Time Slots.
Cell Sizes.
Source
Another technical difference relates to the fact that GSM uses short time slots, so each handset communicates in frequent short bursts. CDMA handsets, on the other hand, communicate using much longer bursts. This impacts on the forms of cell-phone interference mitigation techniques that can be employed by AGPS solutions in the two cases as will be discussed in a later section.
Cell Sizes.
A third technical difference relates to the fact that the GSM technology has a limitation on its cell sizes of around 35 kilometers in radius. CDMA cell sizes, on the other hand, are only limited by transmission power and relevant standards (such as CDMA code-phase search ranges). Hence, they can be much larger. If the coarse location assistance is derived from the cell location, its uncertainty can be much larger in a CDMA system than in a GSM system. In practice, however, this is not a significant factor because the most-demanding AGPS environments tend to be in inner-cities where cell sizes can be limited to a few kilometers in radius.
AGPS Standards
Both the CDMA and the GSM communities have developed standards for control plane AGPS messaging (TIA/EIA/IS-801-1, 3GPP2 C.S0022-0-1, 3GPP TS 25.331) and for minimum operational performance of AGPS handsets (TIA 916, 3GPP2 C.P9004-0, 3GPP TS 25.171 V6.0.0). There is considerable similarity between the assistance fields included in the two protocols. The minimum performance standards are measured in both cases using five separate statistical tests.
The five tests are of sensitivity, nominal accuracy, dynamic range, multipath scenario, and moving scenario with periodic update. The nominal accuracy tests are for static accuracy under typical signal strength conditions rather than weak signal conditions and with no multipath present. Performance in the presence of multipath is tested separately, as are the performances under weak signal conditions and under typical land-based dynamic conditions.
The five tests are of sensitivity, nominal accuracy, dynamic range, multipath scenario, and moving scenario with periodic update. The nominal accuracy tests are for static accuracy under typical signal strength conditions rather than weak signal conditions and with no multipath present. Performance in the presence of multipath is tested separately, as are the performances under weak signal conditions and under typical land-based dynamic conditions.
One difference between the two performance standards is the handset must respond within 16 seconds in the CDMA case but has 20 seconds to respond in the GSM case. In both cases the required sensitivity is –147 dBm, and the horizontal positioning accuracy, although defined differently, is similarly around 30 meters.
Another difference is the GSM standard allows for either precise or coarse time assistance. When only coarse time assistance is provided, the sensitivity test is conducted with one satellite at –142 dBm. This is a recognition there is a performance penalty for not providing precise time assistance.
The other main difference is that, in the case of MS-Assisted (UE-Assisted) operation, the CDMA standard calls for the absolute code-phase accuracy to be tested, whereas the GSM standard calls for the location to be computed in accordance with a defined algorithm and for the accuracy of the result to be tested. This is more significant than it seems because the code-phase test is of absolute code-phase accuracy rather than relative code-phase accuracy. To pass this test, the time assistance must be used to determine the measurement instant with nanosecond precision.
The reason for this requirement is the location server can combine GPS and CDMA measurements in performing a hybrid fix only if the absolute GPS code-phase measurements are known for a precise time (according to the CDMA handset's local clock).
These standards have emerged from complex techno-political negotiations between network operators, handset manufacturers, technology providers, and semiconductor manufacturers. They represent negotiated compromises between these various groups rather than a true consensus as to real-world requirements. In particular, the author considers the sensitivity requirement to be lacking in stringency. For reliable positioning under most indoor conditions, sensitivity of at least –150 dBm is essential and better than –153 dBm is desirable. Sensitivity of better than –185dBm is ideal.
Cell-Phone Interference
One of the technical problems facing the GPS cell-phone developer is the interference to GPS reception from the very strong cellular transmissions of the handset. This is an even more serious issue given that the GPS front-end and antenna performance typically is compromised as a result of the severe physical constraints on the design. It is further exacerbated by the need for extreme sensitivity.
The ideal solution to this problem is to provide sufficient filtering in the GPS RF path to permit concurrent operation of the receiver and handset transmitter. However, the significant benefits that flow from such an approach come at a cost. In particular, the more complex RF design results in additions to the bill of materials that add cost and space.
The ideal solution to this problem is to provide sufficient filtering in the GPS RF path to permit concurrent operation of the receiver and handset transmitter. However, the significant benefits that flow from such an approach come at a cost. In particular, the more complex RF design results in additions to the bill of materials that add cost and space.
Source
0 comments:
Posting Komentar