High packet latency in WLAN system level simulation example

Hi,

I am new to wi-fi protocol and simulations. I have started working with the basic WLAN system level simulation example of MATLAB - Link

The latency in this example even after commenting out the wireless channel fading part and changing the "MACFrameAbstraction" to "false" and "PHYAbstractionMethod" to "none" comes out above or near to 0.2 sec for both the AP and STA.

How can I model the system for ideal scenerio and reduce this latency values?

Thanks,

Garvit

Answer 1

[Pruthvi Charka]

Hi,

I understand that you are trying to reduce the latency values.

The example you have pointed out calculates the latency observed at the receiver node's application.

You can try any combination of the following suggestions to reduce the latency values.

Queueing delay reduction:

• This is the amount of time spent by the packet in the queue (of the MAC layer) waiting for its turn for transmission.
• To reduce this delay, application traffic rate should be as low as possible so that the packets do not get accumulated waiting for other packets to be sent out. See this example to know how to add custom application traffic with configurable data traffic rate. Simulate an 802.11ax Network with Uplink and Downlink Application Traffic

Channel Contention reduction:

• This is the amount of time spent by the MAC layer contending for the medium before it can start the transmission on the wireless channel
• Contention time can be reduced by lower number of transmitters and lower contention parameters. You can try setting either only downlink or only uplink traffic and using low values for CWMin, CWMax, and AIFS values. See wlanDeviceConfig for contention parameters configuration. See Simulate an 802.11ax Network with Uplink and Downlink Application Traffic example for setting unidirectional traffic.

Packet Transmission Time reduction:

• This is the time taken for the packet to be transmitted from the transmitter to the receiver (Note that propagation delay over the air is not modeled in the example).
• Packet transmission time can be reduced by using higher PHY data rates. Higher PHY data rates can be acheived by setting high values for any of MCS, ChannelBandwidth, NumTransmitAntennas, and NumSpaceTimeStreams. See wlanDeviceConfig for these configuration parameters.
• Note that at higher PHY data rates, distance between nodes may need to be reduced due to lowered range of transmission.

Hope this helps!

[Garvit]

Hi Pruthvi,

The link under Queueing delay reduction and second link under Channel Contention reduction are giving me page doesn't exist error.

Can you please reshare the relevant examples?

Thanks,

Garvit

[Garvit]

Hi Pruthvi,

In your response you mentioned adjusting both PHY data rates and application traffic rate. I have replaced the line

associateStations(accessPoint,station,FullBufferTraffic="on");

with -

associateStations(accessPoint,station);
trafficSourceUL = networkTrafficOnOff(DataRate=800000,PacketSize=1500);
addTrafficSource(station,trafficSourceUL,DestinationNode=accessPoint,AccessCategory=0); % UL traffic from STA to AP

trafficSourceDL = networkTrafficOnOff(DataRate=800000,PacketSize=1500);
addTrafficSource(accessPoint,trafficSourceDL,DestinationNode=station,AccessCategory=0); % DL traffic from AP to STA

From what I can understand, the networkTrafficOnOff object will regulate the application traffic rate. I have tested changing these values from 1000 kbps to 8000 kbps but I don't see any impact on latency values.

I am still not able to figure out how to increase PHY layer data rates. Can you explain that to me?

Thanks,

Garvit

[Pruthvi Charka]

Hi,

I see that the link in my previous answer wasn't working. I have edited my previous message and corrected the links. You should be able to access those links now.

I see that you modified the code to use networkTrafficOnOff for UL and DL directions. As I said in my previous answer, application traffic rate should be as low as possible.

• Note that the DataRate argument of networkTrafficOnOff object is in units of Kbps. So the value you've set 800000 is in Kbps and represents 800Mbps which is quite high.
• Try setting the DataRate value to just 8000 (which means 8000 Kbps). You should be able to see a drop in the latency provided that this is the only change made in the example.

PHY data rate is the transmission capacity of the PHY for a particular configuration. IEEE® 802.11™ standard defines the PHY data rates for different PHY types. To achieve higher PHY transmission capacity, you have to set high values for any of MCS, ChannelBandwidth, NumTransmitAntennas, and NumSpaceTimeStreams. You can find information for these configuration parameters in wlanDeviceConfig.

Hope this helps!

[Garvit]

Hi Pruthvi,

Thanks a lot for the assistance, I was able to reduce the latency to below 1 msec values with MCS = 15 and Transmit Antenna = 2 using HT-mixed Transmission format. I require some clarification on following points -

1) The chipset I am trying to mimic is working on (2,2) MIMO so when I set transmit antenna = 2 does it mean that receive antenna also becomes as 2 and the node works as (2,2) MIMO?

2) Channel models are not making any impact on latency currently, I am not sure if it should or shouldn't. can you provide some inputs on this part? currently the distance between access point and station is 10 meters and only these 2 nodes are there in system.

3) I am actually trying to simulate 802.11n network but I am working with 802.11ax example. Is there any chances of getting errors in results due to this part? Currently, I am just trying to setup parameters in a similar way the 802.11n device works.

4) In the next step, I want to see the impact of having multiple access points in system on latency for these 2 nodes. In the simulations can I just add access points in the system which broadcasts its message and not associate any stations with it?

Thanks,

Garvit