Signals and Communication Technology For further volumes
tải về 7.55 Mb. Chế độ xem pdf
|
| -15
-10 -5 0 15 dB fading channel 5 dB fading channel carriers -26 and 26 -10 -8 -6 -4 -2 0 2 4 6 8 10 (MHz) f || Hf () (dB) Figure 5.5: Sample channels used in simulations systems is a channel inversion at the sender. Carriers which are subject to high attenuation are amplified at the transmitter side, thus yielding a flat spectrum at the receiver input. If attenuation is high, a high amount of power is used for weak carriers. An extension of this approach is the option to completely leave out a certain number of carriers (in our simulation six of 48) and use the saved power to boost the remaining carriers. As the transmission is using forward error correction, this appears to be a specific kind of puncturing. Curves C and D give the simulation results for the options channel inversion at transmitter and channel inversion with suppressing the six weakest carriers at transmit- ter, respectively. It turns out that this approach is worse than other options, because even in the case of carrier suppression, with the given transmission functions much energy is wasted for transmitting information in inefficient parts of the frequency band. The information theoretic point of view shows us that this naive adaptation approach has to be suboptimal. Therefore, process- ing as simulated in curve B is the best choice among these options, as can be clearly seen in the figures. However, careful evaluation of the characteristics for different channel characteristics has shown that the interleaver specified in IEEE 802.11 is not perfectly suited to the application scenario discussed here. As the interleaver was selected to match primarily the characteristics of office environments, it can be further optimized for the home scenario. Thus, a modified interleaver has been developed [8]. Curve E shows the simulation results when assuming perfect soft decision as in curve B in combination with the improved interleaver. The performance comparison yields similar results for other channel profiles and transmission parameters (modulation and convolutional encoder). In gen- eral, the difference between the modes A - E increases with fading depth and data rate. Regarding channel profiles with deep fades, this modification yields a gain of more than 2 dB in contrast to the wide spread simplified solution of a serial connection of equalizer and Viterbi decoder with 3 bit quantization. However, the gain which can generally be achieved by these link adaptation methods has to be seen in combination with other methods. If dynamic chan- |