【信号去噪】基于平方根升余弦滤波器实现信号去噪附
matlab代码
1 简介
1.Rcosfir:design a raised cosine FIR filter.(rcosfir是低通滤波用的,以便后面加载频。)
B = RCOSFIR(R, N_T, RATE, T) designs and returns a raised cosine FIR filter.
A raised cosine filter is typically used to shape and oversample a symbol stream before
modulation/transmission as well as after reception and demodulation. It is used to reduce
the bandwidth of the over sampled symbol stream without introducing ISI.
R(滚降因子), The rolloff factor, determines the width of the transition band. R has no units.
The transition band is (1-R)/ (2*T) < |f| < (1+R)/(2*T).
T(符号持续时间,一般是采样频率的倒数) is the input signal sampling period, in seconds.
RATE(过采样率) is the number of points in each input symbol period of length T. RATE must
be greater than 1. The input sample rate is T samples per second, while the output sample
rate is T*rate samples per second.
N_T (用来控制滤波器长度的参数,此值越大抽头越多,也就越精确,滤波器的阶数等于2*N_T+1,
而阶数与功能的实现程度及复杂度有关。阶数越高,根升余弦的波形更接近于真实的模拟波形,但这样
运算量也比较大。阶数的具体大小,可根据几次取不同数值时的仿真结果来选择。) is a scalar or a
vector of length 2. If N_T is specified as a scalar, then the filter length is 2 * N_T + 1 input
samples. If N_T is a vector, it specifies the extent of the filter. In this case, the filter length
is N_T(2)-N_T(1) +1 input samples (or (N_T(2)-N_T(1))* RATE+1 output samples).
The default value for N_T is 3. The default value of RATE is 5. The default value of T is 1.
2.什么是平方根升余弦成形滤波器?为什么用此滤波器?
B = RCOSFIR(R, N_T, rate, T, sqrt),升余弦成形滤波器是常用的低通filter,如果N_T是数值不是向
量的话,length(B)=2*N_T_rate +1。前面介绍的升余弦滤波器的传输函数是整个系统的合成传输函数
H(f),即从发送端开始,经信道到接收滤波器的整个传输函数。接收端的滤波是对整个传输函数的补
偿,它使得整体传输函数成为一个能够满足奈奎斯特第一准则(实现无码间串扰)的传输函数,例如升
余弦函数。通常,把它放置在收发两端,即将接收滤波器和发送滤波器设计(匹配)为平方根升余弦函
数(升余弦函数的平方根)。若不考虑由于信道引起的码间串扰,两个平方根升余弦函数相乘(相当于
时域卷积)就得到升余弦形式的合成的系统传输函数。