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Indoor Radio Planning: A Practical Guide for GSM, DCS, UMTS, HSPA and LTE, Second Edition by Morten Tolstrup

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11.11 Timing Issues in DAS for Tunnels

For long optical fiber deployments in a DAS system one must be careful with the delay of the DAS and timing in the system. This is something that is often forgotten but in an extreme case this could have a fatal impact on the service if one fails to realize this.

It is a fact that all cables, coaxial or optical fiber, will have a certain delay; the velocity of the electric signal on coaxial cable and the light in the optical fiber will simply add delay depending on the specification of the cable or optical fiber. In addition to this the DAS hardware, Main and Remote Unit will also add delay to the signal due to internal signal processing, amplifiers, etc.

The effect of this delay will be to cause the mobile network to react as if the mobiles served by the DAS are more distant than the actual physical distance from the cell/ DAS antenna. In DAS systems with active elements, Remote Units, Repeaters, antennas, etc., radiating from the same cell as the adjacent system, covering the area ordering the antenna, you might cause some unwanted effects such as inter symbol interference, co-channel interference, etc. depending on delay offset of the simulcasted signals the type of mobile service the system supports.

For normal indoor DAS solutions this timing offset effect will rarely be noticed, but for installations with several kilometers of optical fiber, and sometimes daisy chaining of active DAS equipment, it is very important to include it in the ...

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