Performance improving of HF modem using TCM
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Conference ICEST 2004, Bitola, FYRM
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Performance improving of HF modem using TCM
- 1. B S N Performance improving of HF modem using TCM Saša Đorđević, Aleksandar Đorđević Mihailo Pupin Institute, Belgrade, Serbia and Montenegro Performance Improving of HF Modem using TCM ICEST 2004,
- 2. B S N HF channel pro’s • • • • Long range transmission Immune to large obstacles Large number of 3kHz voice channels HF equipment is relatively inexpensive Performance Improving of HF Modem using TCM ICEST 2004,
- 3. B S N HF 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 Performance Improving of HF Modem using TCM ICEST 2004,
- 4. B S N Typical HF propagation example Ionospheric layers Propagation paths The Earth Performance Improving of HF Modem using TCM ICEST 2004,
- 5. B S N HF modem design approaches • Single tone modem • Parallel tone modem • “CHIRP” tone modem Performance Improving of HF Modem using TCM ICEST 2004,
- 6. B S N Single tone modem • • • • Single carrier frequency Simpler design Relatively high bit rate (2400bps) Very low robustness Performance Improving of HF Modem using TCM ICEST 2004,
- 7. B S N “CHIRP” tone modems • • • • Continuous frequency sweep in voice band Relatively complex design Very low bit rate (up to 75 bps) Extremely robust Performance Improving of HF Modem using TCM ICEST 2004,
- 8. B S N Parallel tone modem • • • • Multiple carrier frequencies (16 to 20) Complex design Relatively high bit rate Rather robust Performance Improving of HF Modem using TCM ICEST 2004,
- 9. B S N 20 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 Performance Improving of HF Modem using TCM ICEST 2004,
- 10. B S N Transmitted composite OFDM signal s (t ) = ∞ N t −1 ∑ ∑c n = −∞ k = 0 e j 2πf k t g k (t ) = 0 n ,k g k (t − n ⋅ ( N tTs )) , t ∈ [0, N t ⋅ Ts ] , other Performance Improving of HF Modem using TCM ICEST 2004,
- 11. B S N 16 tone parallel modem Doppler-shift correction tone 7dB 605 16 DQPSK data carriers 935 Frequency 2585 [Hz] 110 Performance Improving of HF Modem using TCM ICEST 2004,
- 12. B S N HF3618 signal structure Doppler-shift correction tone 20 DQPSK data carriers 7dB 495 605 1595 2585 Frequency [Hz] 110 Performance Improving of HF Modem using TCM ICEST 2004,
- 13. B S N “Guard time” Active symbol periode “Guard time” 13.5ms 4.5ms 13.5ms Performance Improving of HF Modem using TCM ICEST 2004,
- 14. B S N HF 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 Performance Improving of HF Modem using TCM ICEST 2004,
- 15. B S N HF channel characteristics, cont’d • Time varying impulse noise – Industrial – Solar induced – From lightning • Deliberate interference (ECM) Performance Improving of HF Modem using TCM ICEST 2004,
- 16. B S N CCIR definitions of HF radio channel Good channel Poor channel Fade rate fd [Hz] 0.1 1 Interpath delay td [ms] 0.5 2 Performance Improving of HF Modem using TCM ICEST 2004,
- 17. B S N Elimination 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 Performance Improving of HF Modem using TCM ICEST 2004,
- 18. B S N 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 Performance Improving of HF Modem using TCM ICEST 2004,
- 19. B S N Additional 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 Performance Improving of HF Modem using TCM ICEST 2004,
- 20. B S N Comparison of simulation results • Simulated 3000 bps modem with 8PSK TCM • Single tone 2400 bps HF modem (MIL-ST-188110A) • 16-tone 2400 bps modem with 8PSK TCM • 16-tone 3600 bps modem with 16PSK TCM Performance Improving of HF Modem using TCM ICEST 2004,
- 21. B S N Performance over CCIR Good channel SNR[dB] 1E+00 0 5 10 15 Single tone 2400 20 25 2400 TCM 30 3000 TCM 35 40 3600 TCM Bit Error Rate 1E-01 1E-02 1E-03 1E-04 1E-05 Performance Improving of HF Modem using TCM ICEST 2004,
- 22. B S N Performance over CCIR Poor channel SNR[dB] 1E+0 0 0 5 10 15 Single tone 2400 20 25 2400 TCM 30 3000 TCM 35 40 3600 TCM Bit Error Rate 1E-01 1E-02 1E-03 1E-04 1E-05 Performance Improving of HF Modem using TCM ICEST 2004,
- 23. B S N Benefit 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. Performance Improving of HF Modem using TCM ICEST 2004,
- 24. B S N Conclusion 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. Performance Improving of HF Modem using TCM ICEST 2004,
- 25. B S N 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 Aleksandar Djordjevic email: sasha@kondor.imp.bg.ac.yu email: djole @kondor.imp.bg.ac.yu Performance Improving of HF Modem using TCM ICEST 2004,
Editor's Notes
- OFDM (Orthogonal Frequency Division Multiplex) signal consists of N subcarrier carriers gk(t) with symbol period of Ts with cn,k as n-th symbol over k-th carrier
- Current 2400 bps modems are 16-tone modems. Basically, the signal is composed of 17 OFDM tones. (16 data carriers and one Doppler-shift correction pilot tone). Pilot tone is set on 605Hz and is 7dB larger then the data carriers creating SNR loss of 1.7dB. Data carriers reside in spectral area from 935 to 2585 Hz with spacing of 110 Hz between each of the signaling tones. The 110 Hz spacing is due to time waveform of transmitted signal (which will be explained later with term “Guard time”)
- HF3618 3000 bps modem uses 20 data carriers with DQPSK modulation, using larger part of the voice band, thus making it more efficient for data transmission, but effectively reducing power of each signaling tone for approximately 1 dB. Doppler shift correction is done by sending three pilot tones set in second, eleventh and twentieth channel in a 0.5 second period prior to data transmission. This method does not reduce SNR, but procedure for establishing connection is longer for 0.5 seconds. Maximum frequency offset which can be overcame by this method can be in range of ±75 Hz
- Because of the intersymbol interference (ISI) there is a "guard time" after each signaling period. Active signal period is 13.5 ms and the guard time is 4.5 ms as shown in figure. With total time of 18ms it has bandwidth of 110Hz as shown before. Due to this time, effective SNR in HF modem is 1.8dB less than in ideal system.
- Fading is so dominant in HF channel, that the mere effect of fading was first noticed in radio broadcast over HF on the early stage of radio communication.
- Time and space variation of signal can be diminished by using time and space diversity. This can be accomplished through time interleaving and using two or more receive points (antennae). These two methods exclude each other and can be used according to the nature of transmission. If voice transmission is used, long interleaving blocks cannot be tolerated, thus urging the use of space diversity. In data communication, time gap is not of great importance so interleaving can be used. Long interleaving blocks cannot be used if modem is part of an ARQ system. Although interleaving can decrease error rate, it can also make periodic error bursts. These errors then must be retransmitted, thus reducing an overall data rate. Due to this reasons simulation is done using very short interleaving blocks. It is shown that space diversity can considerably decrease BER without need for long data interleaving [2]. This combination of space diversity and short interleaving blocks are ideal for both voice and data transmission.
- There are four types of modems taken into account for this comparison.
- This figure shows that parallel tone modems are more robust than single tone. It can be shown that even 50% higher data rate can be used over same channel with the same if not better results. The only area where single-tone modem has better performance than 3600 bps is that of SNR>25dB. A 3000 bps modem has lower performance than 2400 bps modem by some 3.5dB lower SNR.
- Over Poor channel single-tone modem has the lowest overall drop of performance and on some SNR values it almost reaches performance of 2400 TCM modem. A 3000 bps TCM modem has performance that was expected, although on some SNR's (10-20dB) it has worse performance in comparison with single-tone modem. Overall, results are as expected, with performance of 3000 bps modem between those of 2400 and 3600 bps modems.
- This simulation shows that by using TCM in some of existing HF modem techniques (such as 16 or 20-tone parallel modem) quality and robustness of data transmission can be improved significantly without decreasing data rate. Also, using modern DSP algorithms a Doppler correction pilot tone can be omitted, thus enabling a lower portion of the voice spectrum to be used for data carriers. In case of HF3618 modem, omission of the Doppler correction tones can lower channel setting time by some 0.5seconds. By adding additional tones data rate of the HF modem can be increased by losing a small amount of overall performance. Using other MPSK schemes are also in consideration. Some companies are already producing 4800 bps HF modems, which are obviously 16-tone non-TCM 16PSK modems.