Currently proposed FDMA/TDMA cellular radiotelephone systems are spectrally inefficient due to an undesirable granularity in their channel allocation.
Traditionally, in cellular systems, as capacity is needed at any cell site, additional frequencies are more or less permanently and fixedly allocated to that cell. In Time Division Multiple Access (TDMA) systems, like the next generation digital cellular system in Europe commonly known as GSM Pan-European Digital Cellular, each carrier has multiple (eight) time slots in each sequentially repeating time frame. Each slot represents a single channel of radio traffic capacity. Thus, channel capacity would ordinarily and naturally be added on a modulo-eight basis with the addition of each carrier, in others words--a granularity of eight.
This granularity-by-eight is spectrally inefficient because overall nominal system capacity and planning is based upon providing the minimum number of traffic channels to serve the expected, offered load in a given area. Yet adjacent coverage areas might well exceed this nominal capacity by one or two orders of magnitude (modulo-eight) due to this granularity. This is a particular problem in FDMA/TDMA systems because a fixed number of time slots per repeating frame are allocated every time a new carrier is allocated to a coverage area.
Others attempted to solve somewhat analogous capacity problems in FDMA systems by load shedding, or sharing carriers between adjacent sectors of a cell, but such frequency sharing disturbs the regular cellular frequency reuse pattern and causes co-channel interference. Cellular systems require a certain geographic separation of reused frequencies to avoid undo interference. In other words, the level of co-channel interference (carrier-to-interference ratio) experienced by a cellular subscriber from a proximal reuse of that same carrier frequency establishes the distance at which co-channel, omni-directional antenna sites are allowed to exist, while the predictability of the directivity of sectored antennas serves to reduce the reuse distance (see U.S. Pat. No. 4,128,740 to Graziano and assigned to the instant assignee). Sharing a carrier frequency from its intended orientation to an adjacent sector of unintended orientation adversely affects the interference in that direction.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.
According to this invention, to even the granularity-induced discontinuities between adjacent coverage areas and to provide additional capacity as needed, certain time slots from one frame are allocated to one coverage area while other time slots in the frame are allocated to another coverage area.