Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1998-09-01
2002-04-30
Pham, Chi (Department: 2731)
Pulse or digital communications
Receivers
Angle modulation
C455S426100
Reexamination Certificate
active
06381289
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed toward a method for demodulating high speed asynchronous time division multiplexed signals and, more particularly, toward a method for demodulating high speed frameless asynchronous time division multiplexed signals.
BACKGROUND OF THE INVENTION
In wireless communication systems utilizing low Earth orbiting satellites, where data packet switching is employed and Time Division Multiplexing is the selected mode of downlink access, it is advantageous to use a small number of broadband carriers in the downlink, as opposed to a large number of narrow band carriers. This means that the data packets destined for a multiplicity of Earthbound user terminals are time multiplexed into a single broadband, high data rate carrier. However, the data demodulation rate at an individual user terminal may be desired to be much smaller than the carrier data rate, also referred to as the bearer data rate, to reduce demodulator complexity and cost. For example, an exemplary bearer data rate may be 450 Mbits/sec, where an exemplary demodulation rate at an individual user terminal may be 2 Mbits/sec.
Also, as modern satellite communication systems are increasingly becoming cellular in character, the satellites, via high gain antennas, are generating narrow beams, also referred to as “spot beams” and creating small cells on the Earth. In a given satellite footprint, or useful field of view of the Earth from the satellite, there may typically be 360 cells. However, unlike conventional terrestrial cellular communication systems, a satellite communication system may not have all 360 downlink beams active at all times; this would require the generation of 360 simultaneous transmit spot beams and would place a great complexity/cost burden on the satellite payload. To mitigate this problem, a satellite communication system may employ cell hopping by transmitting fewer beams than the number of cells in a satellite's footprint. Typically, the number of hopping beams might be 24 for the above example of 360 cells in a given footprint.
The capacity of a hopping downlink beam to deliver traffic to a cell is directly proportional to its dwell time at the cell: CAPACITY=(BEARER DATA RATE) (DWELL TIME). As shown in
FIGS. 1
a-b
, in conventional, or synchronous, TDM (Time Division Multiplexed) systems, the time domain is divided into fixed-length/fixed-boundary frames
10
, which are further subdivided into fixed-length/fixed-boundary slots
12
and sub-slots
14
. If cell hopping were employed in a conventional TDM system, cells
1
-n would typically be visited with some fixed periodicity with respect to the frame
10
, e.g., once per frame (see
FIG. 1
a
), and the dwell period at a cell (T
slot
) would typically be synchronized to the slot
12
. Within a given slot
12
, capacity is allocated to different receivers
1
-k by allocating a fixed sub-slot
14
to a given receiver
1
-k (see
FIG. 1
b
), with the dwell period at a receiver (T
subslot
) synchronized to the subslot
14
. The capacity allocation is made on a demand-assigned basis through a call set-up protocol, which is relatively time consuming and inflexible.
In the conventional, i.e., synchronous, TDM system described above with respect to
FIGS. 1
a-b
, all cells
1
-n have the same dwell time T
slot
and the cell visits, i.e., slots
12
, occur at times known a priori to the receivers
1
-k in each cell
1
-n. Variations, still within the commonly accepted definition of a synchronous TDM system, may exist as follows: (a) some cells may have different visitation periods than others, e.g., twice a frame or once every two frames; (b) the slot
12
durations may have non-uniform but fixed lengths; or (c) more than one subslot
14
may be allocated to a receiver. The distinguishing feature of a synchronous TDM system is that by acquiring time synchronization to the system clock, a receiver in any cell has accurate knowledge of the time when it will be accessed.
In synchronous TDM systems, once capacity is allocated to a receiver, it cannot be rapidly redeployed. If the receiver does not use the allocated capacity, it is wasted. Accordingly, modern broadband systems offering bandwidth on demand services tend to favor Asynchronous TDM (ATDM) where the dwell times at a cell and the access times to a given receiver can be dynamically changed without notifying the receiver. Thus, ATDM systems alleviate the overhead and delay of call set-up required in a synchronous demand-assigned TDM system.
ATDM systems belong to one of two categories; framed and unframed. As shown in
FIGS. 2
a-c
, in a framed ATDM system, although there is a fixed-length/fixed-boundary frame
10
and slot
12
-structure (see
FIG. 2
a
), the downlink beam hops between cells
1
-k within a given slot
12
(see
FIG. 2
b
). In framed ATDM, only cells that have packets to be delivered are visited by the beam, and the dwell time (T
burst
) at each cell
1
-k is just sufficient to deliver the packets destined for that particular cell
1
-k. The cells that can potentially be visited in a given slot
12
are predetermined, but not all cells are necessarily visited in every slot
12
; a cell being visited only if there are packets to be delivered. Thus, the cells visited in a particular slot
12
comprise a random subset of a fixed set of cells. As a consequence of the random visitation times, the start and end times of an access (T
burst
) to a cell are also random. However, the access to a given cell always occurs within a predetermined fixed-length/fixed-boundary slot
12
(T
slot
)
Within the access to a particular cell, T
burst
, random numbers of packets are transmitted to random numbers of receivers
1
-k, or user terminals (
FIG. 2
c
). The flexible bandwidth on-demand feature of ATDM systems stems from the fact that the mean data rate to a given terminal is proportional to the mean number of packets transmitted to the terminal in unit time, with the number of packets transmitted to the terminal capable of being changed dynamically without the cooperation, or prior knowledge, of the receiver. Thus, the data rate, or bandwidth, to a given terminal can increased by simply increasing the number of packets transmitted to the terminal in each access (T
rx
) and/or increasing the visitation rate to the cell containing the terminal.
One of the limitations of a framed ATDM system, is that the slot length T
slot
limits the capacity peak density that can be created on the ground. As the slot length T
slot
is fixed, it limits the maximum dwell time T
burst
on the busy cells, since the dwell time T
burst
on a given cell cannot exceed the slot time T
slot
. On the other hand, if the capacity demand on the ground is highly non-uniform, in some slots there will be idle time after all cells in the slot have been visited leading to capacity wastage. Therefore, in order to achieve operational flexibility in creating capacity peak densities, some broadband satellite systems are opting for frameless ATDM, where all time limitations of the frame and slot are eliminated.
FIGS. 3
a-b
illustrate the “point and shoot” access to user terminals in a frameless ATDM system. Both the cell revisit time T
revisit
and the cell dwell time T
cell
of the downlink burst are random. Further, there is no fixed association between a hopping beam and a cell, ie., a cell may be visited by any available hopping beam, although this is not explicitly shown in FIG.
3
.
In the above-described synchronous TDM and framed ATDM systems, specific beams were assigned to specific cells, However, which beam visits a cell to deliver a packet is not of great importance in terminal design; what is important is the degree of time predictability of the burst, which is tabulated below.
TABLE 1
Time Predictability of Different Access Schemes
Access Scheme
Burst Dwell Time at Cell
Cell Revisit Time
Synchronous TDM
Known Exactly
Known Exactly
Framed ATDM
Known to be within a slot
Known to within
whose time of occurrence is
a frame period ±
known exactly
half a slot pe
Bayard Emmanuel
Coats & Bennett P.L.L.C.
Ericsson Inc.
Pham Chi
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