388 Chapter 14
Real Time TDMA Formats
Enhanced Observed Time Difference (E-OTD)
Enhanced Observed Time Difference (E-OTD)
Overview
The Enhanced Observed Timing Difference (E-OTD) positioning method relies upon measuring the time at
which signals from the Base Transceiver Station (BTS) arrive at two geographically dispersed locations–the
mobile phone/station (MS) and a fixed measuring point known as the Location Measurement Unit (LMU)
whose location is known. The position of the MS is determined by comparing the time differences between
the two sets of timing measurements.
Multiple ESGs are used with a remote interface to provide mobile manufactures the ability to test handsets
equipped with the E-OTD feature. The system will simulate a real world network complete with a serving
cell (Agilent E5515C (8960 Series 10) Wireless Communications Test Set) and neighboring cells (ESG
Vector Signal Generator) that can be used to test E-OTD equipped handsets.
The ESG has no front panel user interface for the E-OTD feature, so signal setup is done using the GPIB
interface.
Basic GSM Frame Structure
TDMA Time Slot
A TDMA frame is composed of eight 577 micro second time slots (0–7). The basic TDMA frame duration is
4.615 msec. Each time slot contains 148 bits of information and contains a guard period of 8.25 bits. This
guard period is provided for the mobile station transmission to be attenuated between bursts as defined in the
3GPP standard. Of the 148 bits, some are used to carry data, others are used for control and synchronization.
Refer to Figure 14-1 on page 389.
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