Frequency Fundamentals of Wireless Networks

Wireless networks can be divided into two broad segments: short-range and long-range. Short-range wireless pertains to networks that are confined to a limited area. This applies to local area networks (LANs), such as corporate buildings, school campuses, manufacturing plants or homes, as well as to personal area networks (PANs) where portable computers within close proximity to one another need to communicate. These networks typically operate over unlicensed spectrum reserved for industrial, scientific, medical (ISM) usage. The available frequencies differ from country to country. The most common frequency band is at 2.4 GHz, which is available across most of the globe. Other bands at 5 GHz and 40 GHz are also often used. The availability of these frequencies allows users to operate wireless networks without obtaining a license, and without charge.

Long-range networks continue where LANs end. Connectivity is typically provided by companies that sell the wireless connectivity as a service. These networks span large areas such as a metropolitan area, a state or province, or an entire country. The goal of long-range networks is to provide wireless coverage globally. The most common longrange network is wireless wide area network (WWAN). When true global coverage is required, satellite networks are also available.

Many of the wireless technologies in the WPAN, WLAN, and WWAN categories transmit information using radio waves. For this to take place, the data is superimposed onto the radio wave, which is also known as the carrier wave, since it carries the data. This process is called modulation. There are many modulation techniques available, all with certain advantages and disadvantages in terms of efficiency and power requirements. The modulation techniques are as follows:

1. Narrowband technology. - Narrowband radio systems transmit and receive data on a specific radio frequency. The frequency band is kept as narrow as possible to allow the information to be passed. Interference is avoided by coordinating different users on different frequencies. The radio receiver filters out all signals except those on the designated frequency. For a company to use narrowband technology, it requires a license issued by the government. Examples of such companies include many of the wide area network providers.

2. Spread spectrum. - By design, spread spectrum trades off bandwidth efficiency for reliability, integrity, and security. It consumes more bandwidth than narrow-band technology, but produces a signal that is louder and easier to detect by receivers that know the parameters of the signal being broadcast. To everyone else, the spread-spectrum signal looks like background noise. Two variations of spread-spectrum radio exist: frequency-hopping and direct-sequence.

• Frequency-hopping spread spectrum (FHSS) - FHSS uses a narrowband carrier that rapidly cycles through frequencies. Both the sender and receiver know the frequency pattern being used. The idea is that even if one frequency is blocked, another should be available. If this is not the case, then the data is re-sent. When properly synchronized, the result is a single logical channel over which the information is transmitted. To everyone else, it appears as short bursts of noise. The maximum data rate using FHSS is typically around 1 Mbps.

• Direct-sequence spread spectrum (DSSS) - DSSS spreads the signal across a broad band of radio frequencies simultaneously. Each bit transmitted has a redundant bit pattern called a chip. The longer the chip, the more likely the original data can be recovered. Longer bits also require more bandwidth. To receivers not expecting the signal, DSSS appears as low-power broadband noise and is rejected. DSSS requires more power than FHSS, but data rates can be increased to a maximum of 2 Mbps.

3. Orthogonal Frequency Division Multiplexing (OFDM) - OFDM transmits data in a parallel method, as opposed to the hopping technique used by FHSS and the spreading technique used by DSSS. This protects it from interference since the signal is being sent over parallel frequencies. OFDM has ultrahigh spectrum efficiency, meaning that more data can travel over a smaller amount of bandwidth. This makes it effective for high-data-rate transmissions. The drawbacks of OFDM are that it is more difficult to implement than either FHSS or DSSS, and consumes greater amounts of power.