Indoor DAS Channel and the concept of Channel impulse response
Introduction
Understanding the characteristics of the communication channel is vital not only for designing wireless communication systems and devices but also for planning and optimizing both outdoor and indoor communication deployments.
The term "channel" can have different meanings depending on the context. In the context of this discussion, the channel refers to the wireless propagation medium between a transmitter and a receiver equipment.
Typically, system designers statistically model this aspect of the communication system in advance, but they have limited control over it during actual deployments. However, they must construct systems that anticipate and mitigate expected communication channel characteristics and anomalies as much as possible. The task of optimizing and adapting to different channels falls to deployment-specific optimization and performance engineers. The success of a communication system or standard largely depends on how it was designed and its ability to adapt to different channels.
Developing channel models that represent different expected communication channels helps in anticipating and mitigating the effects that reduce communication quality, such as:
- Expected path loss range: Determined by the distance the signal travels and the propagation through various media.
- Multipath nature: Caused by the propagation environment, including reflections off buildings, mountains, cars, and other obstacles.
- Mobility: Communication devices and systems in motion face the additional complexity of frequency deviation due to the Doppler effect.
- Expected noise: Resulting from controlled and uncontrolled transmitters, frequency reuse, adjacent channels, etc.
Representation and Measurements
One concept that we should know while trying to understand the channel is the concept of channel impulse response (CIR).
CIR provides full description of how the channel affects the basic communication signal component, The Impulse signal. Since any communication signal can be represented mathematically in the time domain into a series of “impulses”. Passing these series of impulses into the CIR channel representation, The expected received signal can be construct from the individual responses of each transmitted impulse which will include the expected effect of the propagation channel.
Additionally, channel characteristics can be equally measured and described in the Frequency Domain using the equivalent of the CIR, the channel Frequency Response.
We observe said effects in time and frequency domains providing insight into how to mitigate the resultant challenges
Cell Tower Case
For Cellular coverage where transmission and receiption happens over longer distances, multipath is signinifcantly apparent compared to indoor coverage cases. This simply means we receive our desired signals along with delayed versions because some paths the signal can take are longer than others. The same effect is observed as frequency fading in the frequency domain also referred to as a frequency selective channel. Frequency fading could potentially degrade the required performance.
As a result, handling and understanding delays prove crucial in successful indoor and outdoor RF planning and optimization.
These effects also vary from one technology standard to another and they They are most noticeable in wideband communications.
Effect of DAS on the indoor channel
When it comes to indoor coverage planning, distributed Antenna Systems (DAS) are typically used to enhance and provide the required network coverage for venues that otherwise cannot be covered by outdoor towers with the required coverage KPIs. While the indoor environment is known to have a particular multipath nature, the addition of DAS into the channel adds a different set of multipath charecteristics that are different in nature and magnitude compared to the standard indoor channel where one transmitter or Access Point transmits a distinctive signal from the other transmitters. In a DAS installation however, the same signal is ditributed and transmitted simultaniously from group of antennas with overlapping coverage areas creating a distinctive type of multipath dominated by the delayed versions of the signal transmitted from the DAS antennas.
Simulating CIR
Engineers use Wireless Design software such as FullRays Design to optimize designs and simulate the expected indoor design outcome. Fullrays uses ray tracing techniques to accurately account for DAS delays and their effects to account for issues that may arrise from using Active amplifiers and fiber cables. a sample of which can be seen in the below graph
Emerging Applications
One emerging application that depends on precise channel knowledge and measurements is the precise indoor positioning. A sounding reference signal (SRS) transmitted on the uplink in of LTE communication system is used to produce an RF signature of the particular indoor location. The SRS is originally used in the LTE standard to estimate the channel quality for a individual users across the channel bandwidth to properly allocate resource blocks avoiding channel fading anomalies. With the advances of Machine learning techniques, Indoor positining applications utilising AI techniques are built on the SRS or its corresponding CIR measurement.