performance improving of hf modem using tcm

Performance Improving of HF Modem using TCM ICEST 2004, BitolaPerformance Improving of HF Modem using TCM ICEST 2004, Bitola

institut MIHAJLO PUPIN

IMP-TELEKOMUNIKACIJE, d.o.o.

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Performance improving of HF modem using TCMPerformance improving of HF modem using TCM

Saša Đorđević, Aleksandar Đorđević

Mihailo Pupin Institute, Belgrade, Serbia and Montenegro




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HF channel HF channel pro’spro’s

• Long range transmission• Immune to large obstacles• Large number of 3kHz voice channels• HF equipment is relatively inexpensive




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HF channel con’sHF channel con’s

• Impulse noise made by both man and nature • Slow multipath fading• Random (solar induced) ionospheric disturbances• Long distance HF communication can not be

available for 100% of a time




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Typical HF propagation exampleTypical HF propagation example

The Earth

Ionospheric layers

Propagation paths




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HF modem design approachesHF modem design approaches

• Single tone modem

• Parallel tone modem

• “CHIRP” tone modem




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Single tone modemSingle tone modem

• Single carrier frequency• Simpler design• Relatively high bit rate (2400bps)• Very low robustness




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““CHIRP” tone modemsCHIRP” tone modems

• Continuous frequency sweep in voice band• Relatively complex design • Very low bit rate (up to 75 bps)• Extremely robust




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Parallel tone modemParallel tone modem

• Multiple carrier frequencies (16 to 20)• Complex design• Relatively high bit rate• Rather robust




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20 tone parallel modem HF361820 tone parallel modem HF3618

• 20 carrier frequencies with DQPSK modulation• Symbol rate is 75 baud per carrier• Total bit rate 20 75 2 = 3000 bps• 3 Doppler shift correction tones are sent prior data

transmission




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Transmitted composite OFDM signalTransmitted composite OFDM signal

n

N

kstkkn

t

TNntgcts1

0, ))(()(

other

TNtetg st

tfj

k

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2




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16 tone parallel modem16 tone parallel modem

605 935 2585Frequency

110

16 DQPSK data carriers

Doppler-shift correction tone

7dB

[Hz]




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HF3618 signal structureHF3618 signal structure

605495 2585 Frequency

110

20 DQPSK data carriers

Doppler-shift correction tone

7dB

[Hz]1595




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““Guard time”Guard time”

13.5ms 13.5ms4.5ms

Active symbol periode“Guard time”




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HF channel characteristicsHF channel characteristics

• In most cases there is no direct radio wave.• Usable information consists of at least two waves

reflected from ionosphere layers,• Because of different paths of this components a

slow Rayleigh fading occurs




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HF channel characteristics, cont’dHF channel characteristics, cont’d

• Time varying impulse noise – Industrial

– Solar induced

– From lightning

• Deliberate interference (ECM)




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CCIR definitions of HF radio channelCCIR definitions of HF radio channel

Good channel Poor channel

Fade rate fd [Hz] 0.1 1

Interpath delay td [ms] 0.5 2




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Elimination of time/space variationsElimination of time/space variations

• Time diversity (interleaving)– Can’t be used in voice transmission due to time gaps

– Can’t be used in ARQ systems

– Creates periodic error bursts

• Space diversity (multiple receiving points/ antennae).– More expensive infrastructure




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Use of trellis-coded modulation (TCM)Use of trellis-coded modulation (TCM)

• TCM implies use of redundant data bits, thus decreasing effective data rate.

• To maintain constant bit rate a different modulation than DQPSK must be used.

• Since every carrier carries two information bits it is logical to use 2/3 convolutional encoder.

• This implies use of 8PSK modulation




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Additional simulation parametersAdditional simulation parameters

• Simulation was done with 2 sec. interleaving on assumption that a space diversity is used

• No Doppler shift correction tones ( use of FFT gives 20 phasors regardless of Doppler shift)

• Eliminating pilot tone “empties” lower portion of voice band with possibility of raising data rate up to 3600 bps with a loss of only 1.8dB in SNR




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Comparison of simulation resultsComparison of simulation results

• Simulated 3000 bps modem with 8PSK TCM • Single tone 2400 bps HF modem (MIL-ST-188-

110A) • 16-tone 2400 bps modem with 8PSK TCM • 16-tone 3600 bps modem with 16PSK TCM




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Performance over CCIR Performance over CCIR GGoooodd channel channel 0 5 10 15 20 25 30 35 40

SNR[dB]

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

Bit

Err

or

Rate

Single tone 2400 2400 TCM 3000 TCM 3600 TCM




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Performance over CCIR Performance over CCIR PoorPoor channel channel 0 5 10 15 20 25 30 35 40

SNR[dB]

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

Bit

Err

or

Rate

Single tone 2400 2400 TCM 3000 TCM 3600 TCM




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Benefit of parallel tone modemBenefit of parallel tone modem

• Possibility to improve current modems with small, if any, modification of hardware, enabling simple upgrade by "flashing" new DSP code into processor.

• Multirate" modems, with selection of modulation type during initial handshaking of modems according to channel characteristics.




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ConclusionConclusion

There are many possibilities that should be accounted for, whether the goal is improving performance or data rate. It is sure that, by applying newly acquired knowledge in DSP and information theory, data rate will increase, as it is sure that a HF band would remain interesting for both military and civil services.




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Institute Mihailo Pupin

IMP – Telecommunications

Volgina 15

11060 Belgrade

Serbia and Montenegro

+381 11 27 72 953

www.telecom.imp.bg.ac.yu

Sasha Djordjevic email: [email protected]

Aleksandar Djordjevic email: djole @kondor.imp.bg.ac.yu

performance improving of hf modem using tcm

Technology

b snsingle tone

b snparallel tone

b snperformance

b snhf channel characteristics

t n n tts

n t ts

dqpsk modulation symbol

complex design