Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-12-15
2002-12-03
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
C375S140000, C375S141000, C370S342000, C370S441000
Reexamination Certificate
active
06490313
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to cellular or mobile communications systems and more particularly to a method for optimally assigning pseudo-noise (PN) offsets to base stations in a Code Division Multiple Access (CDMA) cellular network based on the IS-95A standard to maximize co-offset and adjacent offset protection and to minimize interference in the network.
BACKGROUND OF THE INVENTION
A CDMA cellular network is a digital spread spectrum communications system. The CDMA network includes several base stations each providing digital service to mobile units located in different geographical regions. Communication between a mobile unit and a base station in an IS-95A-based CDMA network occurs on reverse and forward CDMA channels. The reverse CDMA channel is a mobile unit-to-base station direction of communication that carries traffic and signaling information. The forward CDMA channel is a base station-to-mobile unit direction of communication that carries pilot, sync, and paging signals in addition to traffic signals.
The reverse CDMA channel includes access channels and reverse traffic channels. The access channels are used by the mobile unit to initiate communication with a base station, and to respond to paging channel requests.
The forward CDMA channel consists of a pilot channel, a sync channel, up to seven paging channels, and up to sixty-three forward traffic channels. Each of these channels is orthogonally spread by an appropriate Walsh function and then spread by the quadrature pair of PN sequences (I and Q) at a fixed rate of 1.2288 million chips per second.
The base station of a sector continuously transmits on the pilot channel of each active forward CDMA channel. A mobile unit operating within the coverage area of the base station uses this continuous transmission for synchronization. The network assigns each base station of the cell site a specific time (or phase) offset of the pilot PN sequence to identify a forward CDMA channel. A given base station uses the same pilot PN sequence offset, or simply PN offset, on all CDMA frequency assignments. For example, all traffic, sync, and paging channels transmitted from a single base station share the same PN offset. An offset index (0 through 511 inclusive) identifies distinct pilot channels. This offset index specifies the offset value from the zero offset pilot PN sequence. Each offset index increment represents the interval between pilot channels in increments of 64 chips (i.e., 52.08 ms).
An active mobile unit maintains four sets of pilot channels when communicating with a base station of a CDMA sector: the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set consists of all the pilot channels that the mobile unit is currently using for demodulation. The Candidate Set contains all of the pilot channels that are not currently in the Active Set but have sufficient signal strength to be considered for soft or softer handoff. The Neighbor Set contains pilot channels that are not currently in the Active or Candidate Set, but may become eligible for handoff (e.g., pilot channels of nearby sites). The Remaining Set is the set of all possible assigned pilot channels in the CDMA network on the same carrier frequency, excluding the pilot channels defined in the other three sets.
Since all PN offsets in a network are time shifted versions of each other, it follows that with appropriate time delay, an incorrect pilot channel from any sector may be mistaken for a pilot channel in the Active Set. However, a large time delay between a mobile unit and a base station implies a large path loss and hence a weak pilot channel signal at the mobile unit Thus, if the PN offsets of different sectors have a large separation between them, a pilot channel signal would have a very high path loss and hence a very trivial probability of appearing within an active search window of another pilot channel. This makes an appropriate assignment of PN offsets crucial to ensure that a wrong pilot channel would be sufficiently weak so as not to cause any problems.
Reusing PN offsets is possible if: (1) a mobile unit being served by a base station is not interfered with by the pilot channel of another CDMA base station using the same PN offset, or (2) a base station in the network can uniquely identify all the pilot channel signals being reported by a mobile unit that it is serving.
The mobile unit uses a network-selected PILOT_INC parameter for the base station to determine which pilot channels to scan from among the Remaining Set, which is the set of all possible pilot channels in the system that are integer multiples of the PILOT_INC parameter on the current CDMA frequency assignment, excluding pilot channels in the other sets. A Remaining Set pilot channel is assigned a lower priority in the scanning order, than an Active, Candidate or a Neighbor Set pilot channel.
The setting of the PILOT_INC parameter by the network is crucial to PN offset index planning. It has an impact on the mobile unit's Remaining Set pilot channel scanning rate, the amount of co-offset and adjacent offset protection available in the network, and the total number of offsets available. The PILOT_INC parameter is an integer having a valid range from 1 to 15. Low values of PILOT_INC provide good co-offset protection, more offsets from which to choose, and less reuse of offsets, and increase the time to scan the pilot channels in the Remaining Set of the mobile unit High values of PILOT_INC, on the other hand, provide good adjacent-offset protection, fewer offsets from which to choose, and more reuse of offsets, and decrease the time to scan the pilot channels in the Remaining Set of the mobile unit
Conventional PN offset planning methods are based on idealized hexagonal grid structures, and accomplished by fitting a highly irregular pattern of cellular base station locations to a tessellated hexagonal grid pattern. PN offsets are assigned by reusing the same PN offset a specified predetermined number of base stations away.
In order to plan for growth, these methods usually group PN offsets into a few groups, typically three for tri-sectored sites in the network. One sector of a base station is assigned a PN offset from one of the three groups. The other two sectors of the base station are assigned PN offsets from the other two groups, respectively. A few PN offsets in each group are reserved for growth of the network when new base stations are added. The remaining PN offsets in each group are used to make assignments. Such assignments by groups are not optimal in terms of reducing inter-sector interference. Moreover, the unused PN offsets constitute a wasted resource until the network grows.
Such methods make initial assignment choices very easy because the assignments can be made without computer assistance or optimized planning. After an initial assignment, irregularities are accounted for by manual modification of the assignment by an experienced engineer with local knowledge of the environment Unfortunately, PN offset planning under these idealized assumptions creates many inefficiencies due to the initial assignment's inaccurate reflection of reality.
The irregularities that produce these inefficiencies are due to several factors. First, the need for base stations in a particular area is highly non-uniform because people do not tend to distribute themselves uniformly over large areas. They tend, for example, to cluster in neighborhoods, at work, and in cities. Second, choices for new base station locations are very limited due to factors such as zoning. Base station locations cannot be chosen in ideal locations even if the user traffic was uniformly distributed over a geographic area Lastly, areas that are covered by base stations are highly dependent upon the propagation environment lrregularities such as terrain, morphology, and reflecting structures produce highly irregular areas of coverage.
Automatic PN offset planning has heretofore found only limited application due to the need to account for s
Boyer Pete Allen
Ganesh Rajamani
Chin Stephen
Fan Chieh M.
Suchyta Leonard Charles
Verizon Laboratories Inc.
Weixel James K.
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