802.11ac Waveform Generation with MAC Frames
Download igadget for mac. This example shows how to generate an IEEE® 802.11ac™ transmission containing MAC frames suitable for performing radio packet error rate (PER) receiver tests.
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Introduction
WLAN Toolbox™ can be used to generate standard compliant waveforms for performing receiver tests. A basic WLAN receiver test scenario is shown in the diagram below.
The device under test (DUT) is stimulated with RF test vectors, usually through a wired link. The packet error rate (PER) is a metric used to test the performance of a receiver at a given receive signal power in the presence of noise, interference, or other impairments. The PER is defined as the number of incorrectly decoded packets divided by the total number of transmitted packets.
The frame check sequence (FCS) within a MAC frame is used to determine whether a MAC frame has been decoded correctly by the receiver, and therefore whether the packet has been received in error. The general MAC frame for IEEE 802.11ac contains the following fields:
- MAC header
- Frame body Download windows for mac.
- FCS
The data to transmit from a higher layer is contained within the frame body of the MAC frame. The transmitter uses a cyclic redundancy check over the MAC header and frame body field to generate the FCS value. The receiver calculates the CRC and compares this to the received FCS field to determine if an error has occurred during transmission.
In this example an IEEE 802.11ac waveform consisting of multiple VHT format packets is generated. The function wlanWaveformGenerator can be used to generate a waveform containing one or more packets. wlanWaveformGenerator consumes physical layer service data units (PSDUs) for each packet and performs the appropriate physical layer processing to create the waveform. A PSDU containing a MAC header and valid FCS can be generated using the function wlanMACFrame. In this example a multi-packet baseband waveform containing MAC packets is synthesized. This waveform may be downloaded to a signal generator for RF transmission and used for receiver PER testing. Source code is provided to download and play the waveform using a Keysight Technologies™ N5172B signal generator. The example processing is illustrated in the following diagram:
IEEE 802.11ac VHT Format Configuration
The format specific configuration of a VHT waveform synthesized with wlanWaveformGenerator is described by a VHT format configuration object. The object is created using the wlanVHTConfig function. The properties of the object contain the configuration. In this example an object is configured for a 160 MHz bandwidth, 1 transmit antenna, 1 space-time stream and QPSK rate 1/2 (MCS 1). Counter strike free download for mac.
Waveform Generation Configuration
The waveform generator wlanWaveformGenerator can be configured to generate one or more packets and add an idle time between each packet. In this example four packets with a 20 microsecond idle period will be created.
The PSDU transmitted in each packet is scrambled using a random seed for each packet. This is accomplished by specifying a vector of scrambler initialization seeds. The valid range of the seed is between 1 and 127 inclusive.
Create a PSDU for Each Packet
For an IEEE 802.11ac data transmission the MAC frame is termed a MAC protocol data unit (MPDU), the MAC header is termed the MPDU header, and the frame body is an aggregated MAC service data unit (A-MSDU). One or more MPDUs are delimited, padded and aggregated to create an aggregated MPDU (A-MPDU). The A-MPDU is delimited and padded to form the physical layer service data unit (PSDU) which is coded and modulated to create the transmitted packet. This process of encapsulation is shown in the following diagram:
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In this example a PSDU is created containing a single MPDU for each packet. The MPDU consists of an MPDU header, A-MSDU frame containing concatenated A-MSDU subframes with random data and valid FCS. The function wlanMACFrame creates an A-MPDU with EOF delimiters and padding, i.e. the PSDU, as specified in [ 1 ]. It also returns the length of the A-MPDU, termed as the APEP Length, which is used to set the
vhtCfg.APEPLength
property of the VHT configuration object. A PSDU is generated for each packet and is concatenated into a vector data
for transmission with wlanWaveformGenerator. The processing to create the concatenated PSDU bits data
is shown in the diagram below:Generate a Baseband Waveform
The concatenated PSDU bits for all packets,
data
, are passed as an argument to wlanWaveformGenerator along with the VHT packet configuration object vhtCfg
. This configures the waveform generator to synthesize an 802.11ac VHT waveform. To generate 802.11n™ HT or other format waveforms different configuration objects are used such as those created by wlanHTConfig or wlanNonHTConfig. The waveform generator is additionally configured using name-value pairs to generate multiple packets with a specified idle time between packets, and initial scrambler states.The magnitude of the baseband waveform is displayed below. Note the number of packets and idle time configured.
The frequency spectrum of the generated time domain waveform,
txWaveform
, can be viewed using the DSP System Toolbox™spectrum analyzer. As expected, the 160 MHz signal bandwidth is clearly visible at baseband.![Mac Mac](/uploads/1/3/3/8/133862478/774671193.png)
Generate an Over-the-Air Signal Using an RF Signal Generator
The baseband waveform created by WLAN Toolbox can now be downloaded to a signal generator to perform receiver tests. Instrument Control Toolbox™ is used to generate an RF signal with a center frequency of 5.25 GHz RF using the Keysight Technologies N5172B signal generator.
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Selected Bibliography
- IEEE Std 802.11ac™-2013 IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications - Amendment 4: Enhancements for Very High Throughput for Operation in Bands below 6 GHz.