Chapter 2

WIRELESS COMMUNICATION IN UNLICENSED

BANDS

2.1	The Indoor Radio Channel ..................................................	5
2.2	Orthogonal Frequency Division Multiplexing....................	8
2.3	The 5 GHz Band..................................................................	11

2.4Error Model for the OFDM Transmission applied

in the 5 GHz Unlicensed Band ...........................................

11

FOR THE UNDERSTANDING of wireless communication, spectrum issues and the transmission scheme of interest for the 5 GHz unlicensed band are discussed in this chapter. The radio channel characteristics are

discussed and a model for error calculation of the Orthogonal Frequency Division Multiplex (OFDM) transmission scheme is developed to provide knowledge about wireless communication and the environment the WLANs of interest are operating in. he design of a wireless network requires an accurate characterization of the radio channel, specifically a precise model that can be used in time-consuming computer simulation. With an accurate channel characterization and with a detailed mathematical model of the channel, the performance and attributes of a radio transmission scheme and protocols are predictable by means of simulation. The characteristics of an indoor radio channel vary from with the environment, which must be considered when modeling the indoor radio channel.

2.1The Indoor Radio Channel

Wireless LANs operate mainly in the indoor environment. Radio propagation in indoor environments is complicated because the direct path between transmitter

6	2. Wireless Communications in Unlicensed Bands

and receiver (i.e., Line of Sight, LOS) is often obstructed by intervening structures (Mangold, 1997). The result is an effect referred to as multi-path fading.

2.1.1Multi-path Fading in Indoor Environment

A radio signal transmitted in an indoor environment reaches a receiver by more than only the LOS path, and from various directions. Before arriving at the receiver antenna, all paths except the LOS go through at least one order of reflection, transmission or diffraction. This is known as multi-path fading. Multipath fading is caused by constructive and destructive interference between the signal waves. They combine at the receiver antenna to a resultant signal, which can vary widely in amplitude and phase. The multi-path phenomenon produces a series of delayed and attenuated echoes for a transmitted signal. Radio channel models describe how this transmitted signal is affected by the radio channel. The frequency band of interest in this thesis is the 5 GHz unlicensed band.

Figure 2.1 shows plots of the amplitude of a typical time domain response |h(t,x)| measured at a receiver location x, and the corresponding frequency response |H(f,x)| obtained from the Fourier transform of h(t,x). The amplitude of the frequency response in dB, and the amplitude of the time response on a linear scale are shown. The frequency response consists of samples at a frequency spacing of 2.5 MHz for a frequency span of 2.0 GHz, which is centered at 5.0 GHz. Such a large frequency span is necessary for a high precision of the time response. The interval between 5.0 GHz and 5.4 GHz is shown in the figures to show the frequency selective fading for the discussed frequency band. The frequency selective nature of the channel can be seen at certain frequencies in Figure 2.1, right.

|h(t)|

|H(f)|

10ns	time	frequency
	5GHz	40MHz
		5.4GHz

Figure 2.1: (Mangold, 1997) Left: time domain response of the multi-path channel with linear scaling.Right: presentation in frequency domain, amplitude in dB. The normalized signal envelope is indicated with a 5dB/unit scale.

2.1 The Indoor Radio Channel

7

There are frequencies at which a transmission may be successful, but other frequencies may be useless for transmissions. The channel is referred to as frequency selective radio channel.

From the frequency response, a periodic time response of a maximum duration of 400 ns can be derived. An impulse response in indoor scenarios is typically smaller than 400 ns. Figure 2.1, left, is an illustration of such a periodic time response. The effects of the multi-path characteristic of the indoor environment can be clearly seen. The time domain response illustrates the multi-path propagation. Multiple delayed peaks in the signal arrive at the receiver antenna later than the first received signal. This is a result of the multi-path channel, which causes frequency selectivity. The phase is linear for most of the frequency band (Mangold, 1997).

2.1.2Time Variations of Channel Characteristics

A radio channel can be modeled as time varying linear filter for a given transmitter and receiver location. In indoor environments, when employing local area radio networks with high data rates, it can be assumed that the channel is slowly time varying compared to the transmission cycles1 of radio networks. For this reason, the impulse response can be assumed time invariant for short time intervals of some milliseconds. The radio channel is interpreted as Wide Sense Stationary (WSS) (Höher 1992).

The time varying nature of the indoor radio channel is caused either by the relative motion between the transmitter and the receiver station, or by movements of objects in the transmission path. It is generally described by the Doppler spread BD. The Doppler effect causes frequency shifts in the received signal, which makes the reception difficult. The Doppler spread BD is a measure of the spectral broadening caused by the speed of changes of the indoor radio channel. The larger the velocities of moving stations or objects in the environment, the larger the Doppler spread. For example, If a pure sinusoidal harmonic wave is transmitted with frequency fc , the signal at the receiver will have spectrum components in the range of fc ± BD/2 where BD = v fc/c, where v is the velocity of objects in near distances or of the antenna, and c is the speed of the electromagnetic waves. The coherence time Tc is a measure of the average time duration over which the indoor radio channel is stationary. The Doppler spread and the coherence time are

1A transmission cycle of a radio network is a transmission of data by one radio station, for example a data frame exchange in 802.11. Typically durations are less than 1 ms.