| China terrestrial digital television broadcasting is still developing and popularizing.Limited signal coverage and hysteretic signal standard seriously reduce the spectrum availability of the UHF band.606?678MHz includes 9 channels,however,only few of them are utilized in certain period and region.It is a great loss for the relatively scarce spectrum resource.In this situation,typical multi-carrier technology orthogonal frequency division multiplexing has a higher transmission rate with higher demodulation signal noise ratio(SNR).Typical single-carrier technology direct sequence spread spectrum has a stronger anti-interference ability,however,the transmission rate is limited.In this paper,a spectrum adaptive access high speed covert digital transmission system is designed on the premise of no influence on the terrestrial digital television broadcasting signal.It utilizes transform domain communication technology to aggregate the unoccupied spectrum and increase spectrum availability.Theoretical innovations focus on increasing spectrum efficiency,reducing peak to average power ratio(PAPR)and enhancing the adaptability on fast time varying multipath fading channels.Based on those innovations,a feasible system is designed,implemented and tested.Specific content is as follows:1.Focused on the spectrum adaptive access and the anti-interference problems,the spectrum condition of 606?678MHz in Beijing is investigated and measured.The existing wireless data transmission systems are analyzed to match the typical spectrum conditions.The main problems include the low transmitting rate or anti-interference ability,affect on the terrestrial digital television broadcasting signals,spectrum exchange between the transmitter and the receiver,etc.The fundamental of the transform domain communication system(TDCS)is illustrated.It has spectrum adaptive access,high level modulation low power spectral density and strong anti-interference ability,which is quantitatively analyzed to-52/-44 d B signal interference ratios(SIR)for typical wideband/narrowband interferences.TDCS is proved to be a better choice for this typical spectrum condition.2.The multi-dimension parallel combinatory signal structure is proposed to solve the low spectrum efficiency problem of TDCS.Firstly,by analyzing the influence of the signal dimensionality on the system spectrum efficiency and bit error rate(BER),the spectrum efficiency improvement is transformed into the problem of increasing the number of the modulation symbols at the expense of minimum BER loss.Secondly,the combination of multi-dimension vector is proposed to extend modulation symbols,where every symbol is generated by a series of phase modulated cyclic shift keying symbols.The phase modulation is limited to ensure the Euclidean distance invariant.Thirdly,the signal design is transformed into the system implementation scheme.The multi-dimension parallel combinatory TDCS is proposed.Compared with the existing methods,the spectrum efficiency is largely improved,BER is also improved to some extent.Finally,a modulation adaptive scheme based on synchronous head is proposed to ensure the system adapt to the changing spectrum environment.The transmission rate and the BER performance are both improved.3.The complementary peak reduction signal(CPRS)method is proposed to solve the high PAPR problem.Firstly,PAPR is proved to be affected by both the spectrum environment and the pseudo random phase sequence.Secondly,the CPRS method is proposed,which uses all interfered frequency bins to generate a peak reduction signal(PRS).PRS is posited with the data signal to reduce PAPR.Compared with the existing methods,the proposed method does not need side information.The computation complexity is largely reduced.The PAPR reduction performance is also improved.Thirdly,a power adjustment coefficient is introduced to solve the power loss problem caused by PRSs,where the power is concentrate in the data signal,to obtain the tradeoff between the BER and PAPR reduction performance.With the low power spectral density characteristic,the system has almost no effluence on the broadcasting signal.Finally,qualitative and quantitative analysis and simulations are performed to verify the feasibility of the proposed method on multi-dimension parallel combinatory signals.4.The high efficiency non-uniform pilot design is proposed to adapt the multipath fading channel with Doppler to ensure the large spread spectrum ratio covert communication.Firstly,the cognitive transmitters and receivers are classified.The effect of the spectrum asymmetry conditions on system performance is limited within 3d B SNR loss.Then,the frequency domain non-uniform pilot is proposed,which ensures the effective channel estimation of the non-continuous spectrum with low PAPR.Secondly,a time-frequency two-dimension non-uniform pilot is proposed to improve the system efficiency.Finally,a compressive sensing measurement matrix is proposed based on non-uniform pilot to solve the large computation complexity and the noise influence of the least square method in the practical channel estimation.5.Based on those innovations,a 606?678MHz spectrum adaptive access high speed data transmission system based on transform domain communication technology is designed with prototype and experimental verifications.The transmission rate,signal PAPR,anti-interference performance,BER performance,multipath Doppler channel estimation performance and the transmission distance are tested.Then system over-all performance is evaluated to verify the technological innovations and the over-all performance of the proposed system.The system can run in the-38 d B SIR 10% widband interference and typical Doppler multipath fading channel with more than 5Mbps transmitting rate and 5km range.The spectrum adaptive access increases the spectrum efficiency without affecting on the nomal terrestrial digital television broadcasting signal.In conclusion,the proposed system satisfies the demonds on 606?678MHz spectrum adaptive access high speed covert data transmiting. |
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