Internal Sources of Interference

Internal Sources of Interference
Thus far, this chapter has focused on sources of interference that are external to the wireless network. As
mentioned in the introduction, a number of challenges arise within a wireless network due to the nature of
wireless transmissions. These sources of interference include multipath and channel noise. Both can be
engineered out of the network.
Figure 5-2: Signal interference on 802.11 wireless networks
Multipath and Fade Margin Multipath occurs when waves emitted by the transmitter travel along a
different path and interfere destructively with waves traveling on a direct line-of-sight path. This is
sometimes referred to as signal fading. This phenomenon occurs because waves traveling along different
paths may be completely out of phase when they reach the antenna, thereby canceling each other.
Because signal cancellation is almost never complete, one method of overcoming this problem is to
transmit more power. In an indoor environment, multipath is almost always present and tends to be
dynamic (constantly varying). Severe fading due to multipath can result in a signal reduction of more than
30 dB. It is therefore essential to provide an adequate link margin to overcome this loss when designing a
wireless system. Failure to do so will adversely affect reliability. The amount of extra RF power radiated
to overcome this phenomenon is referred to as fade margin or system operating margin. The exact
amount of fade margin required depends on the desired reliability of the link, but 802.11 protocols usually
have a 15 to 20 dB fade margin, ensuring a 95 percent confidence interval.[6]
One method of mitigating the effects of multipath is ensuring antenna diversity. Since the cancellation of
radio waves is geometry dependent, using two (or more) antennas separated by at least half of a
wavelength can drastically mitigate this problem. Upon acquiring a signal, the receiver checks each
antenna and simply selects the antenna with the best signal quality. This reduces, but does not eliminate,
the required link margin that would otherwise be needed for a system that does not employ diversity. The
downside is that this approach requires more antennas and a more complicated receiver design. Another
method of dealing with the multipath problem is via the use of an adaptive channel equalizer. Adaptive
equalization can be used with or without antenna diversity.
After the signal is received and digitized, it is fed through a series of adaptive delay stages that are
summed together via feedback loops. This technique is particularly effective in slowly changing
environments such as transmission over telephone lines, but it is more difficult to implement in rapidly
changing environments like factory floors, offices, and homes where transmitters and receivers are
moving in relation to each other. The main drawback is the impact on system cost and complexity.
Adaptive equalizers can be expensive to implement for broadband data links.
Spread spectrum systems are fairly robust in the presence of multipath. Direct sequence spread
spectrum (DSSS) systems reject reflected signals that are significantly delayed relative to the direct path
or strongest signal. This is the same property that enables multiple users to share the same bandwidth in
Code Division Multiple Access (CDMA) systems. However, 802.11's DSSS does not have enough
processing gain and orthogonal spreading codes to do this. Frequency-hopping spread systems (FHSS)
also exhibit some degree of immunity to multipath. Because an FHSS transmitter is continuously
changing frequencies, it always hops to frequencies that experience little or no multipath loss. In a severe
fading environment, the throughput of an FHSS system will be reduced, but it is unlikely that the link will
be lost completely. OFDM systems such as 802.11a and 802.11g transmit on multiple subcarriers on
different frequencies at the same time. Multipath is limited in much the same way in that it is limited in an
FHSS system. Also, OFDM specifies a slower symbol rate to reduce the chance a signal will encroach on
the following signal, minimizing multipath interference.
Channel Noise When evaluating a wireless link, three important questions should be answered:
l How much RF power is available?
l How much bandwidth is available?
l What is the required reliability (as defined by the bit error rate [BER])?
In general, RF power and bandwidth effectively place an upper bound on the capacity of a
communications link. The upper limit in terms of data rate is given by Shannon's Channel Capacity
Theorem, as shown in Equation 5-1:
(5-1)
where:
l C = channel capacity (bits per second)
l B = channel bandwidth (hertz)
l S = signal strength (watts)
l N = noise power (watts)
Note that this equation means that for an ideal system, the BER will approach zero if the data
transmission rate is below the channel capacity. In the real world, the degree to which a practical system
can approach this limit is dependent on the modulation technique and receiver noise.
For all communications systems, channel noise is intimately tied to bandwidth. All objects that have heat
emit RF energy in the form of random (Gaussian) noise. The amount of radiation emitted can be
calculated by Equation 5-2:
(5-2)
where:
l N = noise power (watts)
l k = Boltzman's constant (1.38×10-23 J/K)
l T = system temperature (K), usually assumed to be 290K
l B = channel bandwidth (hertz), predetection
This is the lowest possible noise level for a system with a given physical temperature. For most
applications, temperature is typically assumed to be room temperature (290K). Equations 5-1 and 5-2
demonstrate that RF power and bandwidth can be traded off to achieve a given performance level (as
defined by BER).[7] This implies that using a lower data rate that occupies a lower channel bandwidth will
provide better range.