Fading
Overview
During wireless transmission of a signal from a transmitter to a (moving) receiver, the signal can experience fading. There are different fading effects such as shadowing or multipath propagation. Shadowing can be caused by objects (e.g. hills, tunnels) that obstruct the signal path between the transmitter and the receiver. The resulting amplitude change seen by the receiver is slow. This kind of fading is thus called slow fading and is modeled using a lognormal fading profile. Multipath propagation is primarily present in urban environments where the transmitted signal can be reflected or scattered from diverse objects such as buildings or moving cars. This creates multiple propagation paths. Along each path, the signal can experience a different delay, attenuation, phase shift or Doppler frequency shift. At the receiver, these signals interfere either constructively or destructively, which results in fast fluctuations of the received signal amplitude. This kind of fading is thus called fast fading and is modeled using e.g. Rayleigh or Rician fading profiles.
Fading can cause poor performance in a communications system, since it strongly influences the signal-to-noise ratio of the transmission channel. Bit error ratios will increase as the signal-to-noise ratio drops due to strong fading. Severe drops in the signal-to-noise ratio may even lead to a temporary failure of communications. For this reason, it is important to test receivers under fading conditions during design and conformance test stages. This requires well-known and repeatable test conditions which can be provided by fading simulators generating realistically faded test signals in the lab.
The MIMO technology relies on statistically independent fading in the multiple transmission paths to increase signal diversity. Thus, MIMO systems need to be tested under (multipath) fading conditions. As MIMO is implemented in all modern communications systems for increasing data throughput, fading simulators must also be able to provide realistic MIMO fading scenarios.
Rohde & Schwarz Test Solution
The R&S®SMU200A and R&S®AMU200A signal generators offer integrated realtime fading in the baseband for realistic simulation of up to four fading channels in just one instrument. Up to 40 fading paths per fading channel can be simulated simultaneously. Each fading path can be individually delayed and attenuated. For each fading path, a fading profile such as Rayleigh, Rician, pure Doppler or Gauss can be selected. In addition, lognormal fading (slow fading) can be superimposed onto these fast fading profiles. Our test solution can thus realistically model rural or urban environments for advanced receiver testing.
The user can select various preset fading scenarios in accordance with test scenarios stipulated in communications standards such as 3GPP, 3GPP LTE, WiMAX™, WLAN, etc. The implemented fading scenarios simulate stationary as well as dynamic propagation conditions (e.g. birth/death or high-speed train scenario). Besides the preset scenarios, the user can always configure custom fading scenarios to meet specific test needs.
Also MIMO fading up to 2x2 is supported within in a single instrument. Up to four complex fading channels can be simulated including channel correlations. MIMO fading for 4x2 or 2x4 can be implemented by connecting two instruments. The user can select preset MIMO fading scenarios specified for LTE (EPA, EVA, ETU profiles) or WiMAX™ (ITU profiles).
Both signal generators offer options for standard-compliant signal generation (as baseband options) for all modern communications systems such as LTE, HSPA+, GSM, WiMAX™, WLAN, etc. The baseband signal is faded digitally and AWGN can be added. Alternatively, an analog or digital I/Q input signal can be faded. The faded signal can be output as an analog or digital I/Q signal or as an RF signal.
Fading Scenarios
The stationary configurations differ in terms of the number of paths, the delay resolution and the available RF bandwidth. The number of configured paths and the configured delays remain constant over time.
| Stationary configurations | Description |
|---|---|
| Standard delay | Simulation of up to 40 paths per channel. The delay resolution is 10 ns. |
| Fine delay 30 MHz | Simulation of up to 24 paths per channel. The delay resolution is 0.01 ns. The RF bandwidth is limited to 30 MHz. |
| Fine delay 50 MHz | Simulation of up to 16 paths per channel. The delay resolution is 0.01 ns. The RF bandwidth is limited to 50 MHz. |
The dynamic configurations are in line with test cases defined in some digital standards (e.g. 3GPP, LTE). The number of paths and/or the delays vary over time.
| Dynamic configurations | Description |
|---|---|
| Birth/death propagation | A path appears (birth) while another path disappears (death) to simulate appearing and disappearing signals. |
| Moving propagation | Path delays change dynamically, e.g. for simulating a moving receiver. |
| Two-channel interferer | Either the path delay changes slowly or the path appears or disappears in alternation. |
| High-speed train | Simulation of a very rapidly moving receiver passing an antenna. |
The following fading profiles can be chosen for simulating different propagation conditions:
| Fading profiles | Description |
|---|---|
| Static path | Simulation of a static path. The signal amplitude is constant. |
| Constant phase | The phase of the signal is rotated, e.g. by 180° for simulating reflection off a metallic surface. The signal amplitude is constant. |
| Pure Doppler | The signal is shifted in frequency simulating a relative speed between transmitter and receiver. The signal amplitude is constant. |
| Rayleigh | The signal amplitude follows a Rayleigh distribution simulating multipath propagation without a direct line of sight. |
| Rician | The signal amplitude follows a Rician distribution simulating multipath propagation with a strong direct line of sight. |
| Lognormal | The signal amplitude follows a lognormal distribution simulating shadowing (slow fading). |
Further special fading profiles are offered as an option including Gauss (sum of two Gaussian distributions) and Gauss DAB (Gaussian distribution, shifted in frequency) both in line with the DAB standard as well as WiMAX™ Doppler (rounded Doppler PSG model) and WiMAX™ Rice (WiMAX™ Doppler plus pure Doppler) in line with the WiMAX™ standard.
Standardization
All modern communications standards stipulate receiver tests under fading conditions. The standards include specific fading test scenarios for modeling pedestrian, vehicle and even high-speed mobility in rural, urban and indoor environments. Various simulation parameters such as number of fading paths, path attenuation, path delays, and fading profile per path are specified in the test scenarios. Our fading solutions provide these test scenarios as preset settings. Standard-compliant test scenarios are available for the following standards: 3GPP, LTE, LTE MIMO, WiMAX™, WiMAX™ MIMO, WLAN, DAB, GSM, CDMA, TETRA, 1xEV-DO, NADC, PCN.
