Wireless Internet Using 2.4 GHz

By Michael F. Young
Former President and CTO (YDI Wireless)

Background

Wireless Internet is currently available almost everywhere using digital cell phone and PCS technologies. However, these capabilities are of no use to a local ISP who may consider using them to offer their clients Internet access, since large companies own the licenses for these radio channels. Further, the throughput on these channels is very slow (less than 10 Kbps). So, while they are ideal for WAP and other slow-speed Internet applications, they are not useful for offering high-speed Internet access. Using the 2.4 GHz band has proven to be an effective and profitable means to offer high-speed wireless Internet access to customers at fixed locations. The three main advantages this band offers is that it is license free, it supports high data rates, and the equipment is affordable. The primary disadvantage is that for long range, clear, unobstructed line-of-sight must exist between the base antenna and the client antenna.

License Free Spread Spectrum

The FCC has set standards under Part 15 of the Rules and Regulations for equipment used in the 2.4 GHz band. (The exact spectrum is 2400 to 2483.5 MHz). If the equipment uses Spread Spectrum techniques, then effective radiated transmit powers up to 64 watts can be used! (More on this in later articles). There are two types of Spread Spectrum techniques used: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). This enables many radios to operate in this band with minimum interference - to a point.

With FHSS, a data packet is first sent on a random channel in the band with the next packet sent, after a pause of a few milliseconds, on another random channel in the band. With 80 channels or more available (one channel per MHz, e.g., 2401, 2402, 2403, etc) signals from multiple radios "hop" around each other. This is how they can operate with other radios in the same band with minimal interference.

DSSS radios operate on a fixed radio channel, but the signal is "spread" on that channel by mixing the signal with a Pseudo-Noise (PN) code. This spreading causes the radio signal with the data on it to occupy a much wider band, and looks more like noise to receivers not designed to "de-spread " that signal.

WIPOPs

A PoP (Point of Presence) is a term that is commonly used in the Internet industry to denote a facility that has wired access to the Internet via a broadband backbone. To offer high-speed wireless Internet access, an ISP needs to set up a base station, sometimes referred to as a WIPOP (Wireless Internet PoP). The antennas usually are set up on a tall building or radio tower. This is done so that customer antennas have clear, unobstructed line-of-sight (LOS) back to the base antenna. Trees and buildings will block the signals.

The following diagram shows a typical configuration of a WIPOP and clients using a single radio channel. A WIPOP can be expanded to dual- or three-channel system. You can find more information and diagrams at here.

A WIPOP consists of at least the following:

• Router connected to the Internet somehow
• Access Point
• Coax cable to antenna
• Amplifier (needed for extended range)
• Antenna

An Access Point (AP) is a radio device with an Ethernet interface. It acts like a "wireless hub" to the client sites, where the air acts like an Ethernet cable. The antenna would be placed on a radio tower, building, or some other high spot that would afford a broad line-of-sight area. The amplifier boosts both transmit and receive signals for extended-range connections.

Client sites have high gain directional antennas aimed back at the WIPOP. They may use an Ethernet station adapter to connect an entire LAN to the WIPOP through an inexpensive router, or a single machine equipped with a wireless LAN (WLAN) card and an antenna. As long as the client antennas have clear LOS and are pointed directly at the WIPOP antenna, they will access the Internet like any other traditionally wired customer, but without the wires or any dependence on the local telephone company!

Standards

Virtually all WLAN cards (both FHSS and DSSS) and Access Points comply with IEEE 802.11 standards, meaning that they offer wireless over-the-air data rates of up to 11 Mbps. The newest version of this standard, 802.11(b), covers the 11 Mbps DSSS equipment. Equipment that has been tested to comply with this standard is said to be Wi-Fi certified (like Hi-Fi, but Wireless Fidelity. Cute, eh?). The Wi-Fi standard and test lab was set up by the Wireless Ethernet Compatibility Alliance (WECA) to certify and promote 802.11(b) WLAN products. Visit our White Papers page for information on these standards.

Fresnel Zone and Range

Radio waves do not travel like laser beams. They emanate away from an antenna like ripples in a pond. Simply put, they need width and height as they spread out, especially in the middle of the signal. This area is called the Fresnel Zone. In addition to clear visual LOS, the Fresnel Zone must be clear for maximum range and signal strength. If something like trees or buildings encroach into the Fresnel Zone, the signal is weakened.

There have been many situations where the WIPOP antennas could be seen clearly (but just barely) over a roof or tree line from a client site but a good radio connection could not be established. This is because those obstacles (roofs and trees) act like a knife-edge, refracting (bending) the signal downward, away from the antennas.

Sometimes using an amplifier at the client site overcomes this extra signal loss. Sometimes it does not. It depends on the distance, antenna gains, and amount of Fresnel Zone encroachment.

Needless to say, without line-of-sight, long range (greater than several hundred yards) connections are almost impossible to achieve. On the other hand, from the WIPOP on the WFAX radio tower in Falls Church, a wireless link has been made to a mountain site 30 miles to the west using a 24 dBi gain grid dish antenna and amplifier.

As a general rule, a medium-range (less than 4-5 miles) client links with clear LOS and Fresnel Zone, and a 24 dBi grid dish antenna and no amplifier will be sufficient with the short coax cables between the radio and antenna. Beyond that, amplifiers are usually needed since the signal gets weaker and it becomes more difficult to get clear Fresnel Zones.

Getting Bandwidth to the WIPOP

There are two ways to get Internet access to the WIPOP: wired or wirelessly. The best way is with a wireless point-to-point link, on the license-free 5.8 GHz band. As a rule, you should not use the 2.4 GHz spectrum for the wireless link back to the wired PoP. Leave the precious 2.4 GHz for the "last-mile" access to the clients. A good link that offers 8 Mbps full duplex (like the YDI EX-1) on the 5.8 GHz band will cost less than $7,000 - about the price to rent a T1 line for a year.

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