- Joint Channel Estimation in Service Area Based OFDM Air Interfaces for Beyond 3G Mobile Radio Systems (2005)
- In the thesis the task of channel estimation in beyond 3G service area based mobile radio air interfaces is considered. A system concept named Joint Transmission and Detection Integrated Network (JOINT) forms the target platform for the investigations. A single service area of JOINT is considered, in which a number of mobile terminals is supported by a number of radio access points, which are connected to a central unit responsible for the signal processing. The modulation scheme of JOINT is OFDM. Pilot-aided channel estimation is considered, which has to be performed only in the uplink of JOINT, because the duplexing scheme TDD is applied. In this way, the complexity of the mobile terminals is reduced, because they do not need a channel estimator. Based on the signals received by the access points, the central unit estimates the channel transfer functions jointly for all mobile terminals. This is done by resorting to the a priori knowledge of the radiated pilot signals and by applying the technique of joint channel estimation, which is developed in the thesis. The quality of the gained estimates is judged by the degradation of their signal-to-noise ratio as compared to the signal-to-noise ratio of the respective estimates gained in the case of a single mobile terminal radiating its pilots. In the case of single-element receive antennas at the access points, said degradation depends solely on the structure of the applied pilots. In the thesis it is shown how by a proper design of the pilots the SNR degradation can be minimized. Besides using appropriate pilots, the performance of joint channel estimation can be further improved by the inclusion of additional a-priori information in the estimation process. An example of such additional information would be the knowledge of the directional properties of the radio channels. This knowledge can be gained if multi-element antennas are applied at the access points. Further, a-priori channel state information in the form of the power delay profiles of the radio channels can be included in the estimation process by the application of the minimum mean square error estimation principle for joint channel estimation. After having intensively studied the problem of joint channel estimation in JOINT, the thesis rounds itself by considering the impact of the unavoidable channel estimation errors on the performance of data estimation in JOINT. For the case of small channel estimation errors occurring due to the presence of noise at the access points, the performance of joint detection in the uplink and of joint transmission in the downlink of JOINT are investigated based on simulations. For the uplink, which utilizes joint detection, it is shown to which degree the bit error probability increases due to channel estimation errors. For the downlink, which utilizes joint transmission, channel estimation errors lead to an increase of the required transmit power, which can be quantified by the simulation results.