• Background

In 1990 the IEEE 802.11  WLAN Working Group was formed to standardize Wireless LANS operating in the Unlicensed 2.4 GHz operating range.   The original standard specified Frequency Hopping Spread Spectrum, Direct Sequence Spread Spectrum and Infrared technologies.  In 1997 the standard was adopted specifying a rate of 1 MBits/sec using BPSK modulation and 2 MBits/sec using QPSK modulation.  In 1999 the IEEE 802.11b standard extended the data rate to 11 Mbits/sec via the use of the CCK (Complementary Code Keying) modulation scheme.  At 11 MBits/sec the range of each access point is around 25 meters, while at 1 MBits/sec the range is around 90 meters.  The higher data rate in this extension attracted the attention of many large companies and was the start of large scale interest and deployments, including airports and hotels around the world.

Similarly, in the year 1999, the IEEE 802.11a standard codified a WLAN operating in the 5GHz frequency range (UNII) with data rates at 54 MBits/sec via the use of OFDM (explore Orthogonal Frequency Division Multiplexing in detail here). Four Hundred and Fifty Five MHz (455 MHz) was allocated in the 5 GHz band to alleviate the anticipated spectrum limitations in the 2.4 GHz band (where 83.5 MHz was allocated).

In the year 2000, the IEEE 802.11g group began studying the possibility of increasing the data rate of WLANs operating in the 2.4 GHz range even more to 20 MBits/sec using new modulation and coding techniques.

A few other relevant WLAN standards:

IEEE 802.11e provides QoS enhancements to the IEEE802.11 MAC required for IP services.

IEEE 802.11f provides multi-vendor Access Point interoperability.

IEEE 802.11h provides power management techniques required for possible European operation in the 5 GHz. band.

IEEE802.11i provides new encryption algorithms for security features.

The general IEEE 802 (LAN/MAN) Working Groups can be found here:

http://grouper.ieee.org/groups/802/dots.html

The general IEEE 802.11 (WLAN) Working Group can be found here:

http://grouper.ieee.org/groups/802/11/index.html

• Link Budget

A fundamental concept in any communications system is the link budget.  The link budget is a summation of all the gains and losses in a communications system.  The result of the link budget is the transmit power required to present a SNR (Signal to Noise Ratio) at the receiver to achieve a target BER (Bit Error Rate).

For a satellite communications link, factors like orbits, coverage footprints and service availability (a function of climate, time of year etc.)   also come into play.  In the case of WLANs, it is sufficient to consider factors such as <link> path loss (as a function of frequency), noise (from all sources), <link> Tx/Rx antenna gains and cabling/system losses and receiver sensitivity (for a given measurement bandwidth).

• Rate vs. Range

From the background given above, it is apparent that the data rate of a given system is a function of the modulation scheme chosen.  What's more, the transmitted data rate will also impact the range.  To understand this, it is important to draw the distinction between composite RF signal power coming out of the transmit Power Amplifier and the decoded Eb/No at the receiver.

The data rate will not effect the composite RF signal power coming out of the transmit PA.  However, in a digital communication system, it is the detected Energy per Bit in relation to the Noise Power Spectral density at the receiver that is important.  As the transmitted data rate is increased (more bits), for a given composite RF signal power (SNR), then there will obviously be less energy per bit at the receiver.

Since the Eb/No decreases as data rate increases, it becomes necessary to decrease the range (increase the SNR) to achieve the same Eb/No.

This relation can be expressed as:

Eb/No (dB) = SNR (dB) * W/R

where:

Eb/No = Energy per bit / Noise Power Spectral Density

SNR = Composite Signal to Noise Ratio

W = Channel Bandwidth

R = Transmitted Data Rate

As can be seen, when the transmitted data rate equals the channel bandwidth, the Eb/No = SNR.  For higher order modulation schemes, the Eb/No is less than the composite RF signal power.

Forward Error Correction (FEC) can be employed to compensate for this to some extent, thereby increasing the range.  However, there is no free lunch, and as shown in the app note on digital modulation (explore Digital Modulation in detail here), FEC is a bandwidth expanding process and in digital modulation, you are either power limited or bandwidth limited.

• Authentication/Encryption vs. FEC

It is fairly common to confuse the concepts of authentication and encryption with channel coding techniques like FEC.  They are actually three completely different techniques with different goals.

FEC is a bandwidth expanding, channel coding technique used to compensate for channel impairments.  One the one hand, FEC can extend the range of a system as it will improve the BER for a given Eb/No.  However, on the other hand, the extra redundancy added to the transmitted data will an require an increase in the data rate to keep the payload constant.  This will drive the range down.  As in all designs, an engineering balance must be reached.

Authentication is a process employed to prevent unauthorized transmissions into the channel (spoofing).  Virtual Private Networks (VPNs) are one way to achieve a level of authentication.

Encryption is a process employed to prevent unauthorized reception of the transmitted message in the channel (listening).   Broadly speaking, encryption schemes can be classified as either "block encryption" or "data stream encryption".  Wireless Encryption Protocol (WEP) is one way to achieve some (loose) level of encryption.

• Diversity Reception / Combining techniques

Diversity is a technique that can be used to improve the robustness of a communications link without incurring the extra overhead of a training sequence like some equalization techniques require.

It is common practice to use diversity antenna arrays (comprised of one or more antennas) at the base station to mitigate fading.  If the receive antenna elements (or antennas) are spaced an appropriate number of wavelengths apart, then it is likely that not all antennas will be in a null.  This is useful for mitigating the basic constructive / destructive effects of multipath induced flat fading.

There are many types of combining techniques that can be used in conjunction with diversity reception.  In the simplest implementation, the antenna with the highest SNR is connected to the demodulator circuitry.  This is called Selection Diversity.  While selection diversity can be used to mitigate nulls in received signal, it will not produce any "diversity gain".  It will however improve the link availability.

In Equal Gain Combining, all of the diversity branches are assigned equal weights but are phased appropriately to add coherently in phase, thus producing diversity gain.

In Maximal Ratio Combining, the output SNR is composed of the sum of the individual branch SNRs, thus providing the most diversity gain, at the expense of complexity.

• Quality of Service (QoS)

Currently IEEE 802.11e is working on an extension to support Quality of Service requirements in WLANs.  This would be accomplished at the MAC layer (explore the Network View in detail here).