Updated LDACS1 BER performances with LPES



|[pic] | |ACP-WGM21/IP-02 |

| |International Civil Aviation Organization |17 Jul 2014 |

| | | |

| |INFORMATION PAPER | |

| | | |

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

21st MEETING OF WORKING GROUP M (Maintenance)

Montréal, Canada 17 – 18 July 2014

|Agenda Item x: |LDACS |

Updated LDACS1 BER performances with LPES

(Prepared by Jun Kitaori, ENRI, Japan)

|SUMMARY |

|We have rewritten programs on the LPES (LDACS Physical layer Experimental System) and we have confirmed |

|that BER performances have improved. This paper describes the LPES configuration for LDACS1 BER |

|measurements, conditions and updated BER performance results. In the measurements, the AWGN channel, |

|frequency shift channel and three fading channels have been used. In the reverse link data frame, high |

|corrected BER remains on the channel with large frequency shift. Even in forward link data frame, high |

|corrected BER remains on the en route fading channel. |

1. INTRODUCTION

Previously, we reported some results of LDACS (L-band Digital Aeronautical Communication System) BER (Bit Error Rate) performances obtained with LPES (LDACS Physical layer Experimental System)[1]. However, the BER performances on the AWGN (Additive White Gaussian Noise) channel were much worse than general performances. After that, we rewrote programs on the LPES and have confirmed that BER performances have improved. This paper describes the experimental system configuration for LDACS1 BER measurements, conditions and updated BER results.

2. system CONFIGURATION with lpes AND CONDITIONS for BER measurement

The system configuration with LPES is shown in Figure 1. This time, TX (signal transmitter) side is configured with Agilent instruments. Agilent N5106A has an arbitrary waveform generator, a fader and a noise generator. It repeatedly generates a modulated baseband signal of an LDACS1 frame format. We can choose the frame format from the FLDF (forward link data frame) or RLDF (reverse link data frame). The baseband signal is affected by a fader, and then it is mixed with Gaussian noise. Agilent E4438C up-converts the baseband signal to an RF signal. The RF center frequency is set to 970 MHz. On the RX (signal receiver) side, the LPES receives the LDACS1 RF signal and calculates BER.

3. BER performance results

First, we show updated LDACS1 BER performances on the AWGN channel in Figure 2. The frame type is FLDF. Three types of modulations, QPSK (Quadrature Phase Shift Keying; drawn by red lines and square icons), 16QAM (Quadrature Amplitude Modulation; green lines and circle icons) and 64QAM (blue lines and triangle icons) are shown in the figure. The CNR (Carrier to Noise Ratio) achieving corrected BER=0 for QPSK is 8dB. Compared to the previous report [1], the CNR has improved by about 9dB. We have replaced the synchronization part in the LPES RX program. An algorithm based on [2] is a better method than the previous implemented method. The CNR for 16QAM and 64QAM are 9dB and 14dB, respectively. When the uncorrected BER of these modulations is less than about 10-2, the corrected BER will be zero.

When frequency shift such as Doppler shift is added to the LDACS1 signal, it will cause bit errors. Secondly, we have obtained BER performance vs frequency shift. Figure 3 describes the BER performances in FLDF (drawn by red lines and square icons) and RLDF (green lines and circle icons for DFE, and blue lines and triangle icons for IE). In the figures starting from Figure 3, filled icons and solid lines indicate corrected BER. Unfilled icons and dashed lines indicate uncorrected BER. The red lines and green lines are obtained with DFE (Decision Feedback Equalizer). The corrected FLDF BER is zero over the frequency range -1500 to +1500 Hz. The maximum frequency shift 1500 Hz corresponds approximately to 900 knots or 1670 km/h when carrier frequency is 970 MHz. In contrast, the range achieving corrected RLDF BER = 0 is around -200 to +200 Hz. The principal difference between FLDF and RLDF is pilot symbol map. Pilot symbol maps in FLDF and RLDF are shown in Figure 4. Four consecutive OFDM symbols without pilot symbols are placed in an RLDF. The DFE doesn’t update channel estimation if no pilot symbols exist in an OFDM symbol. Therefore, many bit errors would occur at the OFDM symbol under worse channel estimation. To reduce channel estimation errors for RLDF, we have implemented another type of equalizer which simply calculates interpolating values between pilot symbols in frequency axis and time axis. In this paper, we call it IE (interpolation equalizer). The blue lines in Figure 3 describe BER with IE. You can see that the range achieving corrected RLDF BER with IE = 0 is around -700 to +800 Hz. The range is wider than the green lines. However, Doppler shift of 800 Hz corresponds to mobile velocity of 891 km/h or 481 knots. The velocity doesn’t satisfy the maximum applicable Doppler shift of 850 knots [3]. Appropriate equalizer should be chosen to reduce bit errors.

We also have measured BER performances under some fading environment conditions. They are listed in Table 1. The conditions are based on the reference [3] Appendix A, but they are modified to suit our equipment specifications. The BER performance results under fading conditions are shown in Figure 5 for ENR (en route), Figure 6 for TM (terminal) and Figure 7 for APT (airport), respectively. The legends in the figures are the same as in Figure 3. Under ENR conditions, FLDF can achieve corrected BER=0 at CNR=9 dB, however, RLDF (with DFE and IE) never reaches corrected BER=10-1. The main reason for the RLDF BER degradation is too large maximum Doppler shift. Our equalizers insufficiently estimate ENR fading channel states. Under TM conditions, FLDF can achieve corrected BER=0 at CNR=15 dB, and IE improves RLDF BER performances better than DFE. Under APT conditions, RLDF BER performances with IE are better than those with DFE, however, the performances in both FLDF and RLDF don’t reach the corrected BER=5x10-4. These corrected BER=5x10-4, in other words, the probability that a frame of 1000 bit data has bit errors after error correction will be 0.5. Retransmissions of correct frames will frequently occur in these situations.

4. conclusions

We have updated the results of LDACS1 BER performances on the AWGN channel. Moreover, we have obtained new LDACS1 BER performances under frequency shifts and some fading conditions. Large frequency shifts make high corrected BER in RLDF. Under ENR conditions, corrected BER on RLDF never reaches 10-1. Our equalizers insufficiently estimate ENR fading channel states. Under APT conditions, corrected BER in both FLDF and RLDF doesn’t reach 5x10-4. Retransmissions will frequently occur in these situations. To reduce retransmissions, some ways are effective. For example, finding a more appropriate equalizer or designing an alternative pilot symbol map for RLDF.

5. acknowledgement

The author would like to thank Mr. Ulrich Epple of the German Aerospace Center for providing the information for a better synchronization algorithm.

References:

[1] J. Kitaori, Results of LDACS PHY BER performances with GNU Radio, ICAO ACP-WGM20/IP-08, Jan. 2013.

[2] J. van de Beek, M. Sandell, and P. Borjesson, “ML estimation of time and frequency offset in OFDM systems,” IEEE Trans. Signal Process., vol. 45, pp. 1800–1805, July 1997.

[3] SESAR Joint Undertaking. Updated LDACS1 System Specification, EUROCONTROL, 2011.

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Figure 1: System configuration

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Figure 2: Updated LDACS1 BER performance on AWGN channel

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Figure 3: BER performance vs frequency shift

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Figure 4: Pilot symbol maps

Table 1: Fading environment conditions

EABIC 3: BER performance vs frequency shift

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Figure 4: Pilot symbol maps

Table 1: Fading environment conditions

Environment |Fading type |Delay [μs] |Attenuation [dB] |Max. Doppler shift [Hz] | |

ENR (en route) |Pure Doppler

Rayleigh

Rayleigh |0

0.3

15 |0

16

22 |997

603

-594 | |TM (terminal) |Rice (K=10[dB]) |0 |0 |498 | |APT (airport) |Rayleigh |0 |0 |330 | |

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Figure 5: BER performance under ENR fading conditions

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Figure 6: BER performance under TM fading conditions

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Figure 7: BER performance under APT fading conditions

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