Overview

Executive Overview

This antenna reference guide explains issues and concerns about antennas used with a Cisco^® Aironet^® wireless LAN system or wireless bridge system. It details deployment and design, limitations and capabilities, and basic theories ofantennas. This document also contains information about the Cisco antennas and accessories, as well as installation scenarios, regulatory information, and technical specifications and diagrams of the available antennas.

Overview of Antennas

Each Cisco Aironet radio product is designed to perform in a variety of environments. Implementing the antenna system can greatly improve coverage and performance.

To optimize the overall performance of a Cisco wireless LAN, it is important to understand how to maximize radio coverage with the appropriate antenna selection and placement. An antenna system comprises numerous components, including the antenna, mounting hardware, connectors, antenna cabling, and in somecases, a lightning arrestor. For a consultation, please contact a Cisco Aironet partnerat:http://tools.cisco.com/WWChannels/LOCATR/jsp/partner_locator.jsp.

Cisco partners can provide onsite engineering assistance for complex requirements.

Radio Technologies

In the mid-1980s, the U.S. Federal Communications Commission (FCC) modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modification authorized wireless network products to operate in the industrial, scientific, and medical (ISM) bands using spread spectrum modulation. This type of modulation had formerly been classified and permitted only in military products. The ISM frequencies are in threedifferent bands, located at 900 MHz, 2.4 GHz, and 5 GHz. This document covers both the 2.4- and 5-GHzbands.

The ISM bands typically allow users to operate wireless products without requiring specific licenses, but this will vary in some countries. In the United States, there is no requirement for FCC licenses. The products themselves must meet certain requirements to be certified for sale, such as operation under 1-watt transmitter output power (in the United States) and maximum antenna gain or effective isotropic radiated power (EIRP) ratings.

The Cisco Aironet product lines utilize both the 2.4- and 5-GHz bands. In the United States, three bands are defined as unlicensed and known as theISM bands. The ISM bands are as follows:

● 900 MHz (902-928 MHz)

● 2.4 GHz (2.4-2.4835 GHz) - IEEE 802.11b

● 5 GHz (5.15-5.35 and 5.725-5.825 GHz) - IEEE 802.11a, HIPERLAN/1 and HIPERLAN/2. This band is also known as the UNII band, and has threesubbands: UNII1 (5.150-5.250 GHz), UNII2 (5.250-5.350 GHz), and UNII3 (5.725-5.825 GHz)

Each set of bands has different characteristics. The lower frequencies exhibit better range, but with limited bandwidth and hence lower data rates. The higher frequencies have less range and are subject to greater attenuation from solid objects.

802.11 Modulation Techniques

The IEEE 802.11 standard makes provisions for the use of several different modulation techniques to encode the transmitted data onto the RF signal. These modulation techniques are used to enhance the probability of the receiver correctly receiving the data and thus reducing the need for retransmissions. The techniques vary in their complexities and robustness to RF signal propagation impairments.

Direct-Sequence Spread Spectrum

The direct-sequence spread spectrum (DSSS) approach involves encoding redundant information into the RF signal. Every data bit is expanded to astring of chips called a chipping sequence or Barker sequence. The chipping rate, as mandated by the U.S. FCC, is 10 chips atthe 1- and 2-Mbps rates and 8 chips at the 11-Mbps rate. So, at 11 Mbps, 8 bits are transmitted for every one bit of data. The chipping sequence is transmitted in parallel across the spread spectrum frequency channel.

Frequency-Hopping Spread Spectrum

Frequency-hopping spread spectrum (FHSS) uses a radio that moves or hops from one frequency toanother at predetermined times and channels. The regulations require that the maximum time spent on any one channel is 400 milliseconds. For the 1- and 2-Mb FHSS systems, the hopping pattern must include 75 different channels, and must use every channel before reusing any one. For wide-band frequency hopping (WBFH) systems, which permit up to 10-Mb data rates, the rules require the use of at least 15 channels, and they cannot overlap. With only 83 MHz of spectrum, WBFH limits the systems to 15 channels, thus causing scalability issues.

In every case, for the same transmitter power and antennas, a DSSS system will have greater range, scalability, and throughput than an FHSS system. Forthis reason, Cisco has chosen to support only direct-sequence systems in the spread spectrum products.

Orthogonal Frequency Division Multiplexing

The orthogonal frequency division multiplexing (OFDM) used in 802.11a and 802.11g data transmissions offers greater performance than the older direct-sequence systems. In the OFDM system, each tone is orthogonal to the adjacent tones and therefore does not require the frequency guard band needed for direct sequence. This guard band lowers the bandwidth efficiency and wastes up to 50 percent of theavailable bandwidth. Because OFDM is composed of many narrow-band tones, narrow-band interference degrades only a small portion of the signal, with little or no effect on the remainder ofthe frequency components.

Antenna Properties and Ratings

An antenna gives the wireless system three fundamental properties - gain, direction, and polarization. Gain is a measure of increase inpower. Direction is the shape of the transmission pattern. A good analogy for an antenna is the reflector in a flashlight. The reflector concentrates and intensifies the light beam in a particular direction similar to what a parabolic dish antenna would do to a RF source in a radiosystem.

Antenna gain is measured in decibels, which is a ratio between two values. The gain of a specific antenna is compared to the gain of an isotropic antenna. An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern (similar to a light bulb with no reflector). dBi is used to compare the power level of a given antenna to the theoretical isotropic antenna. The U.S. FCC uses dBi in its calculations. An isotropic antenna is said to have a power rating of 0 dB, meaning that it has zero gain/loss when compared to itself.

Unlike isotropic antennas, dipole antennas are real antennas. Dipole antennas have a different radiation pattern compared to isotropic antennas. The dipole radiation pattern is 360 degrees in the horizontal plane and 75 degrees in the vertical plane (assuming the dipole antennais standing vertically) and resembles a donut in shape. Because the beam is “slightly” concentrated, dipole antennas have a gain over isotropic antennas of 2.14 dB in the horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi (in comparison to an isotropic antenna).

Some antennas are rated in comparison to dipole antennas. This is denoted by the suffix dBd. Hence, dipole antennas have a gain of 0 dBd (=2.14 dBi).

Note that the majority of documentation refers to dipole antennas as having a gain of 2.2 dBi. Theactual figure is 2.14 dBi, but is often rounded up.