Typically, in radio systems, DAS is used as coverage extension and improvement. There are a great number of design principles to consider when it comes to optimizing performance in a public safety or narrowband radio DAS. In the interest of time, this entry will focus on three:
- Power Control and LInk Balancing
- Near/Far Problem
You need to understand how much 'headroom' you have when designing and commissioning the system. Headroom is the difference between the per channel power and the total power of all the channels (composite power) per frequency band (VHF, UHF, 700 MHz, 800 MHZ, 900 MHz). WHat is the benefit of calculating headroom?
Every repeater or DAS remote has a maximum output power. Typically, these are 1W, 2W, 5W varieties. If the amplifier is capable of 5W and there is one channel, then then the designer can use the full power. If there are 10 channels in the band, then each channel should be set to use 0.5W of the output power amplifier. Since the output power per channel is 1/10th of full power, the DAS antenna density must account for this to achieve the desired coverage.
In addition, when commissioning and setting the power levels and gain through the system, it is important to set as per the design. If full gain and automatic gain control are used to control the power levels, then the power per channel fluctuates with the number of channels in use. This causes the “coverage” in the DAS to constantly change or what we call “breathing”. This is considered poor practice.
Power Control and Link Balancing
The key to any radio system is having a balanced link budget. Best practices use a link budget study to determine if a radio system is balanced. We typically determine the maximum allowable pathloss in both DL and UL and it helps us determine the density of antennas required in a DAS.
We also want to determine the downlink and uplink link budget for the DAS. We must know what types of radios are being used on the DAS (e.g. mobiles, portables or both) as well as their output power. We know from above that the 10 channel system has a DL output power of approximately 0.5W per channel. The portable radio might transmit back at a full 2, 3 or 4W. Therefore, the uplink side of the system may not require the same amount of gain as the downlink, pending the review of both paths. Both paths DL and UL have to be fully planned and understood.
Imagine one user transmitting right underneath the antenna (shouting into the DAS) and another user far away from the antenna (whispering into the DAS). You still need to have enough dynamic range to accommodate both, right? This is the near–far problem.
Near/far, also known as the hear-ability problem, is the effect of a strong signal from a near signal source making it hard for a receiver to hear a weaker signal from a further source due to adjacent-channel interference. Such a situation is common in wireless communication systems.
Every UL amplifier has a certain amount of power it can receive (hear) before it starts to implement Automatic Limit Control. If the signal is too strong (the ALC being only so effective) the receiver can/will shut down. This is known as receiver overload or receiver blocking.
Many public safety repeaters and DAS systems are built the same as their cellular counterparts. This is likewise poor practice. Cellular has a certain amount of power control. That power control means that ALC is never implemented (under normal conditions) and receiver blocking never happens. However, this is not the case for radio systems.
P.25, for instance, does not have power control. Therefore, it is possible that a 4W portable radio could transmit directly underneath a DAS antenna, a location that has considerably less pathloss than a user 150 feet from the DAS antenna.
The dynamic range of the DAS, the point of when the ALC kicks in on the repeater or DAS remote, the amount of ALC and the point of receiver blocking must be known and accounted for in a DAS design and implementation.
A carefully thought-out design is key to the optimization of a public safety or narrowband radio DAS. Here are Cartel we strive to produce the most elegant deigns for your DAS. Considering how Headroom, Link Balancing, and the Near/Far problem factor into performance is just part of the equation. If you have more questions about how to plan or improve your public safety DAS, please contact us.
~Wallace Hollingshead, Engineering Manager