AN EXPERIMENTAL EVALUATION OF THE PERFORMANCE OF 2-BRANCH SPACE AND POLARIZATION DIVERSITY SCHEMES AT 1800 MHZ

This paper presents the results of an experimental evaluation of the mean signal level and envelope cross-correlation of 1800 MHz base station signals received in two-branch spatial and polarization diversity schemes, Measurements have been conducted with the experimental base site located in i) two urban sites with different degrees of urbanization, ii) a residential area, iii) a rural area, and iv) near a motorway. In each of these locations, the effect of the random orientation of a typical mobile radio telephone handset has been studied by examining the characteristics of signals received from a mobile collinear antenna inclined at angles of 0 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees to the vertical. Furthermore, the diversity gain at 90% signal reliability has been evaluated for each diversity scheme by simulating selection, equal-gain and maximal-ratio combining techniques using the recorded signals as inputs. Results have shown that 20 lambda separation in the horizontal plane or 15 lambda in the vertical plane is sufficient to obtain a cross-correlation of less than 0.7 for most of the time at 1800 MHz. Similar cross-correlation results were obtained for polarization diversity. When the antenna is inclined at 45 degrees, a 6 dB degradation in signal level was recorded for space diversity schemes. However, the diversity gain is unaffected by tilt and remains unchanged at 5-6 dB for horizontal and 3.5-4.5 dB for vertical separation. For polarization diversity, only a little degradation is experienced because most of the energy lost on the vertical branch is recovered on the horizontal branch. The diversity gain is between 1-2 dB at 0 degrees tilt and increases to 3-5.2 dB at 45 degrees inclination. This suggests that the performance of polarization diversity will be comparable to that of space diversity when the transmitting antenna is inclined to the vertical. Empirical equations relating diversity gain to mean signal level differences and cross-correlation coefficients are also presented.