Key Techniques Of The Voice Transmission In Shortwave Channel

Shortwave communication, which is one of the wireless communications and reliesmainly on the reflection of the ionosphere for long distance transmission, is a kind ofeffective and economical means of emergency communication. It is widely used inmeteorology, scientific exploration, and disaster relief, commercial and other fields.Meanwhile, it is also essential communication in sea voyage. Therefore, the reliabilityof voice transmission has important practical application value in emergencycommunication. In this paper, the voice transmission in shortwave channel is studied.Firstly, the voice transmission link based on the third generation standard ofshortwave communication is studied, including Forward Error Correction (FEC) coding,gray coding, symbol mapping, scrambling etc. We study the key techniques of thereceiver, including demodulation, descrambling, channel estimation, equalization,decoding etc. In the simulations, we make a comparison of the performance ofconvolutional code and the biting convolutional code; and compare channel estimationalgorithms of the Least Square (LS), the based on the autocorrelation of pilot and thebased on Discrete Fourier Transform (DFT); and contrast the Minimum Mean SquareError (MMSE) frequency domain equalization with the Decision Feedback Equalization(DFE). As the requirement of real time in voice communication, the system withoutinterweaving cannot resist the deep fading of channel, and the continuous bit error willreduce the performance of the decoder. In this paper, diversity reception is used toreduce bad influence of the channel fading on the performance of the system.Secondly, the diversity reception schemes of distributed antenna are studied. Forsingle user multi-relay system, through the theoretical derivation, we analyze differentreception schemes from the view of the outage probability and Diversity MultiplexingTrade-off (DMT). In the simulations, we compare the performance of the SelectionCombining (SC), the Maximal Ratio Combing (MRC) in all branches and the randomselection combing. We obtain the fact that the scheme of MRC in all branches can getthe highest diversity gain and the optimal performance of outage probability, but havethe highest receiving complexity. Further, the scheme of a certain number of branch combing can achieve the compromise between performance and complexity. Shortwavevoice link effectively reduces the channel fading effect on the performance of thesystem using multipath diversity receiving.Again, the relay allocation based on maximum rate is studied in multi-usermulti-relay system. Through the theoretical derivation and analysis, we obtain theoptimal relay allocation strategy based on the maximum rate by setting forwardingthresholds for relay. In the case of independent channel, under the condition of the highSNR, we get the fact that the system has the maximum transmission rate when therelays are allocated equivalently in users. Under the condition of the low SNR relativeto the threshold, when the relays are allocated equivalently in users, the diversity gain ofthe single user is less than the highest diversity gain of the users in other relayallocations, so the outage possibility of the user is increased. Therefore, the systemcannot get the maximum transmission rate when the relays are allocated equivalently inlow SNR.Finally, in the case of the correlated channel, the relay allocation based onmaximum rate is studied in multi-user multi-relay system. Also, we get the sameconclusion with the independent channel. However, the correlation of channel can resultin a loss of the system transmission rate. Under the condition of the weak correlation ofchannel, the correlation has little impact on the rate of the system. With the increasingof the correlation coefficient, the correlation of channel has brought great loss to thetransmission rate under the condition of the strong correlation.In conclusion, we mainly simulate the voice transmission link based on the thirdgeneration standard of shortwave communication, study the diversity reception schemesof the distributed multiple receiving antennas, and analyze the relay allocation strategyin multi-user multi-relay system.