A second impairment common in a wireless communications channel is interference from adjacent carriers. While government regulations and standards bodies regulate emissions into adjacent bands to limit interference, regulations do not entirely eliminate it. As a result, modern receivers apply filtering to account for adjacent-channel interference. In Figure 3, a single tone adjacent to a WCDMA carrier is illustrated.

The impact of interference on the modulated signal depends upon both the interference power and its frequency offset. In general, interference is more problematic when it is higher in power and closer to the band of interest. To illustrate this, we compare constellation plots of a 16-QAM signal in the presence of a single-tone carrier that is 4 MHz from the center frequency.

As Figure 4 illustrates, EVM increases with the power of the adjacent-channel interference. In the rightmost graph, we see that as EVM increases dramatically the receiver loses carrier lock.

One of the most interesting characteristics of the wireless environment is multipath signal propagation. To give you an idea how multipath propagation affects a receiver, imagine listening to voice at the end of a long hallway. In this scenario, the speaker's voice will travel along multiple paths as it bounces off of walls, chairs and other objects. Because some paths take longer to travel than others, your ear (the receiver) will hear sound that has been slightly distorted.

In a wireless communications channel, the echo characteristic described above is also true of electromagnetic waves. In this environment, reflected waves will arrive at the receiver slightly after the direct path signal. The result is a phenomenon called intersymbol interference (ISI) and it degrades receiver performance. A diagram illustrating this effect is shown in Figure 5.

In NI LabVIEW, multipath dis-tortion can be modeled through a Rician fading profile. The Rician profile is a mathematical model used to represent multipath characteristics where there is still a direct line of sight from the transmitter to the receiver. Note that a similar multipath fading profile, Rayleigh, is also commonly used to model multipath propagation.

When applying a Rician fading profile, it is important to consider two characteristics of the environment. The first characteristic, Doppler shift, describes the apparent frequency offset caused when a carrier's wavelength “appears” different due to receiver mobility. The second characteristic is the Rician parameter; also known as the “K” parameter. This parameter represents the ratio between the direct path strength and aggregate reflected path strength. The smaller the K parameter, the smaller the amplitude of the direct path signal.

To illustrate this affect, Figure 6 compares the K parameter for a 16-QAM signal with a symbol rate of 3.84 Msymbols per second (Msps). We applied a Doppler shift of 5 KHz for each plot.

As Figure 6 illustrates, the weaker the line-of-sight signal characteristic, the more prominent the effects of multipath fading. It also indicates that ISI causes symbol spreading, increasing the EVM.