Use this cell to set the length of the raised cosine window. The range is 0 to 16 * OSR samples, where 16 is the guard interval length of an OFDM symbol when OSR is 1. For example, if you set OSR to 4, the maximum value of the windowing length is 64. Entering 0 samples means no windowing will be applied. A raised cosine time domain window is applied to the baseband signal to reduce out-of-band power.
A baseband filter is applied to reduce the transmitted bandwidth, increasing spectral efficiency.
For signals generated with digital signal processing, baseband filters are often finite impulse response (FIR) filters with coefficients that represent the sampled impulse response of the desired filter. FIR filters are used to limit the bandwidth of the input to the I and Q modulators.
The standard does not specify what type of filter must be used, but the transmitted signal must meet the spectral mask requirements. Gaussian filtering with a BT of 0.5 was chosen for a measured example. This filter is commonly used in the industry for 802.11b. Other selected filters will also filter the signal enough to fit within the spectral mask.
Five options for baseband filtering can be selected in the Filter Type menu:
None- No filter.
The Gaussian filter does not have zero Inter-Symbol Interference (ISI). Wireless system architects must decide just how much of the ISI can be tolerated in a system and combine that with noise and interference. The Gaussian filter is Gaussian shaped in both the time and frequency domains, and it does not ring like the root cosine filters do. The effects of this filter in the time domain are relatively short and each symbol interacts significantly (or causes ISI) with only the preceding and succeeding symbols. This reduces the tendency for particular sequences of symbols to interact which makes amplifiers easier to build and more efficient.
Root cosine filters, also referred to as square root raised cosine filters, have the property that their impulse response rings at the symbol rate. Adjacent symbols do not interfere with each other at the symbol times because the response equals zero at all symbol times except at the center (desired) symbol time. Root cosine filters heavily filter the signal without blurring the symbols together at the symbol times. This is important for transmitting information without errors caused by ISI. Note that ISI does exist at all times except the symbol (decision) times.
In the frequency domain, this filter appears as a low-pass, rectangular filter with very steep cut-off characteristics. Due to a finite number of coefficients, the filter has a predefined length and is not truly "ideal." The resulting ripple in the cut-off band is effectively minimized with a Hamming window. This filter is recommended for achieving optimal ACP in 802.11a signals. A symbol length of 32 or greater is recommended for this filter.
Allows you to select a simple unformatted text file (*.txt) of coefficient values, characterizing a user-defined filter. Each line in the file contains one coefficient value. The number of coefficients listed must be a multiple of the selected oversampling ratio. Each coefficient applies to both I and Q components.
The symbol length of the filter determines how many symbol periods will be used in calculating the symbol. The filter selection influences the symbol length value.
The Gaussian filter has a rapidly decaying impulse response. A symbol length of 6 is recommended. Greater lengths have negligible effects on the accuracy of the signal.
The root cosine filter has a slowly decaying impulse response. It is recommended that a long symbol length, around 32, be used. Beyond this, the ringing has negligible effects on the accuracy of the signal.
The ideal low pass filter also has a very slow decaying impulse response. It is recommended that a long symbol length, 32 or greater, be used.
For both root cosine and ideal low pass filters, the greater the symbol length, the greater the accuracy of the signal. Try changing the symbol length and plotting the spectrum to view the effect of the symbol length on the spectrum.
This cell sets the filter's bandwidth-time product (BT) coefficient. It is valid only for a Gaussian filter.
B is the 3 dB bandwidth of the filter and T is the duration of the symbol period. BT determines the extent of the filtering of the signal. Occupied bandwidth cannot be stated in terms of BT because a Gaussian filter’s frequency response does not go to zero, as does a root cosine filter. The range of this cell is 0.1 to 1. The default value is 0.5. As the BT product is decreased, the ISI increases.
This cell sets the filter's alpha coefficient. It is valid only for root cosine filters.
The sharpness of a root cosine filter is described by the filter coefficient, which is called alpha. Alpha gives a direct measure of the occupied bandwidth of the system and is calculated as occupied bandwidth = symbol rate X (1 + alpha). If the filter had a perfect (brick wall) characteristic with sharp transitions and an alpha of zero, the occupied bandwidth would be = symbol rate X (1 +0) = symbol rate. An alpha of zero is impossible to implement. Alpha is sometimes called the "excess bandwidth factor" as it indicates the amount of occupied bandwidth that will be required in excess of the ideal occupied bandwidth (which would be the same as the symbol rate).
At the other extreme, take a broader filter with an alpha of one, which is easier to implement. The occupied bandwidth for alpha = 1 will be: occupied bandwidth = symbol rate X (1 + 1) = 2 X symbol rate. An alpha of one uses twice as much bandwidth as an alpha of zero. In practice, it is possible to implement an alpha below 0.2 and make good, compact, practical radios. Typical values range from 0.35 to 0.5, though some video systems use an alpha as low as 0.11.
This cell sets the filter's bandwidth. It is valid only for ideal lowpass filters.
It is valid only for user-defined filters.
Click the 
button in this cell to select a simple unformatted
text file (*.txt) of coefficient values, characterizing a user-defined
filter. Each line in the file contains one coefficient value. The number
of coefficients listed must be a multiple of the selected oversampling
ratio. You can also edit the filter's coefficient in the coefficient table
below this cell.