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Nhóm tiếp theo gồm có (chủ yếu) của các yếu tố quy mô. Lỗi sẽ gây ra âm thanh khó chịu (cái gọi là chim non trông thật), nhưng chúng có thể được che giấu đến một điểm nhất định về mức độ âm thanh nếu được phát hiện bởi một cơ chế thích hợp. | CDMA 289 a Pedestrian v 1 m s SNR no PC 3.5 10 co p. 0 Vehicular v 10 m s Mpwr SNR pc SNR no PC 4 -10. 3 -10 . . . . 1.1 1.15 1.2 SNR dB SNR dB Figure 5.20 Illustration of the dynamics of fast UL PC and its effect to fading compensation. Hence while a slow power control algorithm removes the near-far problem and reduces the local mean intercell interference a fast power control algorithm additionally may cancel the effect of the fast fading to a certain degree depending on the velocity of the MS. This results in a gain with respect to the required Eb N0 since the channel to be considered is approximately a pure Gaussian instead of a fading one. Simulation results reported in Holma and Toskala 2001 show that for a pedestrian UMTS subscriber in a low delay spread environment the Eb N0 requirement for a speech-like service can be reduced by fast power control from 11.3 dB to 5.5 dB. On the other hand an interference rise of about 2 dB is caused reducing the effective gain from about 6 dB to about 4 dB. For a vehicular subscriber with a velocity of about 50 km h and above there is no additional gain by fast power control however interleaving becomes efficient reducing the effect of fast fading. Summing up the discussion for PC one obtains the following main results UL PC removes the near-far problem caused by intracell interference and leads to a significantly improved probability distribution of the local mean SIR as illustrated in the example of Figure 5.17. Compensating the near-far effect by PC reduces not only the intracell but also the intercell interference an MS at a reduced power causes less interference in other cells. This effect is included in Figure 5.17. 290 CDMA Fast UL PC can cancel fast fading nearly completely for pedestrian subscribers. Depending on the environment this results in a gain of some decibels with respect to the required Eb N0 since the channel to be considered is approximately a pure Gaussian instead of a fading one. At higher velocities
