Open topic with navigation

WLAN

802.11ac Waveform Settings

These waveform settings are applicable when you select 802.11ac as the PHY Specification.

MIMO

Basic

PHY Specification

Capability

Comment

Generation Mode

Frame Type

Idle Interval

Head Idle Interval

Bandwidth

Combined Waveform

Freq Segment Spacing

Number of Frames

Total Sample Points

Number of Data Symbols in One Frame

RF Burst Duration in One Frame

Overall Waveform Duration in One Frame

Spectrum Control

Oversampling Ratio

Downclocking Ratio

User Defined Downclocking Ratio

Mirror Spectrum

Windowing Length

FilterType

BT

Alpha

Bandwidth

Length(symbol)

Filter Coefficient

Marker

Marker1 Source

Marker2 Source

Marker3 Source

Marker4 Source

Routing

MIMO

Enables rapid configuration of MIMO using Mx1, Mx2, Mx4, or Mx8 within the VHT PPDU format. When applied for MIMO configuration, all WLAN 802.11AC settings are internally restored to their default values. Subsequently, Spatial Mapping Schema is set to Spatial Expansion and the Number of Transmit Chains and the Number of Spatial Streams (Nss,u) of User 0 are automatically set in accordance with the selected Mx configuration. For instance, selecting Mx2 results in both the Number of Transmit Chains and Nss,u being configured to 2.

Basic

PHY Specification

Refer to Waveform > PHY Specification section.

Capability

Set the capability for WLAN 802.11ac to either Basic or Advanced.

• Basic capability supports configuration of all WLAN 802.11ac signal parameters except the channel coding function. This makes it suitable for component testing.

  Parameters under Basic capability are a subset of the Advanced capability.

• Advanced capability supports generation of fully channel-coded waveforms and is suitable for both receiver testing and component testing.

  The following parameters are supported only for the Advanced capability.
  - Channel Coding State in User Setup > User Configuration tab
  - BBC Interleaving and LDPC Tone Mapper in User Setup > User Configuration tab
  - MAC layer function in Payload tab
  - Beacon option for Frame Type (R&D to confirm)
  - MPDU in User Setup for VHT PPDU Format or VHT NDP Format
  - Multi User in Transmission Mode
  - Partial AID setting in for VHT PPDU Format or VHT NDP Format
  - Coding Rate and Data Rate in User Setup
  When Capability is set to Basic:
  - Scrambler is fixed to ON. This can be set to ON or OFF in Advanced capability.
  - Channel Coding State is fixed to OFF. This can be set to ON or OFF in Advanced capability.
  - LDPC Tone Mapper in User Setup is fixed to OFF. This can be set to ON or OFF in Advanced capability.

Comment

Enter an alpha-numeric comment of up to 32 characters. The comment resides in the file header and can include spaces and special characters.

Generation Mode

Select the type of frame to be generated.

• VHT PPDU format is the standard WLAN802.11ac format. It is constructed with legacy preamble, VHT preamble and VHT data portion.

• VHT NDP format is same as the VHT PPDU format without VHT data portion.

• Non-HT refers to the one defined by the standard as Non-HT Duplicate Transmission, which repeats the 802.11a signal in each 20 MHz segment.

Frame Type

Select the frame type.

When you select Beacon, the tab Beacon Frame is displayed in User Setup, giving you access to additional parameters for configuring the Beacon frame type.

Idle Interval

Set the length of the idle time between frames.

No signal is transmitted during the idle interval, but the MAC layer operates as if a signal is being transmitted.

Head Idle Interval

Set the idle interval ahead of frames, in unit of seconds.

Bandwidth

Set the occupied bandwidth for 802.11ac to 20 MHz, 40 MHz, 80 MHz, 160 MHz, or 80+80 MHz.

Combined Waveform

Choose whether to use combined or separate waveforms for the two 80-MHz frequency segments.

Freq Segment Spacing

Set the frequency spacing between two segments of 80+80 for the combined waveform.

Number of Frames

Set the number of frames.

Total Sample Points

Query only.

Displays the number of samples (or data points) in the waveform, which depends on the Bandwidth.

The number of sample points varies with the Oversampling Ratio and is related to the number of packets, the frame mode, the data rate, and the length of the user data. The maximum number of samples a waveform can have depends on the ARB memory capacity of the signal generator's baseband generator. You cannot edit this setting.

Number of Data Symbols in One Frame

Query only.

Displays the number of OFDM symbols in the data portion of one frame.

You cannot edit this setting.

RF Burst Duration in One Frame

Query only.

Displays the time duration (in seconds) of the burst in one frame. The burst duration is equal to the preamble portion plus the data portion.

You cannot edit this setting.

Overall Waveform Duration in One Frame

Query only.

Displays the time duration (in seconds) of the overall waveform in one frame. The overall waveform duration is equal to the RF burst duration plus the idle interval.

You cannot edit this setting.

Spectrum Control

Oversampling Ratio

Specify the number of times that the baseband signal is oversampled.

Downclocking Ratio

Specify a downclocking ratio.

The downclocking ratio reduces the bandwidth of the signal such that it can operate in the TVWS (TV White Space) spectrum. The TVWS spectrum provides a longer range and better indoor penetration for signal propagation at lower frequencies.

The following formula shows the relationship between downclocking ratio, oversampling ratio, bandwidth, and sample clock:

ARB Sample Clock = Bandwidth X Oversampling Ratio / Downclocking Ratio

User Defined Downclocking Ratio

Specify a downclocking ratio.

Mirror Spectrum

Reverse the spectrum of the waveform. This is useful for systems with external up conversion where the signal spectrum is mirrored by the up conversion process.

• On: The Q channel is inverted, resulting in a mirrored spectrum.

• Off: The spectrum is not inverted.

Windowing Length

Set the duration of the transition time (Ttr) in the windowing function. Ttr creates a small overlap between consecutive subsections to smooth the transitions between them. Smoothing the transition is required to reduce the spectral sidelobes of the transmitted waveform.

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.

FilterType

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.

Five options for baseband filtering can be selected in the Filter Type menu:

• None: No filter.

• Gaussian: 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 Raised Cosine: Root raised cosine, 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 the center (desired) one. 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 not exist at all times, only at the symbol (decision) times.

  You can change the values of the Root Raised Cosine Filter to optimize for better spectral characteristics (sharper roll-off on the band edges resulting in less adjacent channel power) vs. better EVM.

  However, to reduce the value of Alpha will make the roll-off sharper but cause worse EVM.

• Ideal Lowpass: In the frequency domain, this filter appears as a low-pass, rectangular filter with very steep cut-off characteristics. The pass band is set to equal the symbol rate of the signal. 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. A symbol length of 32 or greater is recommended for this filter.

• User Defined: 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.

BT

Set the filter's bandwidth-time product (BT) coefficient.

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. Common values for BT are 0.3 to 0.5. As the BT product is decreased, the ISI increases.  

Alpha

Set the filter's alpha coefficient.

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.

Bandwidth

Set the effective bandwidth for the ideal low pass filter.

Length(symbol)

The symbol length of the filter determines how many symbol periods will be used in the calculation of 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.

Filter Coefficient

When you select User Defined as the Filter Type, click the > button displayed with this field 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.

Marker

Marker1 Source

Frame Start - It indicates the beginning of each frame. It starts at the beginning of the Head Idle Interval.

Marker2 Source

RF Blanking - It controls On/Off of the RF signal. There is a 500 ns pre-blanking before the Preamble part and a 335 ns latency after the Data part for Marker2.

Marker3 Source

Frames - It indicates the period of each frame. The Head Idle Interval is included in the frame, and the Idle Interval is excluded.

Marker4 Source

Preamble Blanking - It indicates the Preamble part of each frame.

Routing

Refer to Waveform > Routing Settings.