What is Channel Emulation?
In wireless communication, a signal transmitted undergoes many changes before arriving at the receiver. In most cases, the quality of the signal at the receiver gets deteriorated from the original signal. This process of signal deterioration is called ‘Fading‘. It is a variation of the attenuation of the signal due to variables like time and geographical position. Channel fading is the process where a communication channel undergoes fading. In this blog, we explore the concept of Channel Emulation and how it is helpful in the validation process.
Types of Fading
Fading can be broadly classified as Slow fading and Fast fading. The terms slow and fast fading refer to the rate at which the magnitude and phase change imposed by the channel on the signal changes. The coherence time is a measure of the minimum time required for the magnitude change or phase change of the channel to become uncorrelated from its previous value.
- Slow fading occurs when the coherence time of the channel is large relative to the delay requirement of the application
- Fast fading occurs when the coherence time of the channel is small relative to the delay requirement of the application
Once the signal has been transmitted, it can either reach the receiver directly or be reflected or diffracted by many objects between them as shown in the figure below. The received signals are either constructively or destructively combined at the receiver.
Causes of Fading
Fading can occur due to various factors and environmental conditions. Some of the common causes are:
- Path Loss
- The fluctuation of the received signal power
- Fluctuations in signal phase
- Variations of Angle of arrival of the received signal
- Reflection and diffractions from various object
- Received power variation created by multipath
- Frequency Shift(Doppler shift)
A few techniques can be used in order to overcome signal fading:
- Diversity reception and transmission.
- MIMO and OFDMA
- Rake receivers.
- Space-time codes.
- Forward Error Correction and Interleaving
Any new device or technology needs to be tested to see how well it performs in the real world. In telecommunication, one way of testing a cell phone and base station would be Over The Air Field trials. To test the capability of a network it is important to see how they perform in different scenarios. But there are many limitations to this method. It can be expensive and time-consuming. Also, the amount of data collected from field trials is extremely limited.
In order to make OTA testing easier, Channel Emulation is a common approach adopted. A variety of channel conditions and complex multiple-antenna algorithms make a channel emulator a key component in developing and testing. In this way, Channel Emulation eliminates the need to test the devices outside in harsh conditions. Instead, the testing can be done inside a lab with a transmitter, receiver, and emulator in between. There are many commercially available Channel Emulators for this purpose. Transmitting signals are fed to the emulator using cables. The emulator will modify these signals to replicate the conditions of a real wireless channel. Configuration can be made to reproduce a variety of wireless scenarios such as urban microcell, rural macro-cell, mobility patterns, and also different weather conditions. The resulting signals are then fed to the receiver using cables. The different scenarios can be easily programmed and also are highly reproducible making it easier to know the errors that may occur and where the design might have gone wrong. The different technologies like 2G, 3G, 4G, and Wi-Fi systems have all been developed using channel emulators.
Another solution would be to use UE Simulators like Simnovator which has an integrated Channel Emulation capability. This will be cost-effective and avoids having another external device. The UEs can be configured to be near, middle, or far coverage areas of the base station. They can also be configured to move with different velocities creating a mobility scenario.
However, the traditional emulation model is not suitable for testing 5G systems. Due to the millimeter-wave bands of 5G, there is a huge amount of bandwidth. Even the number of antennas on the transmitter and receiver is extremely large in comparison to 4G systems and integrated Channel Emulation can only be used for antennas in the range of 16×16 MIMO. 5G technology might easily have 256 antennas on the base station side and 64 antennas on the UE side and connecting that many cables from the transmitter to the emulator and then the emulator to the receiver becomes quite tedious. Even if that many cables are connected, it is not possible to connect phased-array antennas with the available mechanism. Phased-array antennas are expected to be an important aspect of 5G, as they will be used to overcome the high path losses at millimeter-wave frequencies. The computational complexity in building the emulator becomes high with an increasing number of antennas and larger bandwidths.
Hence a lot of research is being carried out in order to design a Channel emulator suitable for 5G. Some of the studies have been briefly described below.
- The new emulation paradigm also emulates the antennas in addition to emulating the wireless channel. In this paradigm, the front end is eliminated. Instead of the transmitter telling its phased-array which direction to beam-form, the transmitter tells the emulator which direction to beam-form. Similarly, instead of the receiver telling its phased-array where to beam-form, the receiver tells the emulator which direction to beam-form.
- Use of Phase Matrix Unit between the Device Under Test (DUT) and Channel Emulator. The number of DUT antennas is high and consequently, the need for fading channel emulator (CE) resources becomes high. An approach can be taken to reduce the number of independent fading channels to be emulated. This can be done by using a phase-shifting and combining unit i.e. Phase Matrix Unit.
- Implementation of a real-time hardware-in-the-loop (HIL) wideband high-velocity channel emulation platform for the performance evaluation and verification of 5G mmWave systems. A novel spectral splitting & stitching method and channel partitioning algorithm were developed to synchronously combine up to eight 160 MHz sub-channels. The HIL platform was calibrated in the time and frequency domains.
A variety of Channel Emulators are now commercially available for 5G testing as well. A suitable design can be chosen based on the requirements. Hence, it can be concluded that Channel Emulation techniques are a cost-effective, time-saving alternative to the tedious field trials.