ACP WGC11 / WP 09
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
Working Group C – 11th meeting
Brussels, Belgium
18 – 20 September 2006
Agenda Item 3 Update on Future Communication Infrastructure Activities
FCS Phase II Results Paper 3 - Detailed Technology Investigations
Prepared by: Glen Dyer, Tricia Gilbert / ITT Industries
James Budinger / NASA Glenn Research Center
SUMMARY
The Future Communication Study (FCS) Phase II detailed investigations described herein were conducted primarily in response to feedback received on FCS Phase I technology investigation results that raised concern about technology performance in the L-band aeronautical channel environment; ground infrastructure cost for an L-Band air-ground (A/G) communications solution; satellite solution performance; and technology performance in the C-Band aeronautical surface environment.
To address the received feedback and to support in-depth evaluation of technologies emerging from the Phase II screening process, several detailed study activities were conducted as part of FCS Phase II technology evaluation. These included investigations specific to the L-Band aeronautical channel including definition of a channel model that could be used for common characterization of waveform performance in the A/G channel; definition of a framework for specifying the infrastructure costs associated with an L-Band system; and analysis of the performance of recommended technologies with the common channel model and their potential to interfere with incumbent users of the band. Also investigated was satellite system modeling (with a focus on availability analysis) and C-Band performance modeling (to evaluate technology performance on the airport surface).
This paper provides an overview of the FCS Phase II detailed investigations and provides recommendations to the group specific to the described results.
1. Background
1.1 As part of the Future Communication Study (FCS) cooperative research program, candidate technologies supporting the long-term aeronautical mobile communication operating concept are being evaluated. After Phase I of the FCS technology investigation, Technology Pre-Screening, feedback from several stakeholders was received. This feedback indicated that there was concern about technology performance in the L-band aeronautical channel environment; ground infrastructure cost for an L-band A/G solution; satellite solution performance; and technology performance in the C-Band aeronautical surface environment.
1.2 With this feedback in mind, Phase II of the FCS technology investigation, Technology Screening, commenced. The Technology Screening process evaluated an inventory of more than 50 technologies consisting of both commercial standards/systems as well as custom aeronautical standards. These technologies were organized into technology families characterized by similarities in user requirements, services offered, and reference and physical architectures. The screening of the technologies considered technology performance with regard to data loading capability, technology communication range, and ability to use protected spectrum. Specific threshold values were associated to these metrics traceable to requirements in the Concept of Operations and Communication Requirements for the Future Radio System (COCR). A result of the screening was the identification of the most promising technology candidates to bring forward as candidate technologies for the Future Radio System (FRS).
1.3 As a result of the technology screening process described in a companion paper, eight technologies have been identified as candidates for a general aeronautical communication solution for the FRS (also called a continental solution because the solution applies to all continental flight domains including airport, terminal and en route). In addition, some additional technologies have been identified as the best performers in the context of specific flight domains that have a unique environment and may warrant separate technology considerations (i.e. oceanic and airport domains). A summary of the results is provided in Table 1 below.
Table 1: Recommended Technologies from Technology Screening
NASA/ITT Screened TechnologiesContinental Solution / W-CDMA
APCO P34
L-band E-TDMA
LDL
B-VHF (at L-band)
Link 16
Inmarsat SBB
Custom Satellite Solution
Oceanic Domain / Inmarsat SBB
Custom Satellite System
Airport Domain / IEEE 802.16
1.4 To further understand the technologies emerging from the screening process and to address specific feedback received on the Phase I technology investigation results, Phase II of the technology investigation included a set of focused and in-depth analyses. The topics of these studies were organized into five major areas including:
· L-Band Technology Performance
· L-Band Interference
· L-Band Technology Cost for Ground Infrastructure
· Satellite Technology Availability Performance
· C-Band Technology Performance
1.5 It is the intent of this paper to provide an overview of the detailed study results specific to the topics above.
2. L-Band Technology Performance
2.1 Consideration is being given to the use of L-Band (960-1024 MHz) to employ the next generation aeronautical communication system. Several screened technologies are being considered for this band, including P34, LDL, WCDMA, B-VHF (at L-Band), and L-Band E-TDMA. Upon review of the technology definitions and developed concepts of use, technology considerations warranting in-depth analysis were identified. The focus of Phase II in-depth technology investigations was specific to P34 and LDL, two technologies that scored well during Phase I technology investigations. The selected analysis topic(s) for each technology was made based on the need to assess those components of the technology that provide the most challenge for application of the technology as a viable solution for aeronautical communication as well as received feedback on Phase I results. Selected analysis topics for the candidate L-Band technologies include:
· P34: Protocol model developed in OPNET to assess P34 net entry and data transfer performance and BER performance in the L-Band channel
· LDL: Bit-Error-Rate (BER) performance in the L-Band channel
2.2 As both P34 and LDL analysis topics include assessment of their performance in the L-band channel, a first step in the L-band analysis work was modeling of this channel. A literature search revealed that while many channel models exist for the terrestrial channel in close proximity to L-Band, there had been no previous activity to develop a channel model that characterizes the L-Band Air/Ground (A/G) channel for radiocommunications. As most standardization bodies consider it a best practice to test candidate waveform designs against carefully crafted channel models that are representative of the intended user environment, a channel model was developed that could be used for common characterization of communications waveform performance in this A/G channel.
2.3 Characterization of the Delay Spread and the Doppler Power Spectrum is essential for generating a useful model for waveform simulation and evaluation of candidate FRS technologies in L-Band. In order to form estimates of the delay spread and associated statistics, a ray-tracing simulation was developed. This simulation models both diffuse and specular reflections from the Earth’s surface. The specific channel modeled was one associated with a worst case scenario, specifically modeling of the A/G channel over mountainous terrain. This terrain, in the en route case, has the potential to provide long multipath delays that either limit the data rate to be transmitted or require special techniques to achieve acceptable performance. The context of the developed model for evaluation of the A/G channel is shown in Figure 1.
Figure 1: L-Band Channel Model Context
2.4 The specific implementation of the model was for mountainous terrain in Aspen Colorado. A topographic map of this terrain is provided in Figure 2.
Figure 2: Aspen Colorado Mountain Terrain Used for L-Band A/G Modeling
2.5 The developed simulation used a method of concentric oblate spheroids to model multipath contributions. Using the transmitter and receiver as focal points, a series of oblate spheroids was generated in the three-dimensional simulation space. The first oblate spheroid in the series was generated so that it just barely intersected the underlying terrain. The semi-minor axis of each successive oblate spheroid was increased by a fixed increment so that the spheroids intersected more and more of the underlying terrain as they were stepped through. The desired product was the set of points on the terrain that were intersected by the oblate spheroids. When plotted, each set of intersection points appears as a distorted annulus approximating the cross section of the spheroid when sliced by the Earth’s surface. Each set of intersection points is mutually exclusive from any other set because any intersection point can only be accounted for once. Each set of intersection points contributes to multipath for a particular delay. Figure 3 illustrates the method of concentric oblate spheroids used to model multipath contributions.
Figure 3: Two Concentric Oblate Spheroids Intersecting the Underlying Terrain
2.6 The contour of terrain trapped between two successive spheroids was used to calculate multipath dispersion for a particular time delay. Each contour consisted of a set of terrain points that represented potential scatterers. Ray tracing was used to determine specular and diffuse multipath. The detailed methodology utilized to identify multipath components is shown in Figure 4.
Figure 4: L-Band Channel Modeling– Identification of Multipath Components
2.7 Implementing the methodology above and employing data reduction and analysis techniques, the mean RMS delay spread was calculated to be 1.4 µs. It is instructive to consider representative technologies at this point since the technology data rate will drive channel model parameter estimation. A rule of thumb frequently applied is that if the mean RMS delay spread is at least one tenth of the symbol duration, then the channel is frequency-selective. In order to illustrate this, two technologies that scored well during the FCS Pre-Screening were considered: LDL and P34. Table 2 shows the corresponding data rates and symbol durations for LDL and P34.
Table 2: Data Rates of LDL and P34
Waveform / Data Rate / Symbol Duration / 1/10th of the Symbol DurationLDL / 62.5 kbps / 16 µs / 1.6 µs
P34 / 4.8 kbps* / 208.3 µs / 20.83 µs
* P34 is an OFDM system. The tabulated data rate is per carrier and is the symbol rate. Overall P34 data rates range from 76.8 – 691.2 kbps
2.8 Using our rule of thumb, P34 should undergo flat fading and LDL presents a borderline case because the mean RMS delay spread is very close to one tenth of the symbol duration. It is important to note that frequency-selective channel models differ in structure from flat fading channel models. For this reason it was decided to develop a frequency-nonselective fading model for P34 and a frequency-selective fading model for LDL. An example of the resulting, more complex model for LDL is shown in Table 3 below.
Table 3: LDL Channel Model Parameters
Tap # / Delay (µs) / Power (lin) / Power (dB) / Fading Process / Doppler Category /1 / 0 / 1 / 0 / Rician / Jakes
2 / 1.6 / 0.0359 / -14.5 / Rayleigh / Jakes
3 / 3.2 / 0.0451 / -13.5 / Rayleigh / Jakes
4 / 4.8 / 0.0689 / -11.6 / Rayleigh / Jakes
5 / 6.4 / 0.0815 / -10.9 / Rayleigh / Jakes
6 / 8.0 / 0.0594 / -12.2 / Rayleigh / Jakes
7 / 9.6 / 0.0766 / -11.2 / Rayleigh / Jakes
2.9 One of the primary results reported is the simulated RMS delay spread. It should be noted that this delay spread can be modeled as a function of the average distance from the transmitter, with increasing delay spreads reported for increasing distances. Specifically, a generalized model, using the method cited in Greenstein, has the form:
where,
· d is the distance in km
· σ0 is the median value of the RMS delay spread at d = 1 km
· ε is an exponent that lies between 0.5-1.0, based on the terrain type
· A is a lognormal variate
2.10 To determine the parameters that are appropriate for a generalized L-Band A/G model in mountainous terrain, RMS delay spreads were predicted for a reference distance of 1 km as well as for the previously mentioned values at 64.37 km (40 miles). The two predicted values that resulted from the simulation work are:
sRMS(1 km) = 0.1 μs
sRMS(64.37 km) = 1.4 μs
2.11 Fitting the Greenstein model to the reference data provides a generalized expression for RMS delay spread, which is found to be:
2.12 Having defined a channel model to support FRS technology performance analysis, the focus is now turned to evaluation of two specific candidate technologies, P34 and LDL. As noted above, P34 in-depth analysis included assessment of P34 net entry and data transfer performance and BER performance in the L-Band channel. To assess net entry and data transfer performance, a simulation was developed. The simulation work included development of an operational scenario, communication nodes and communication links. The selected scenario for evaluation was the NAS Super Sector, as defined in the COCR. In this scenario, one fixed station node was used to model the ground station and 95 mobile nodes were used to model aircrafts. The defined communication link for this model implemented the P34 Scalable Adaptive Modulation (SAM) air interface. This included 50 kHz channels and QPSK modulation (providing 76.8 kbps). This is the lowest defined P34 data rate, but should be satisfactory for “closing the link” for the sector size defined in the COCR. A depiction of the simulation context for this analysis is shown in Figure 5.
Figure 5: P34 Simulation Context
2.13 Using this context, a P34 configuration that was simulated was fixed-network equipment (FNE) to mobile radio (MR). The MR to MR and repeater modes were not simulated. The simulated configuration aligns with the P34 concept of use for aeronautical application defined during Phase I of the FCS technology investigation. The custom OPNET development included modeling of the P34 PHY, MAC, LLC and SN layers, as illustrated in Figure 6.
Figure 6: Modeled Elements of P34
2.14 Simulation model results are shown in Figure 7. These figures show the response time of the P34 simulation to the offered load for each of the transmitted message. Note that the sub-network latencies over P34 protocols (SNDCP, LLC CP, LLC UP, MAC) meet COCR latency requirements. Specifically, although there are some startup outliers, 95% of delay measurements are under 0.7 seconds.