 deliver reasonable quality (speech or data) in a GSM network. Some problems are created as stated at different distances from the BTS for each mobile communicating with the BTS. Delay times in round-trip propagation and the attenuation of the signal are different. To solve this problem, timing advance (up to 63 bit times) can be used to compensate for various distances and delays in the air. In effect, this creates an overall average of delay that the systems can deal with. By having a station transmit a few bit times early, the BTS is able to compensate for the arrival (either early or late) and address the proper time slot. To illustrate how to compensate for the variable delays and the timing that is required, the timing advance is used, as shown in Figure 6-3. In this scenario, a normal burst for two different mobiles is being sent through the airwaves. The first mobile's burst arrives in time and is slotted into time slot number 6 properly. However, transmitter number 2's burst arrives late, and overlaps time slots. It is partially in the time slot for number 7 and a portion of the burst falls into the time domain for time slot number 0. The base station notices that the burst is arriving late. Therefore, it sends a directive on the downlink control channels to the mobile. The directive tells the mobile to transmit its data earlier SO that the delay is compensated for and the burst arrives within the appropriate timing, as shown in Figure 6-4. Now the transmitted Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio and MS-PCUSN Interfaces Chapter 6 too late! Figure 6-3 The data bursts are arriving late and 0123456 0012 overlapping a time slot. BURST overlaps with other timeslot BURST Figure 6-4 012345 TA solves the problem. BURST arrives on time due to delay early Base station tells mobile to send early. normal burst will arrive in ti