ISSUES

Now let us look at some of the issues WLAN makers must deal with to continue
the growth momentum that the technology has achieved in the market. It is also
necessary to understand the reason for the degradation of the WLAN signal. One of the major issues for WLANs is the degradation of radio signals by loss and
reflection. In an ideal radio channel, the received signal would consist of only a
single direct path signal, which would be a perfect reconstruction of the transmitted
signal [38–84]. However in a real channel, the signal is modified during
transmission in the channel. The received signal consists of a combination of
attenuated, reflected, refracted, and diffracted replicas of the transmitted signal. On
top of all this, the channel adds noise to the signal and can shift in the carrier
frequency if the transmitter, or receiver is moving (Doppler effect). Understanding
these effects on the signal is important because the performance of a radio system
is dependent on the radio channel characteristics.
In the following sections some general issues related to WLANs are discussed
together with their solutions. Also different channel degradation conditions and
health hazards for wireless systems are explained.
1.6.1 General Issues
General issues related to WLANs and solutions to these issues are shown in Figure
1.9. One of the major issues is security; the IEEE 802.11 standardization body is
working on this well known issue. At the time of writing the security enhancement
standard was expected to be completed by early 2005.
Another issue is spectrum. The ISM band of 2.4 GHz has become a garbage
band. Several different types of equipment including microwave ovens work in this
frequency band and they obviously interfere with each other which affects the
performance of WLANs. IEEE 802.11a works in the 5 GHz band, which is also an
unlicensed band, but efforts are ongoing towards harmonizing different wireless
technologies planned to work in this frequency band.Another issue is the actual data rate versus the hype (e.g., IEEE 802.11b
promises 11 Mbps but actual data throughput is 5 Mbps at best). The reason for
this is the overhead from the TCP/IP and MAC layer and collisions that occur. It is
extremely important that the customers are informed or educated about it. Mobility
is an issue which has been taken care of by IEEE 802.11f for within one network;
mobility between different networks with service continuity remains an issue for
the future. QoS is being tackled by IEEE 802.11e. Network management is also an
important issue for which several vendors provide solutions.
Battery life, size of device, and integration within the device are also issues of
high importance. IEEE 802.11 provides power management but vendors have also
come up with good implementations and innovative ideas to save energy and thus
battery life; one example is Broadcom. Intel Centrino on the other hand provides
an integration of WLAN and the CPU; this kind of integration might be the
direction in which we will see the future wireless/mobile communications product
moving.
1.6.2 Attenuation
Attenuation is the drop in the signal power when transmitting from one point to
another. It can be caused by the transmission path length, obstructions in the signal
path, and multipath effects. Figure 1.10 shows some of the radio propagation
effects that cause attenuation. Any objects which obstruct the line of sight signal
from the transmitter to the receiver, can cause attenuation.
Shadowing of the signal can occur whenever there is an obstruction between
the transmitter and receiver. It is generally caused by indoor and outdoor obstacles
—in-building obstacles (e.g., furniture), buildings, and hills— and is the most
important environmental attenuation factor.
Shadowing is most severe in heavily built-up areas, due to the shadowing from
buildings. However, hills can cause a large problem due to the large shadow they
produce. Radio signals diffract off the boundaries of obstructions, thus preventing
total shadowing of the signals behind hills and buildings. However, the amount of
diffraction is dependent on the radio frequency used, with low frequencies
diffracting more than high frequency signals. Thus high frequency signals,
especially, ultrahigh frequencies (UHFs), and microwave signals give for line-ofsight
conditions the highest signal strength. To overcome the problem of
shadowing, transmitters are usually elevated as high as possible to minimize the
number of obstructions.
Shadowed areas tend to be large, resulting in the rate of change of the signal
power being slow. It is termed slow-fading, or log-normal shadowing because the
distribution of the logarithm of the amplitude is normal.1.6.3.1 Rayleigh Fading
In a radio link, the RF signal from the transmitter may be reflected from objects
such as hills, buildings, or vehicles. This gives rise to multiple transmission paths
at the receiver.
The relative phase of multiple reflected signals can cause constructive or
destructive interference at the receiver. This is experienced over very short
distances (typically at half wavelength distances), and thus is given the term fast
fading. These variations can vary from 10 to 30 dB over a short distance. Figure
1.10 shows the level of attenuation that can occur due to the fading.
The Rayleigh distribution is commonly used to describe the statistical time
varying nature of the received signal power. It describes the probability of the
signal level being received due to fading in case there is no LOS.
1.6.3.2 Frequency Selective Fading
In any radio transmission, the channel spectral response is not flat. It has dips or
fades in the response due to reflections causing cancellation of certain frequencies at the receiver. Reflections of nearby objects (ground, buildings, trees, etc.) can
lead to multipath signals of similar signal power as the direct signal. This can result
in deep nulls in the received signal power spectrum due to destructive interference
for some frequencies.
For narrow bandwidth transmissions, if a strong notch in the channel
frequency response occurs at the transmission frequency then the entire signal can
be lost. This can be partly overcome in two ways.
By transmitting a wide bandwidth signal or spread spectrum as CDMA, any
dips in the spectrum only result in a small loss of signal power, rather than a
complete loss. Another method is to split the transmission up into many small
bandwidth carriers, as is done in a COFDM/OFDM transmission. The original
signal is spread over a wide bandwidth; thus any nulls in the spectrum are unlikely
to occur at all of the subcarrier frequencies. This will result in only some of the
subcarriers being lost, rather than the entire signal. The information in the lost
subcarriers can be recovered provided enough forward error corrections are sent.
1.6.3.3 Delay Spread
The received radio signal from a transmitter typically consists of a direct signal,
plus reflections of objects such as buildings, mountings, and other structures. The
reflected signals arrive at a later time than the direct signal because of the extra
path length, giving rise to a slightly different arrival time of the transmitted pulse,
thus spreading the received energy. Delay spread characterizes the magnitude of
time spread in the received multipath signal; it is defined as the second-order
moment of the channel power profile (spread-in-time of the received power).
In a digital system, multipath effects can lead to inter-symbol interference.
This is due to the delayed multipath signal overlapping with the following symbols.
This can cause significant errors in high bit rate systems, especially when using
time division multiplexing (TDMA).
Inter-symbol interference can be minimized in several ways. One method is to
reduce the symbol rate by reducing the data rate for each channel (i.e., split the
bandwidth into more channels using frequency division multiplexing). Another is
to use a coding scheme that is tolerant of inter-symbol interference such as CDMA.
1.6.3.4 Doppler Shift
When a wave source and a receiver are moving relative to one another the
frequency of the received signal will not be the same as the source. When they are
moving towards each other the frequency of the received signal is higher than the
source, and when they are moving away from each other the frequency decreases.
This is called the Doppler effect. An example of this is the change of pitch in a
car’s horn as it approaches then passes by. This effect becomes important when
developing mobile radio systems. The level of the frequency offset due to the Doppler effect depends on the
effective speed source of the transmitter with respect to the receiver and on the
speed of the propagation of the wave. Doppler shift can cause significant problems
if the transmission technique is sensitive to carrier frequency offsets (for example,
narrowband and OFDM) or the relative speed is higher (for example in low earth
orbiting satellites). With wideband DSSS systems there is less sensitivity for this
phenomenon.
1.6.4 UHF Narrowband
Narrowband is a term used to describe RF signals sent over a narrow band of
spectrum, typically 12.5 KHz to 25 KHz. UHF narrowband systems transmit on
both licensed and unlicensed frequencies, and systems based on this technology
operate at a higher power than spread spectrum systems, typically at 1 to 2 watts.
Because of the higher power, these systems have the longest transmission range of
all the WLAN technologies. However, these products have been hobbled by lack
of vendor interoperability, lower speeds, and the requirement for site licenses for
some of the licensed frequency bands.
1.6.5 Infrared
Infrared technology is an invisible beam of light that uses signals much like those
used in fiber optic links today. Infrared is reliant upon line-of-sight links between
the transmitter and receiver. Physical impediments such as walls will block the
transmission of signals, limiting infrared WLANs largely to in-room
communications. Because of the limitations of infrared technology, it is not used in
many implementations today. Infrared technology was one of the three
technologies under the IEEE 802.11 specification, but under the newer 802.11b
specification only Direct Sequence (one of two spread spectrum technologies)
technology is used.
1.6.6 Health Consideration
Until a few years ago, the analysis of possible harmful effects of electromagnetic
radiation on people was devoted mainly to power lines and radar, because of the
huge power levels involved in those systems [1, 90]. Even when mobile telephone
systems appeared, there was no major concern, as the antennas were installed on
the roofs of cars. With the development of personal communication systems, in
which users carry mobile telephones inside their coat pockets, with the antenna
radiating a few centimeters from the head, safety issues gained great importance
and a new perspective. Much research in the literature focuses not only on the
absorption of power inside the head, but also on the influence of the head on the
antenna’s radiation pattern and input impedance. However, these works have addressed only the frequency bands used in today’s systems—that is, up to 2-GHz
(mainly on the 900- and 1,800-MHz bands)—and only very few references are
made to systems working at higher frequencies, as it is in the case of wireless
broadband communications like WLANs.
The problems associated with infrared technology are different from those
posed by microwaves and millimeter waves. Eye safety, rather than power
absorption inside the head, is the issue here, because the eye acts as a filter to the
electromagnetic radiation, allowing only light and near-frequency radiation to enter
into it, and the amount of power absorption inside the human body is negligible.
Exposure of the eye to high levels of infrared radiation may cause cataract-like
diseases, and the maximum allowed transmitter power seems to limit the range to a
few meters. If this is the case, safety restrictions will pose severe limitations on the
use of infrared in wireless broadband systems, as far as general applications are
concerned. The question in this case is not that there are always problems during
system operation (e.g., mobile telephones), but the damage that may be caused if
someone looks at the transmitter during operation.
Microwaves and millimeter waves have no special effect on eyes, other than
power absorption. In WLANs, antennas do not radiate very near (1 or 2 cm) to the
user as in the mobile telephone case, thus enabling power limitations to be less
restrictive (also the case if mobile multimedia terminals are used as they are in
PDAs). However, if terminals are used in the same form as mobile telephones, then
maximum transmitter powers have to be established, similar to those for the
current personal communication systems. The standards for safety levels have
already been set in the United States and Europe, as the ones used for UHF extend
up to 300 MHz (IEEE/ANSI and CENELEC recommendations are the references).
Thus, it is left to researchers in this area to extend their work to higher frequencies,
by evaluating SAR (the amount of power dissipated per unit of mass) levels inside
the head (or other parts of the human body very near the radiating system), from
which maximum transmitter powers will be established. This may not be as
straightforward as it seems, however, because the calculation of SAR is usually
done by solving integral or differential equations using numerical methods
(method of moments or finite difference), which require models of the head made
of small elements (e.g., cubes) with dimensions on the order of a tenth of the
wavelength. This already requires powerful computer resources (in memory and
CPU time) for frequencies in the high UHF band, and may limit the possibility of
analyzing frequencies much higher than UHF. On the other hand, the higher the
frequency, the smaller the penetration of radio waves into the human body, hence
making it possible to have models of only some centimeters deep. This is an area
for further research.