, the beamformer gain in the direction of the desired transmitted signal (i.e., the look angle) is usually maximized while nulls are placed in the directions of strong interfering signals. At the transmitter, the beam-pattern of each antenna is dynamically adapted to maximize the overall gain in the direction of the desired receiver and to minimize interference at other receivers. Thus, a major advantage of a multiple-antenna transmitter is the reduction of multipath and interference effects without an attendant increase in the overall transmitted power compared to a single-antenna transmitter.; The capability of independently varying the phase of each transmit or receive path plays a key role in beamforming systems. Most previous RF- and LO-only phase shift implementations suffer from limited tuning range and a tradeoff between phase selector coarse granularity and output phase resolution, respectively. In this work, two novel techniques for beamforming transmitters that alleviate the aforementioned problems are presented.; The first approach achieves a 360° phase shift range by coarse-tuning using the LO and fine-tuning at RF. A 5.2GHz CMOS RF phase shifter based on a tunable all-pass filter is designed and fabricated. It achieves more than twice the phase shift range of previously published single-stage CMOS phase shifters, and offers competitive performance compared to its GaAs counterparts. Embedding the phase shifter in the signal path of a wireless beamforming transmitter and employing both RF and LO shifting provides a full 360° phase shift range. The second approach achieves a full phase shift range by Cartesian combining, with the phase-shifting and upconversion functions merged. A 5GHz phase-shifting modulator is fabricated in 0.18mum CMOS. Both techniques show promising potential for deployment in wireless beamforming transmitter applications due to their capabilities to achieve a 360° phase shift range with few limitations or drawbacks.