来自：Matlab文档
WARNING: rcosflt is not recommended. Use RCOSDESIGN instead.
Y = rcosflt(X, Fd, Fs, TYPE_FLAG, R, DELAY)
Fd (Hz)：X的输入频率
Fs (Hz)：Y的输出频率【Fs必须是Fd.整数倍】
TYPE_FLAG指定滤波器的选型或者设计
R ：roll-off factor 指定滤波器的过渡频带
DELAY ：从滤波器的开始到脉冲响应峰值的时间延迟
R：the roll-off factor specifies the excess bandwidth of the filter. R must be in the range [0, 1]. For example, R = .5 means that the bandwidth of the filter is 1.5 times the input sampling frequency, Fd. This also means that the transition band of the filter extends from .5 * Fd and 1.5 * Fd. Since R is normalized to the input sampling frequency, Fd, it has no units. Typical values for R are between 0.2 to 0.5.
DELAY ：determines the group delay of the filter. The group delay is the opposite of the change in filter phase with respect to frequency. For linear phase filters, the group delay is also the time delay between the input signal and the peak response of the filter. DELAY also determines the length of the filter impulse response used to filter X. 延时计算：Fs/Fd * 2 * DELAY + 1.
Y ：is the output of the upsampled, filtered input stream X. Y的长度：Fs/Fd * (length(X) + 2 * DELAY).
TYPE_FLAG： is a string which may contain any of the option strings listed below delimited by a '/' For example, the 'iir' and 'Fs' flags may be combined as 'iir/Fs'. While some of the pairs of option substrings are mutually exclusive, they are not mutually exclusive in general.
'fir' Design an FIR filter and use it to filter X. When the 'filter' TYPE_FLAG is not used, an FIR filter is designed and used to filter X. See the 'filter' TYPE_FLAG description for the behavior when the 'fir' and 'filter' TYPE_FLAGs are used together. This option is exclusive of the 'iir' substring
'iir' Design an IIR filter and use it to filter X. When the 'filter' TYPE_FLAG is not used, an IIR approximation to the equivalent FIR filter is designed and used to filter X. See the 'filter' TYPE_FLAG description for the behavior when the 'iir' and 'filter' TYPE_FLAGs are used together. This option is exclusive of the 'fir' substring.
'normal' Design a normal raised cosine filter and use it to filter X. The filter coefficients are normalized so the peak coefficient is one. This option is exclusive of the 'sqrt' substring.
'sqrt' Design a square root raised cosine filter and use it to filter X. The filter coefficients are normalized so that the impulse response of this filter when convolved with itself will result in an impulse response that is approximately equal to the 'normal' raised cosine filter. The difference in this approximation is due to finite filter length. This is a useful option when the raised cosine filtering is split between transmitter and receiver by using the 'sqrt' filter in each device. This option is exclusive of the 'normal' substring.
'Fs' X is input with sample frequency Fs (i.e., the input signal has Fs/Fd samples per symbol). In this case the input signal is not upsampled from Fd to Fs but is simply filtered by the raised cosine filter. This is useful for filtering an oversampled data stream at the receiver. When using the 'Fs' substring, the length of vector, Y is length(X) + Fs/Fd * 2 * DELAY.
'filter' Means the filter is provided by the user. When using the 'filter' TYPE_FLAG, the input parameters are:
Y = rcosflt(X, Fd, Fs, TYPE_FLAG, NUM)：filters with a user- supplied FIR filter. When the TYPE_FLAG contains 'filter' and the 'fir' type substrings, the FIR filter indicated by NUM is used to filter X.
Y = rcosflt(X, Fd, Fs, TYPE_FLAG, NUM, DEN, DELAY)：filters with a user-supplied IIR filter. When TYPE_FLAG contains both 'filter' and 'iir' type substrings, the IIR filter defined by numerator, NUM, and denominator, DEN, is used as to filter X. The DELAY parameter is used to force rcosflt to behave as if the filter were designed by rcosflt using the same DELAY parameter. The DELAY parameter should match the DELAY parameter used to design the filter defined by NUM and DEN in the RCOSINE function. The default value of DELAY is 3. The raised cosine filter should be designed using the RCOSINE function.
Y = rcosflt(X, Fd, Fs, TYPE_FLAG, R) ：filters the input signal X using a raised cosine filter and default DELAY parameter, 3.
Y = rcosflt(X, Fd, Fs, TYPE_FLAG) ：filters the input signal X using a raised cosine filter and the following default parameters
DELAY = 3
R = .5
Y = rcosflt(X, Fd, Fs)： filters the input signal X using a raised cosine filter and the following default parameters
DELAY = 3
R = .5
TYPE_FLAG = 'fir/normal'
Y = rcosflt(X, Fd, Fs, TYPE_FLAG, R, DELAY, TOL) ：specifies the tolerance in IIR filter design. The default value for TOL is .01.
[Y, T] = rcosflt(...) returns the time vector in T.