List of Parameters and Terms for PN Offset Planning in CDMA
There are various parameters and terms which come into play when discussing PN offsets and their function in CDMA.
All base station digital transmissions are referenced to a common CDMA system-wide time scale that uses the Global Positioning System (GPS) time scale, which is traceable to and synchronous with Universal Coordinated Time (UTC).
The mobile station shall establish a time reference which is used to derive system time. This time reference will be the earliest arriving multipath component being used for demodulation.This reflects the assumption that the mobile station’s fix on system time is always skewed by delay associated with the shortest active link.
The Pilot PN sequence offset (index), in units of 64 PN chips. It ranges from 0 to 511. Every transmit sector will have an offset assigned to it. This parameter is set for each sector in the pilotpn field.
The pilots associated with the Forward Traffic Channels assigned to the mobile station.It is the base station that assigns all active set pilots to mobile stations.
The pilots that are not currently in the Active Set but have been received by the mobile station with sufficient strength to indicate that the associated Forward Traffic Channels could be successfully demodulated. As a property of the Mobile Assisted HandOff (MAHO), the mobile station promotes a Neighbor Set or Remaining Set pilot to the Candidate Set when certain pilot strength criteria are met and then recommends the pilot to the base station for inclusion in the Active Set.
The pilots that are not currently in the Active Set or the Candidate Set and are likely candidates for handoff. Neighbor Set pilots are identified by the base station via Neighbor List and Neighbor List Update messages.
The set of all possible pilots in the current system on the current CDMA frequency assignment, excluding pilots in the other sets. These pilots must be integer multiples of PILOT_INC.
These parameters represent the search window sizes associated with Neighbor Set and Remaining Set pilots.The mobile station centers the search window for each pilot around the pilot’s PN sequence offset using timing defined by the mobile station’s time reference. In general, a neighbor search window, SRCH_WIN_N, will be sized so as to encompass the geographic area in which the neighbor may be added (a soft handoff “add” zone or “initial detection area”). The largest a neighbor search window need be is sufficient to cover the distance between the neighbors, , plus an accommodation of the time-of-flight delay (approx. 3 chips).
This parameter represents the search window size associated with the Active Set and Candidate Set pilots.The mobile station centers the search window for each pilot around the earliest arriving usable multipath component of the pilot. Note that in contrast to the neighbor or remaining set search windows, the active/candidate search windows “float” with the desired signals. That is to say that the center position of the search window is updated every scan to track the new location of the earliest arriving multipath component.
To better illustrate the relationships between search windows, consider the following scenario:
A mobile station monitors a neighbor pilot. The neighbor search window is centered on the neighbor pilot offset. This centering is relative based on timing derived from the time reference. When the pilot strength of a neighbor pilot recommends promotion to the candidate set, then the search window will be tightened to the active search window size. The active search window is sized to compensate for delay spread only and is, therefore, smaller than the neighbor search window. In addition, the active search window locks onto and tracks the candidate pilot.
The pilot arrival time is the time of occurrence of the earliest arriving usable multipath component of a pilot relative to the mobile station’s time reference.
The mobile station reports pilot strength and phase measurements for each active and candidate pilot in the Pilot Strength Measurement Message when recommending a change in the handoff System Time status (i.e. mobile assisted handoff). The mobile station computes the reported PILOT_PN_PHASE as a function of the PILOT_ARRIVAL and the PILOT_PN.
The pilot arrival component represents the time delay of the pilot relative to the time reference or, in other words, how skewed the pilot is from the mobile’s concept of system time. Both the PILOT_ARRIVAL and PILOT_PN_PHASE measurements are in chips (15 bits, 0 to 32,767 or 215-1) while the PILOT_PN is in offsets (9 bits, 0 to 511). The difference (6 bits) corresponds to the 64 chip interval
between successive PN offsets.
Note also that the mobile does not identify pilots by their offset index directly, but by their phase measurement. If the pilot arrival was larger than 32 chips (1/2 of a pilot offset or 4.8 miles), then this could undermine the ability of the base station to properly translate pilot phase into pilot offset index (given a PILOT_INC of 1).
The pilot PN sequence offset index increment is the interval between pilots, in increments of 64 chips. Its valid range is from 1 to 15.
The mobile station uses this parameter in only one manner, to determine which pilots to scan from among the Remaining set. Only valid pilots (i.e. those pilots that are multiples of PILOT_INC) will be scanned. For the mobile station, PILOT_INC impacts only the scanning rate applied to Remaining pilots. It accomplishes this by reducing the number of Remaining pilots that need to be scanned.
For the base station, its effect is different. In the base station, it is used in properly translating pilot phase back into pilot offset index. The consequence is that the operator may artificially increase the separation between valid time offsets. By selecting a PILOT_INC of 2, for instance, an operator chooses to limit the number of valid offsets to 256 (i.e. 0, 2, 4,…, 508, 510) instead of 512. The increased separation means that the pilot arrival must be larger before adjacent offset ambiguity is possible and consequently the likelihood of a strong adjacent interferer is reduced.