DESIGN OF A SPECIFIC CDMA SYSTEM FOR AIR TRAFFIC CONTROL APPLICATIONS UNIVERSIDAD DE LAS PALMAS DE GRAN CANARIA UNIVERSIDAD POLITÉCNICA DE MADRID
2 1.- Motivation and objectives 2.- Operational aspects 2.1.- Proposal for the TMA / taxiing scenarios: C band Duplex options Topics covered 2.2.- Proposal for the en route scenario: VHF band Coexistence with legacy systems Transmit Beamforming Codes with arbitrary spectral nulls 3.- Deployment in two steps Outline
3 New needs require new systems CDMA technologies might complement current narrowband VHF transmissions. Main advantage: very mature and available technology Two available bands: VHF and C band. Global solution Provide a flexible simulation tool to test different alternatives in realistic aeronautical environments Contribute to EUROCONTROL initiatives, ad-hoc working groups … 1.- Motivation and objectives Communication capacities 20102020 POTENTIAL NEW SYSTEMS Communication strategies Eurocontrol 8,33 kHz + VDL2 SATCOM Communication needs
4 CDMA standard transmitters to obtain representative results Aeronautical channel simulator: en route, TMA, Taxiing Advanced CDMA receivers MultiUser Detection covering also ISI increase Doppler Correction. Adaptive implementation Multiple antennas at the ground station: beamforming and spatial diversity 1.- Main goal of the project: flexible simulation tool CDMA Standard Tx Aeronautical channel + interferences CDMA Advanced receivers
5 2.- Operational aspects. Proposal for the C band Very high attenuation: link budget analysis recommends coverage around 25 Km. Limited to TMA / Parking Available bandwidth: 60 MHz, from 5.090 to 5.150 GHz. WCDMA (5 MHz Bw) is the most suitable option. Larger bandwidth, larger gain. Two duplex options: FDD vs TDD
6 TDD. Time Division Duplex. Extra time guard interval is required for time alignment of different users in uplink. Maximum time guard depends on the cell size. To cover 25 Km, time guard should be 688 chips instead of the UMTS 96 chips. 96 chips means 3.7 % efficiency loss per slot 688 chips means 26.9 % efficiency loss per slot FDD. Frequency Division Duplex No such limitation. Pay attention to the assigned bands of both links Data transmission Guard Key disadvantag e
7 Further considerations regarding the spreading factor in TDD /FDD and aeronautical channel delay spread. Typical delay spread En route: 33 sec. Arrival, parking: 7 sec. Taxi: 0.7 sec CDMA symbol length TDD (16 chips/Symbol)=4.16 sec (indoor applications) FDD (256 chips/Symbol)=66.6 sec (outdoor applications) Conclusions TDD suffers strong ISI (performance degradation). This point may be compensated by the use of advanced MultiUser Detectors FDD is more suitable for these scenarios. MultiUser Detectors are not realistic. The interference limitation behaviour may be reduced by using beamforming We have simulated both systems. Evaluated pros & cons.
8 Our proposal for the C band. UMTS-FDD. Topics covered in our research MRC detector. SingleUser detector Adaptive implementation to follow channel variations Adaptive beamforming Several antennas at the ground stations Tx beamforming (Downlink) Rx beamforming (Uplink) Increase SNR (or coverage) Reduce intra and inter-cell interferences
9 2.- Operational aspects. Proposal for the VHF band Large coverage (around 300 Km). Ideal for en route scenario. FDD is mandatory with both links separated around 12 MHz (current technology provides enough isolation). Main problem: mutual interference with existing narrowband systems. Narrower band CDMA is recommendable: type IS-95 (or its Multicarrier version CDMA-2000).
10 2.- VHF band: coexistence with legacy narrowband systems: voice, VDLx Interference of NB over CDMA is not a big deal Interference of CDMA over VDL is easy to solve VDLs use the same channels for all the sectors CDMA has to avoid these channels Interference of CDMA over voice channels is the key issue Voice channels use different frequencies per sector The use of these channels is dynamic depending on the activity factor It is mandatory to guarantee null degradation of these voice channels to allow the deployment of the new system
11 Analog Communications Envelope Detector Original SIR=10 dB SIR=0 dB Demonstration: Interference on Voice Channels SIR=20 dB Received Samples with Interference (Ratio is given at the output) Received Samples as Recorded (without Interference)
12 2.- VHF band: coexistence with legacy narrowband systems: Our proposal Minimize interference by the combination of two techniques CDMA transmit beamforming to eliminate interference of no co-located victim Transmit a CDMA modified spectrum with spectral nulls in some specific channels New codes design CDMA 1 CDMA 2 Victim 1 no suffering interference by spatial filtering Victim 2 Suffering interference by spatial filtering Freq. CDMA 2 Victim 2 Interference eliminated by frequency filtering CDMA 2
13 3.- Deployment in two steps VHF AM, VDL Current state C band CDMA VHF AM, VDL First step C band CDMA VHF AM, VDL CDMA Second step C Band: WCDMA- FDD VHF: IS-95 like FDD (new codes) Exploit: spatial dimension !!