An extract of IEEE 802.11n-2009 Modulation Coding Scheme for Wireless N is shown in the diagram. It shows the maximum speeds possible per spatial stream for a given modulation method and the applied coding scheme.

Modulation Method

The modulation method expresses the way the raw user data carried by the radio channel is applied to the main channel frequency in use by changing one or more of its wave properties.

The modulation is applied at the transmitting radio. A corresponding radio at the receiving end demodulates or decodes these wave properties from the radio channel frequency and reconstitutes the raw user data applied at the transmitter.

Each modulation method uses one of more of the properties of radio waves at the same time to represent the meaning of the applied data. Forgetting the actual radio wave properties involved you can think of each modulation property as adding a new signalling path in parallel. Providing that each property can be changed in the same unit of time each extra path doubles the data rate possible on the channel. So N paths (N properties) produces a total channel speed N times faster than a channel modulation method that used just one radio wave property. By doing these multi property modulation schemes the radios used effectively increase the radio channel data capacity.

Coding Scheme

Because of the nature of radio wave propagation there are inherent errors in trying to interpret the radio signals as they pass to the receiver and because of this a coding scheme is used which effectively adds extra coding bits to the raw data as a means of interpreting and correcting the data presented at the receiving radio. These forward error correcting codes offer efficiency in the transmission on the channel avoiding the overhead of re-transmission. They do however reduce the true channel capacity for real user data.

The code rate of a forward error correction code is the proportion of the data-stream that is useful that is not used for error correction. If the code rate is k/n, for every k bits of useful information, the coder generates totally n bits of data, of which n-k are redundant.

If R is the total bit rate or data signalling rate of the channel, the useful bit rate is R x k/n.

Often in router specifications the possible data rates achieved are expressed in terms of the corresponding index entry name used in the main MCS table for Wireless N specifications. So you see references like MCS0 to MCS7 for example. To understand the MCS data table for Wireless N here are some examples.

How to Calculate Real Data Rates From Wireless N Specs

How to Calculate Real Data Rates from Wireless N Specs From the partial MCS table shown above an MCS index 3 (MCS 3)

– uses one spatial stream
– has a modulation method 16-QAM
– has a coding rate of 1/2
– has a total bit rate of 26 Mb/s @ 20 Mhz channel width for 800 nano second guard interval
– has a total bit rate of 54 Mb/s @ 40 Mhz channel width for 800 nano second guard interval

If we used a single antenna radio on 20 Mhz channel width (half duplex, one direction at a time) we can expect 13 Mb/s effective data rate because the coding rate is 1/2.

How to Calculate Real Data Rates From Wireless N Specs from the partial MCS table shown above an MCS index 11 (MCS 11)

– uses two spatial streams
– has a modulation method 16-QAM
– has a coding rate of 1/2
– has a total bit rate of 52 Mb/s @ 20 Mhz channel width for 800 nano second guard interval
– has a total bit rate of 108 Mb/s @ 40 Mhz channel width for 800 nano second guard interval

If we used 2×2 MIMO antennas radios on 20 Mhz channel width (full duplex, both directions at once) we can expect 26 Mb/s effective data rate because the coding rate is 1/2 but this time we have two spatial paths.

Router Power Standards and Signal Reach

The physical reach of a wireless signal depends on a number of factors. At it source, the router signal strength and quality are influenced by the radio power and antenna gain combination. Most domestic routers operate at power levels of from 10 to 20 dBm and have small external or internal omni antennas giving a signal gain of 1.5 to 3 or sometimes 5 dBi. From a raw power point of view the more radio power and the greater antenna gain the further the wireless signal will reach. So basically more radio power plus higher gain antennas means greater signal reach.

Making high power quality linear amplifiers for wifi radios is not easy or cheap to do. The better quality branded routers offer low noise wireless’s and this is reflected in the price you pay. The best way to increase wifi signal reach is by adding a better higher gain antenna at the router where possible. This is one way to extend the router signal to provide even greater coverage.

Wireless N technology also improves the design of radios and antennas built into the network hardware. The signal range of Wireless N routers often exceeds that of older forms of Wi-Fi, helping to better reach and maintain more reliable connections with devices further away or outdoors. Additionally, 802.11n can operate on signal frequencies outside the band commonly used by other non-networked consumer gadgets, reducing the likelihood of radio interference inside the home.

Wireless N Netgear N300 Wireless Gigabit Router with shareable USB Storage

The Netgear N300 Wireless N Gigabit Router is ideal for both professional and personal Internet use, providing Gigabit for ultra-fast wired connections and Wireless-N for better wireless speed and range. The router provides Wireless-N speed for simultaneous downloads, streaming voice and music and online gaming, in addition to basic Internet applications.

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Wireless N 300 Mb/s Gigabit Router with shareable USB Storage