Erik Haas and Michael Schnell German Aerospace enter - DLR J. Prinz,.Rihacek, and M. Sajatovic Frequentis Nachrichtentechnik G.m.b.H.
Overview urrent VHF Band Situation OFDM Multi-arrier Modulation Multi-arrier DMA Future B-VHF Environment Data Link System Structure ssues That Need to be Solved for B-VHF 2
urrent VHF Band Situation General Facts VHF OM Band: 118-137 MHz 4 Mainly voice communications 4 Previously 760 channels with 25 khz channel spacing 4 Now also additional 8.33 khz channel spacing Line-of-sight (LOS) Airborne Radio hannel 4 Good balance between coverage and transmitting power 4 ell size up to 200 nautical miles FDMA (One Frequency per Sector) 4 Half duplex channel ( Listen-before-PTT access ALOHA) 4 Voice party line exists between all users on a given channel DSB-AM Modulation 4 25 khz channel spacing (~8 khz RF BW) 4 8.33 khz channel spacing (~5 khz RF BW) VHF Voice Band in Europe is Already ongested! 3
urrent VHF Band Situation Theoretical 25 / 8.33 khz channel spacing All channels continuously allocated & used Power Analog 25 khz VHF AM-hannel 8.33 khz VHF AM-hannel 25 khz Frequency Digital 25 khz VHF VDL-hannel 4
Requirements for a New System oncurrent usage in the same frequency band without interference with DSB-AM / VDL system Full duplex (options: FDD, TDD) Priority management Efficient radio frequency spectrum utilization to meet (future) voice and data requirements Reduced susceptibility to radio frequency interference mproved communications systems security No increase of user workload Robustness against jamming and attacks of phantom controllers 5
Starting Facts for a New System t is difficult to find additional frequencies for VHF data links At any given location, only a part of all VHF channels are allocated and used for AT 4 The VHF channel assigned to the current sector is definitely occupied, as well as the VHF channels from near-by sectors. 4 VHF channels used in distant sectors might be seen as unoccupied 4 What an aircraft sees depends on the flight level 4 The aircraft will always see less than 760 active channels At any given moment, only a part of the allocated VHF channels are actually used, i.e., the duty cycle on each channel is less than 100% 6
urrent VHF Band Situation Practical 25 / 8.33 khz channel spacing Only a part of the allocated channels are used Not all channels are seen with full power all the time Power Analog 25 khz VHF AM-hannel 8.33 khz VHF AM-hannel 25 khz Frequency Digital 25 khz VHF VDL-hannel 7
Orthogonal Frequency Division Multiplexing OFDM Distribution of a high rate data stream on many orthogonal subcarriers with low data rate N FFT sub-carriers f F S B f c B : Bandwidth F S : Subcarrier Spacing f c : arrier Frequency 8
Orthogonal Frequency Division Multiplexing OFDM Distribution of a high rate data stream on many orthogonal subcarriers with low data rate OFDM X Data Data Serial to Parallel onverter Parallel to Serial onverter S R FFT FFT Add Guard nterval Remove Guard nterval Y Parallel to Serial onverter Serial to Parallel onverter Digital to Analog onverter Analog to Digital onverter x(t) y(t) hannel h(τ,t) n(t) nverse OFDM 9
Advantages of OFDM Multi-arrier Modulation ncrease of spectral efficiency Decrease of equalization complexity Only one carrier frequency necessary Efficient modulation algorithm available (FFT, FFT) System can be designed to fit requirements perfectly 4 Bandwidth 4 Transmission channel No continuous spectrum required 4 Sector-oriented exclusion of VHF frequencies used for DSB-AM & VDL in the near-by sectors is possible to minimize interference 10
Principle of M-DMA Several users occupy the same set of subcarriers L different orthogonal spreading sequences (.) are used to separate the users ( k ) ( k ) ( k ) S S x() t OFDM The spreading sequences are Walsh-Hadamard codes Maximum number of users K = L Number of subcarriers N c = L 11
M-DMA Example 1 +1 +1 1 2 +1 +1 2 3-1 OFDM hannel OFDM -1 3 4 +1 User Data Spreading M-Modulation Transmission M-Demodulation De-Spreading +1 User Data 4 12
M-DMA with M&Q Modification Standard M-DMA 13
M-DMA with M&Q Modification 1 1 M OFDM 1 1 M M nterleaving OFDM x() t M 14
M-DMA with M&Q Modification 1 Each user in every group has M data symbols available Extension to Q independent user groups Maximum number of users K = QL Number of subcarriers N c = QML FDMA User Group 1 OFDM User Group Q OFDM 1 M M ( K L+ 1) 1 ( K ) 1 ( K L+ 1) M nterleaving OFDM x() t M ( K ) 15
Advantages of M-DMA M-DMA introduces multiple-access component Number of users can be exchanged versus available bandwidth per user Robust against narrowband interferers / attacks / jamming Narrowband interference suppression Performance improvement through spreading gain Resistant against slow / fast frequency selective fading channel 16
Future B-VHF Environment Transition Phase hannel allocation remains unchanged for DSB-AM & VDL channels Distant VHF channels can be locally re-used for the new B-VHF system Old DSB-AM / VDL equipment remains untouched Power Analog 25 khz VHF AM-hannel 8.33 khz VHF AM-hannel 25 khz Frequency Digital 25 khz VHF VDL-hannel B-VHF hannel 17
Future B-VHF Environment ompletion Phase DSB-AM & VDL channels progressively replaced by the new system Distant VHF channels can be locally re-used for the new B-VHF system Old DSB-AM / VDL equipment remains untouched Power Analog 25 khz VHF AM-hannel 8,33 khz VHF AM-hannel 25 khz Frequency Digital 25 khz VHF VDL-hannel B-VHF hannel 18
ssues That Need to be Solved for B-VHF DSB-AM / VDL channels which can not be utilized for the current aircraft location need to be known by the B-VHF system Avoidance of interference with existing DSB-AM / VDL channels OFDM system structure Multi-user access technique for up- and downlink 4 DMA, FDMA, TDMA 4 Sub-channel mapping to users / user groups Multiplexing technique for half-duplex voice / full-duplex data 4 FDD Frequency Division Duplex 4 TDD Time Division Duplex Synchronization, channel estimation & equalization Protocol layer structure Simultaneous support for data & voice communications 19
Questions??????????? 20