The basic RF signal is known as a carrier signal, it is used to carry useful information. In terms of wireless, a wireless LAN signal carries data.
To add this data to the wireless carrier signal, the frequency of the original carrier signal must be preserved but some altering occurring to signify a 1 or 0 in the signal. This is down through RF modulation
RF modulation has a few goals:
- Carry data at a predefined rate
- Reasonably immune to noise
- Practical to transmit and receive data
RF modulation can only modify a few attributes of a signal though to accomplish these goals, these attributes are Frequency, Phase, and Amplitude.
The techniques require some of the provided bandwidth to be used to accomplish sending and receiving data.
Low bit rate signals such as AM and FM are trivial to module, and are called narrowband signals.
Wireless local area networks require high bit rate signals, and require more bandwidth to modulate the signal. Data is sent out over a wide range of frequencies known as the spread spectrum.
At the first physical layer, wireless local area networks can be broken down into two common spread spectrum categories, DSSS and OFDM.
Direct Sequence Spread Spectrum (DSSSS)
DSSSS is used in the 2.4Ghz band. A small number of fixed channels support phase modulation and scalable data rates. The channels are wide enough to augment the data and spread it out to avoid disruption.
Orthogonal Frequency Division Multiplexing (OFDM)
OFDM is used in both 2.4 and 5Ghz networks. A single 20Mhz channel contains data sent in parallel over multiple frequencies. The channel is divided into multiple subcarriers with both phase and amplitude modulated with quadrature amplitude modulation to move data efficiently.
AP to Client Compatibility
For wireless communication to be compatible between an access point and a wireless station, it must use a wireless mechanism where both devices are compatible.
IEEE 802.11 defines mechanisms such as RF signals, modulation, coding, bands, channels and data rates to provide a standardised communication medium.
The original specification was published in 1997 but there have been many modifications to it over the years. These can commonly known as 802.11a, b, g, n, ac, ax, and so on.
The new amendants have modified parts of the standard, introducing new modulations and coding schemes that are used to carry the data over the airwaves.
For 2.4Ghz, 802.11 evolved with 802.11b and 802.11g that defined maximum data rates of 11 Mbps and 54 Mbps. The new data rates brought more complicated modulation methods that resulted in an increased data rate.
5Ghz saw the 802.11a band that brought 20 Mhz channel to the specification.
The 802.11ac amendment in 2013 that brought higher data rates with more advanced modulation techniques, wider channel widths and greater data aggregation. 802.11ac can only be used on the 5Ghz band
Another exclusive for the 5Ghz band was 802.11ax that offer even more compatible modulation schemes over what 802.11ac offers. It can help reduce interference through the use of better transmit power control and BSS marking.
Before 802.11ac and 802.11ax was the final band to be introduced that supported 2.4Ghz and 5Ghz, that was 802.11n. It helped scale wifi performance to a maximum of 600 Mbps.
802.11b
Compatible with both 2.4Ghz and 5Ghz
Supports data rates of 1, 2, 5.5 and 11 Mbps
Channel width of 22 Mhz
802.11g
Compatible with both 2.4Ghz and 5Ghz
Supports data rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbps
Channel width of 22 Mhz
802.11a
Compatible with 5Ghz
Supports data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
Channel width of 22 Mhz
802.11n
Compatible with 2.4Ghz and 5Ghz
Data rates of up to 150Mbps on each spatial stream, with up to four separate streams.
20 or 40 Mhz channel widths
802.11ac
Compatible with 5Ghz only
Data rates of up to 866 Mbps on each spatial stream, with up to four separate streams
20, 40, 80, or 160 Mhz
802.11ac
Compatible with 5Ghz only
Data rates of up to 1.2 Gbps per spatial stream, with up to four seperate streams
20, 40, 80 or 160 Mhz
Multiple Radios
Before 802.11n was introduced, a wireless device was limited to one signal sender and one signal receiver. These two components formed what is known as a radio chain or a single in single out system.
802.11n, 802.11ac, and 802.11ax take advantage of a MIMO system, multiple in, multiple out that can form multiple radio chains. Data can be divided between multiple radio sender and receivers to increase performance greatly.
With the multiple multiple output system that 802.11n, 802.11ac and 802.11ax can utilise, their devices are often characterised by the number of radio chains available.
This can be show in the form 2×4, 2 transmitters, 4 receivers. 2 x 2, 2 transmitters, 4 receivers and so on.
In some device specifications the number of spatial streams that a device can support is affixed to the end, for example for two devices it would be 4×4:2, 4 transmitters, 4 receivers, 2 spatial streams.
These multiple radios can used in multiple ways. Extra radios can be used to improve the quality of the signal between the access point and the station, or multiple radios can be used to serve multiple clients simultaneously.
Throughput can be increased to a device by multiplexing the data streams across two or more radio chains on the same channel. This is known as spatial multiplexing.
These radios can transmit simultaneously due to their radios being spaced apart. With each chain having its own antenna, the signals will be spaced apart far enough that it is likely they will be out of phase with the other antennas.
There is a fair processing overheard for spatial multiplexing, however the increased performance from doing so is a worthwhile reward.
If a station connects to an access point where the number of radio chains do not match up, the devices will negotiate a common compatibility by informing each other of their radio capabilities. They will use the lowest number of spatial streams that they have in common with each other.
Beamforming
Usually when a transmitter sends a signal, there are receivers that have an equal opportunity to receive that signal, there is nothing the transmitter does to selectivity prefer a receiver
802.11n, 802.11ac, and 802.11ax offer a method to customise the transmitted signal to prefer a certain receiver over another receiver.
Transmit Beamforming, TxBF, allows the phase of the signal to be altered as it is fed into each transmitting antenna so the resulting signal arrives at the destination in phase at a specific receiver. The can improve the signal quality and the signal to noise ratio.
The conditions of each receiving device can differ, the transmitter uses feedback from the far end device to tweak the beamforming technology to make the appropriate phasing for the signal.
Maximal-Ratio Combining
When an RF signal is received on a device, it may be degrade or distorted due to a variety of conditions. When MIMO is used to send multiple copies of that signal to a receiving device, the receiving device can use those multiple copies to try store the signal to its original readable state.
Maximal ratio combining uses multiple antennas and radio chains to combine the signals together to produce the best copy of the signal the receiving device can obtain at any one time.
Maximising AP to Station Throughput
To pass data successfully between an AP and station, they must be using the same modulation method and the best data rate possible in their current environment. Where there is noise a lower data rate may be preferred, otherwise if the signal quality is good a higher data rate may be successful in transmission.
Where wireless devices move around, the transmitter and receiver will continue to negotiate a modulation method based on their current conditions.
Where a user may be close to the transmitter, a more complicated modulating and coding scheme to achieve data rates may be chosen. Where a user moves further away from the transmitter, a less complicated and simpler coding and modulation scheme may be used to achieve a more stable signal at the cost of data rates.
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