NASA A/G Communications Technology Assessment Study First Order Analysis of Required Bandwidth for the Next-Generation Aeronautical Data Link March 14, 2006

2 Background The Draft ICAO position for the World Radiocommunication Conference 2007 requests additional allocations for mobile route communications –These additional allocations could enable, among other things, provisioning of a next-generation aeronautical communications system, specifically designed for data communications –Eurocontrol and the FAA, with the support of NASA, have entered into a bilateral study to determine the user requirements for, the technical capabilities of, potentially applicable technologies, and the roadmap for attaining this next-generation communications system User requirements, including expected number of users and required data rate, are specified in the Communications Operating Concept and Requirements for the Future Radio System (abbreviated as the COCR) This paper presents the results of a first order analysis that estimates the required bandwidth for the next-generation A/G communications system –This may be useful to spectrum management professionals faced with a requirement to justify the aforementioned reallocation of spectrum

3 Analysis Process Link Budget Derive Radio Coverage Using MLM data for all sectors provided by MITRE Calculate PIAC per radio site Coverage above FL180 for US Place En-route Radio Sites Account for channel access mechanism Calculate required channel rate # Freq required Freq reuse factor Total BW Calculate Required BW Use COCR loading calculations Add VDL2 overhead Estimate information throughput (service rate)

4 Derive Basic Radio Coverage Area from Link Budget The first step is to presume some physical layer parameters, and derive the radio communications coverage area from link budget calculations –Since this is a Future Radio System, to be deployed at L-Band, it is assumed that the physical layer of VDL M2 has been redesigned to be FSK modulation with a 100 Kbps data rate. –Link Budget (shown below) closes at 160nmi Assumes FSK modulation and data rate is selected to meet capacity requirements. A binary modulation was chosen due to the increased path loss at L-Band – the reduction in required Es/No more than offsets the increased noise due to larger BW Calculated from statistics derived from multiple iterations of the IF-77 Electromagnetic Wave Propagation Model (Gierhart-Johnson) model for slightly rolling plains

5 Place Radio Sites The second step is to develop a lay down for radio coverage in a region of interest –We have selected the continental United States, and show a notional radio placement to provide coverage above FL180 Radio Site ID (arbitrary number, used for tracking in spreadsheet)

6 Calculate PIAC per radio site The third step is to derive an estimate of the Peak Instantaneous Aircraft Count (PIAC) in each radio coverage volume To formulate this estimate, –Used 2020 PIAC number from MITRE MLM data This provides a number of aircraft in the en-route sectors of each center (ARTCC) as projected to the year 2020 –Assume uniform distribution within ARTCC –Derive PIACs for each ARTCC above Flight Level 180 This step is necessary as the radio coverage area (assumed) was for flight level 180 and above, and the MITRE data sometimes includes aircraft below this flight level –For each radio site estimate % ARTCC(s) coverage This step allows an estimate of the percent of aircraft in each center that fall in a particular radio sites coverage volume –Calculate PIACs for each radio site

7 Use COCR loading analysis to derive capacity requirements –COCR analysis includes message overheads through the Network layer (assumes ATN OSI overheads), models arrival rates for each message type and treats the A/G network as an M/G/1 server Add VDL-2 message overhead to account for sub-network loading 1 Derive Required Information Throughput 1 Loading is not particularly sensitive to the sub-network overhead. For example, inspection of the graph to the right does not show appreciable differences between the fully loaded message set and the COCR loading analysis, which leaves off at the network point of attachment. This is not the case for channel access mechanism, which can significant impact the required data rate.

8 Estimated Required Information Throughput Given the previously calculated PIAC values, and the transfer function between PIAC and loading (previous chart) map data requirements to radio sites as shown below. Information Throughput Required Radio Site ID

9 For a CSMA with collision detection with random backoff, a relationship between offered load and information throughput can be derived as S = Ge -2G –Relationship is plotted below –Using the above plot, actual channel BW required is ~5 times the information throughput (this is considered conservative for VDL M2 – published results indicate an achieved efficiency closer to 32%) Calculate Required Channel Rate Peak value at ~18%

10 Estimated Required Channel Rate Given the previously calculated information throughput, and transfer function (previous slide) we can calculate the required channel signaling rate. –Map shows required signaling rates between 122 and 430 kbps Channel Rate Required Radio Site ID

11 Calculate Required BW Use the following steps to estimate required BW –Assume a base radio data rate that is commensurate with serving a common denominator number of users – 100 kbps was selected Sensitivity analysis showed lower data rates were more efficient, but would not meet throughput requirements –For each radio site, given the required data rate, scale by the base data rate and take the ceiling. This is the number of the frequencies needed for each radio site –Count all radio sites frequencies to obtain total number of radio frequencies required –Scale by frequency reuse factor to account for reuse data rate –Multiply total number of frequencies by base data rate and efficiency (bps/Hz) to arrive at the BW required Derived BW required is approximately 10 MHz –Recall that this is just for en-route airspace, above FL 180. As coverage is pushed down to the surface, more BW would be required *Reuse factor of 1.23 is very conservative. In essence, we have assumed that coverage is provided from FL180 through FL450, and that the reuse factor is being driven by aircraft at the highest altitudes. *

12 Closing Thoughts The predicted bandwidth required was for green field spectrum –The need to engineer for compatibility with existing systems (if an overlay system in envisioned) will drive up spectrum requirements Use of pulsed signaling with low duty cycle is a technique that might be effective in promoting compatibility with DME equipment. This would drive up the spectrum requirements of the new system –Some efficiencies can be gained by use of a deterministic channel access mechanism Has the potential of reducing required bandwidth for the information transfer requirements of the future radio system However, the considerations required for co-site compatibility still apply and will work to drive spectrum requirements up Predicted BW was only for en-route communications. Coverage was assumed to be required above FL180 only. Additional channels/BW would be required to push coverage down to the surface

NASA A/G Communications Technology Assessment Study Supplemental

14 Calculate PIAC per Radio Site