Broadband technologies are taking a predominant role in the emerging information society, and, in particular, broadband satellite communication systems are being broadly employed to respond to the growing requirements of the information society. More specifically, based on global access and broadcasting capabilities, satellite communication systems are well suited to provide broadband services to remote locations and highly mobile users (e.g., broadband services provided to rural areas and to ships, aircraft, trains, etc.), as well as to major metropolitan areas of high population density and high broadband demands. Accordingly, the overall demand for broadband capacity continues to increase exponentially, however, bandwidth availability limitations of satellite systems continues to be a predominant issue in the growth of this communications technology.
In order to satisfy the growth in demand for high availability broadband capacity, broadband satellite communications systems that deploy high throughput satellites are becoming more prevalent. High throughput satellite (HTS) is a classification for a communications satellite that provides upwards of more than 20 times the total throughput of a classic FSS geostationary communications satellite (e.g., throughputs of more than 100 Gbit/sec of capacity are currently being deployed, which amounts to more than 100 times the capacity of a conventional Ku-band satellite). Such satellites typically utilize the same amount of allocated orbital spectrum, and thus significantly reducing cost-per-bit. The significant increase in capacity of an HTS system is achieved, in part, by a high level frequency re-use and spot beam technology, which enables frequency re-use across multiple narrowly focused spot beams (usually in the order of 100's of kilometers). The signal strength received at the earth surface within a narrow spot beam footprint, from a multiple beam satellite, however, varies with the distance between the receiving terminal and the beam center on the ground. For a given transmitter power of the satellite transponder, the effective isotropic radiated power (EIRP) in dBW, at the earth surface, decreases relative to the beam center, in accordance with the satellite antenna gain pattern. In other words, the gain or received power for a terminal located at or near the beam center is at a peak level, and that power or gain decreases (relative to the beam center) as a terminal location moves away from the beam center towards the beam edge (e.g., in some current systems, by as much as approximately 4 dB from beam center to beam edge).
Accordingly, based on terminal location and the higher realized gain, a terminal at or near the beam center can receive data transmitted at a higher modulation and coding scheme and with less transmit power, which increases the bandwidth efficiency to that terminal (e.g., increases the realized bits/Hz/sec transmitted to that terminal at a lower transmit power). Similarly, based on terminal location and the lower realized gain, a terminal further away from the beam center (and at a worst case near the beam edge) requires a higher transmit power level to receive and reliably decode transmissions at the higher modulation and coding schemes, and thus cannot realize the same bandwidth efficiency as the terminals closer to the beam center. Moreover, in current systems, however, all carriers within a given beam of a wideband transponder (e.g., sharing a single transponder) are typically transmitted at a common power level. In that regard, based on a terminal location, the modulation and coding scheme that the terminal is able to receive and reliably decode is limited (e.g., based on the associated intermodulation distortion and signal to noise ratio at that terminal). With a common power level transmission across all carriers of a beam, however, the modulation and coding must be set to accommodate the worst case link margin scenario (e.g., for the terminals at the beam edge). Such a scenario, therefore, fails to take into consideration the advantaged terminals (based on their location with respect to the beam center) and the fact that such terminals do not require the transmitted power level for the assigned modulation and coding. Accordingly, such a scenario results in an inefficient utilization of the available transmit power across the beam, and correspondingly an inefficient utilization of the total bandwidth capacity potential of the beam based on the total available power.
What is needed, therefore, is an approach for optimizing the power utilization of a satellite spot beam transponder, and thereby optimizing the modulation and coding scheme assignments and the associated data rate for terminals across the entire transponder beam, including terminals located at the more disadvantaged locations within the beam (e.g., at the beam edges as opposed to the beam center).