Field experiments on closed loop mode transmit diversity in W-CDMA forward link

This paper experimentally investigated the bit error rate (BER) performance using closed loop (CL) mode transmit diversity, which was discussed in ongoing standardization of the third generation partnership project (3GPP) for the application of dedicated physical channels (DPCHs) in the forward link of wideband direct sequence code division multiple access (W-CDMA). We evaluated three CL modes using weighting factors W_1,n = A_1,ne^jφ1,n and W_2,n = A_1,ne^jφ2,n that are calculated according to feedback information (FBI) bits sent via the reverse link: Mode 1: φ_1,n = 0, φ_2,n = {±π/4,±3π/4}, A_1,n = A_2,n = √1/2, Mode 2: φ_1,n = 0, φ_2,n = {0,π}, A_1,n = A_2,n = √1/2, Mode 3: φ_1,n = φ_2,n = 0, (A₁, A_2,n) = {(1,0),(0,1)}. The laboratory experimental results clarified that the received timing difference of transmitted signals from two antennas should be smaller than approximately 1/4 chip duration in order to limit the loss of transmit power to within 0.3 dB. It was also clarified that the total required transmit power for achieving the average BER of 10^-3 was increased as the fading correlation between two antennas becomes larger due to the decreased diversity effect in a slow fading environment such as when the maximum Doppler frequency f_D = 5 Hz. However, it was increased as the fading correlation becomes small, in a fast fading environment above approximately f_D = 160 Hz because the combined signal power was decreased since the phase difference of the two received signals increased due to the tracking inability of the CL control. The field experimental results showed that when FBI bit errors occur in a fast transmit power-controlled channel in the reverse link, the total required transmit power of a DPCH without antenna diversity, reception at a mobile station was decreased by approximately 1-2 dB compared to the case without transmit diversity, while this improvement decreased to approximately 0.5 dB with antenna diversity reception.