HT WLAN Systems Flashcards
802.11N?
Called HT Clause 20 Uses: Mimo, OFDM Greater range Increased throughput Backward compatible with 802.11 a, b, and g radios Allows channel bonding
MIMO?
Multiple Input Multiple Output
What does MIMO do?
Requires multiple radios and antennas, called radio chains
- At Transmitter
- At Receiver
Sends multiple radio signal at the time
Takes advantage of multipath
Advanced DSP used to process the signals
Has multiple radio chains
Radio Chain?
A radio chain is defined as a single radio and all of its supporting architecture, including mixers, amplifiers, and analog/digital converters.
More radios -> more spatial multiplexing (SM)
Spatial Multiplexing?
MIMO radios can send multiple independent data streams.
Due to antenna separation, each stream goes through different multipath.
Throughput is greatly increased with SM
Example: 3x3:2 MIMO means:
- 3 Transmitter
- 3 Receiver
- 2 Unique Data Stream
MIMO Diversity: Antenna Diversity?
Antenna diversity is a method of using multiple antennas to survive the negative effects of multipath
Antenna diversity is NOT ____?
Spatial multiplexing
Switched Diversity?
Antenna diversity can be just simple antenna switching.
- Listening on multiple antennas
- Selecting the best channel
Maximal Ratio Combining?
Signals ma also be linearly combined
- Increased effective SNR
- Useful when using non-MIMO transmitter
STBC?
Space Time Block Coding
What does STBC do?
Is a method where the same information is transmitted on two or more antennas.
STBC is a type of ?
Transmit diversity:
- Does not directly increase data rate
- Receive sensitivity of the radio system improves
- Signal can be detected in low SNR
Cyclid Shift Diversity?
Some extra delay is cyclically added to the each transmitted signal
Delays reduce correlation between received multipath signal.
This technique can be used with non HT radios
Transmit Beamforming?
Uses multiple antennas to with varying amplitude and phase.
Forms RF beam in desired direction and time
Can be used when there are more transmitting antennas than there are spatial data streams
Effectively increased SNR of the combined signal at the receiver
Gives greater coverage
Needs knowledge of the channel, needs feedback from receiver.
Channel bonding in 5Ghz U-NII Bands
Use of 40Mhz HT channels in the 5 GHz frequency bands natural choice
There are more 20Mhz channels that can be bonded together in various pairs.
Channel Bonding in 2.4 Ghz ISM Band?
Deploying 40 Mhz HT channels at 2.4Ghz does not scale well in multiple channel architecture
Only one 40Mhz channel can be used
Forty MHz Intolerant?
AP Using 802.11n 40MHz channel will be forced to 20MHz channel
Guard Interval(GI)
Gap bettween two symbols
Protects against InterSymbol Interference (ISI)
Modulation And Coding Scheme?
802.11n data rates are defied with a modulation and coding scheme matrix
Data rate varies by which factors?
Modulation Coding Number of spatial streams Channel size Guard interval
How many MCs are there?
77MCs for both 20Mhz and 40Mhz channels
HT PHY?
Physical layer convergence procedure is created in PHY
PPDY consists of?
PHY Preamble
PHY Header
3 Types of HT?
Legacy
HT Mixed
HT Greenfield
HT MAC?
New MAC techniques improves throughput and power management
- Frame aggregation
- Interframe spacing (IFS)
- Black Acknowledgement
All these reduces MAC overhead
Block Acknowledgment?
Legacy
Block Acknowledgement
Multiple Traffic ID Block Acknowledgement
Legacy?
Each unicast frame needs to be ACKed
Block Acknowledgement?
One ACK for multiple MPDUs
Multiple Traffic ID Block Acknowledgement?
Only one ACK required for all the MPDUs inside an A-MPDU
