8	2. Wireless Communications in Unlicensed Bands
inversely	proportional to each other (Tc ≈ 1/BD). In indoor environments, the

base band signal bandwidth is larger than the Doppler spread and the effects of time variations are negligible at the receiver. Therefore, and in accordance with the WSS channel, the wide-band propagation characteristics of radio waves within buildings can be characterized as slowly fading channels, which are stationary for transmission cycles. What is not stationary, i.e., slowly changing, is the interference caused by neighboring radio transmissions from other transmitting stations. Interferences are the limiting parameter in indoor radio systems and need to be considered in simulation, or analytical models, as accurate as possible.

2.2Orthogonal Frequency Division Multiplexing

Orthogonal Frequency Division Multiplexing (OFDM) is the transmission scheme used by the wireless LANs that are considered in this thesis. This transmission scheme is discussed in the following.

Radio transmission within buildings typically experience multi-path propagation. In a conventional serial data system, short data symbols are sequentially transmitted. The frequency spectrum of each data symbol is allowed to occupy the entire available bandwidth. As described in the previous section, in indoor environments the received signal arrives as an unpredictable set of reflections and direct waves each with its own degree of attenuation, delay and phase shift. This leads to Inter-Symbol Interferences (ISI) between consecutively transmitted data symbols due to the signal delay spread at the receiver. Multi Carrier Modulation (MCM) techniques like OFDM transmit data by dividing the high symbol rate stream into several low symbol rate streams, and by using these sub-streams to modulate different sub-carriers (Mangold 1997). The symbol duration of each sub-stream is then higher than the channel delay spread and the maximum excess delay of delayed signals. By using a large number of OFDM sub-carriers, immunity against the multi-path effect can be provided (Cimini, 1985). OFDM, having densely spaced sub-carriers with overlapping spectra of the modulated subcarriers, abandons the use of steep band pass filters to detect each sub-carrier as it is used in classical Frequency Division Multiplexing (FDM) schemes. It offers therefore a high spectral efficiency. There are extensions of OFDM, which are not used in HiperLAN/2 and 802.11a, towards a more flexible MCM. The enhanced MCM is based on Wavelet transforms, where the individual sub-carriers have different bandwidths. This aims to provide an accurate adaptation of the transmission scheme, in terms of data-throughput per sub-carrier, to the time-variant radio channel. In indoor radio environment, signals coming from multiple indirect paths added to the direct path mean that the condition of orthogonality

2.2 Orthogonal Frequency Division Multiplexing

9

between sub-carriers is no longer fulfilled, which results in Inter-Channel Interference (ICI). However, multi-path propagation can cause ISI as well. The part of the OFDM symbol that carries the information is in the following referred to as block instead of symbol, to distinguish it from the symbols per sub-carrier. Adding a guard interval Tg before the block period Tb can circumvent both effects. A new block duration, Tb'=Tg+Tb is then obtained. This duration represents what is known as OFDM symbol duration. The guard interval is typically smaller than Tb/4. If Tg is longer than the maximum channel excess delay, the subcarriers are still mutually orthogonal inside the effective block interval (Tg ... Tb'). Adding a guard interval means that a cyclically extended OFDM symbol is transmitted. Pre-pending a cyclic prefix in an OFDM symbol aids to remove the effects of the multi-path channel by making the OFDM symbol to appear periodic in time. Only with (nearly) periodic discrete-time signals, a convolution of two signals is equivalent to multiplying the Fourier transforms (frequency responses) of the respective signals, here an OFDM symbol and the channel impulse response. OFDM symbols are created by an Inverse Fast Fourier Transform (IFFT) of the data to be transmitted. The frequency response of an OFDM symbol generated with an IFFT is the original data, thus each original data symbol is multiplied by a single complex number when it is transmitted over the radio channel. Equalization at the receiver becomes very simple, or even avoidable.

There are two drawbacks of the cyclic prefix worth to be mentioned (Mangold et al., 2001f). One is that redundant data is transmitted over the radio channel reducing the maximum data throughput on top of OFDM. The other drawback is that the cyclic prefix of duration Tg leads to a power loss, as the receiver only uses the energy received during the time Tb. The energy corresponding to Tg is discarded. A power loss αg must be taken into account:

	αg =	Tb	(2.1)
		Tb'

In HiperLAN/2 and 802.11a, one 52-sub-carrier OFDM symbol occupying 16.6 MHz has a duration of 4 µs, and Tg=800 ns (or, optionally in HiperLAN/2, Tg=400 ns) results in αg = 0.8 (respectively 0.9). With Tg=800 ns, the useful received signal energy per symbol, Eav, is 20 % less than the received signal energy without cyclic prefix. Now, it is to be differentiated between background noise N and the cumulated interference level ΣI. The interference ΣI is as well created by electromagnetic harmonic sinusoidal waves, as is the wanted signal. Integrating the signal energy and the interference energy over a shorter time, i.e., Tb' instead of Tb reduces the received signal energy by αg, and to an unknown extend the

10	2. Wireless Communications in Unlicensed Bands

cumulated interference level. Note that signals arrive from different locations, and transmitters. Therefore, and because of the tolerant specifications in HiperLAN/2 and 802.11a in terms of frequency accuracy, the worst-case scenario is assumed that ΣI is not affected by the guard interval, and

E		αgC
av	=		.	(2.2)
N0		( N + ∑I )

The 802.11a and HiperLAN/2 Physical Layers (PHY) are almost identical. They apply a 52-carrier OFDM with convolution coding and linear modulation schemes that can be adaptively chosen based on QoS requirements and radio channel conditions. 48 carriers out of 52 are used for data transport (IEEE 802.11 WG, 1999a; ETSI, 2000a). The remaining four sub-carriers of the OFDM symbols are used for pilot symbols. The 802.11a task group of the 802.11 working group accepted the OFDM transmission scheme defined for HiperLAN/2, which facilitates the development of coexistence and interworking mechanisms between the two systems. It is worth noting that the IEEE 802.11 standard specifies a MAC protocol without the definition of the PHY for 5 GHz. 802.11a as a supplement of 802.11 specifies the PHY for the 5 GHz OFDM transmission (IEEE 802.11 WG, 1999a). The Table 2.1 summarizes numerical values for the main parameters of the OFDM transmission system as defined by the two standards.

Table 2.1: Numerical values for the OFDM parameters of 802.11a and HiperLAN/2 (IEEE 802.11 WG, 1999a; ETSI, 2000a)

Parameter	Value
Sampling rate 1/T :	20 MHz
OFDM block duration Tb :	64·T = 3.2 µs
Guard interval duration Tg :	16·T = 0.8 µs (0.4 µs optional in HiperLAN/2)
OFDM symbol duration Tb‘=Tg +Tb :	80·T = 4 µs
Number of data sub-carriers :	48
Number of pilot sub-carriers :	4
Total number of sub-carriers Ntotal :	52
Sub-carrier spacing Df :	1/Tb=0.3125 MHz
Spacing betw. the outmost sub-carriers :	(Ntotal-1) · Df =15.9375 MHz

2.3 The 5 GHz Band

11

2.3The 5 GHz Band

The 5 GHz unlicensed band comprises frequency bands between 5.15 GHz and 5.825 GHz. Figure 2.2 illustrates this spectrum as it is defined for the U.S. and Europe.

A spectrum of 300 MHz has been released in the U.S. for the Unlicensed National Information Infrastructure (U-NII) band. A spectrum of 455 MHz is available in Europe. In an unlicensed band, regulators permit the operation of any radio communication systems, in contrast to an allocation of spectrum on a licensed base. The restrictions that regulators put on the candidate systems are radio parameters such as limits of the radiated power, out of band emissions, antenna characteristics and the communication services that are supported.

Different center frequencies fc are defined for the 5 GHz unlicensed band. In the U.S., three U-NII bands are defined between 5.15 GHz and 5.825 GHz leading to 12 frequency channels for operation. In Europe, current regulations allow the operation of wireless LANs at 19 channel frequencies. However, 11 more channels will be available in the U.S. by end of 2003.

The channelization of 20 MHz is not mandatory in the U.S., as part of the regulation. Higher antenna gains are permitted with corresponding reduction of transmitter power. In Europe, wireless LANs must use full spectrum range in order to share the spectrum with radar systems, based on Dynamic Frequency Selection (DFS) and Transmitter Power Control (TPC). However, in the lower part of the spectrum, below 5350 MHz, wireless LANs are permitted to operate without these complicated schemes (REGTP, 2002).

2.4Error Model for the OFDM Transmission applied in the 5 GHz Unlicensed Band

For the OFDM transmission technique applied in the 5 GHz unlicensed band, an error model was developed in Mangold et al. (2001f). This model makes use of an analytical approximation of packet errors of any length, which was developed in Qiao and Choi (2001). This approximation is summarized in Appendix C, and is in this thesis referred to as the Qiao-Choi transmission error probability analysis. The error model for the OFDM transmissions is of importance for this thesis, because it is implemented in the WARP2 simulation tool, as described in Appendix A. The WARP2 simulation tool is used for the analysis of 802.11 in this thesis.

12	2. Wireless Communications in Unlicensed Bands
In the following, network	elements such as Access Point (AP), Central Control-

ler (CC), Wireless/Mobile Terminal (WT/MT) or Station are referred to as a station. A combination of coding and modulation schemes is called a Physical layer Mode (PHY mode), which may be dynamically selected for transmission by a station. A burst is called the single transmission of a frame including preambles and headers initiated by one station. A burst comprises a preamble and a subsequent frame sent as a number of OFDM symbols. Preambles are used for time and frequency synchronization and frame identification. As part of the error calculation, it is assumed that a frame always is transmitted completely at a specific PHY mode and at a constant power level. After a preamble, there follows a number of OFDM symbols sent at the basic and most protected PHY mode, for example, the SIGNAL field as part of the Physical Layer Control Protocol (PLCP) header in 802.11a. This is neglected for the sake of simplicity of the model. See Figure 2.3 for an illustration of a burst. The power of a burst received at the addressed station is referred to as C; whereas the sum of unwanted interference is generally noted as ΣI. Background noise is represented as N, which is in the area of -95dBm for HiperLAN/2 and 802.11a OFDM receivers.

U-NII Regulation, U.S. (12 freq. channels):

< 50 mW

(or 4dBm+10log(B)) antenna gain <6dBi

5150 .. 5250 MHz and

5250 .. 5350 MHz

	11 more channels in 2003
	< 250 mW	< 1000 mW
		(17 dBm+10log(B))
	(or 11 dBm+10log(B))
		antenna gain <23 dBi
	antenna gain<6dBi
		5725 .. 5825 MHz

B = 26 dB emission bandwidth

European Regulation (19+4 freq. channels):

<1000mW, indoor/outdoor

<200mW, indoor

5150 .. 5350 MHz	5470 .. 5725 MHz	ISM band
20 MHz channelization	20 MHz channelization	(< 25mW)

	Tx	52-carrier OFDM power density spectrum,
		as defined for HiperLAN/2 and 802.11a
	power

out of channel emissions

radiated power

frequency

Figure 2.2: The 5 GHz band for wireless LANs in the U.S. and Europe. Higher antenna gains are permitted with corresponding reduction of transmitter power. In Europe, wireless LANs must use the full spectrum in order to share the spectrum with radar systems efficiently (an exception from this rule is defined in REGTP (2002)).