## 2 Fast Telemetry and Communication Method Based on Doppler Diversity Reception  
In the traditional acquisition and tracking methods, the received signals have to be acquired and processed to make the residual Doppler frequency offset within the acquisition band of the narrow-band phase-locked tracking loop. Generally, residual Doppler has to be limited within 20 Hz. All of these propose very high requirements on signal acquisition. The Doppler capture search for hypersonic vehicles range from MHz to 10 Hz with spanning \(10^{5}\) orders of magnitude, which consumes a longer time and cannot meet the requirements of actual fast telemetry and communication.  
From the perspective of communication, the large dynamic Doppler is a fast time-varying factor relative to the wireless channel and it is a disadvantageous factor against communication. Small Doppler can be adjusted through channel estimation and equalization, but large Doppler cannot be adjusted in the same method. Doppler diversity is a new diversity method that views Doppler frequency offset as an advantageous factor [8; 9] and acquires gains through diversity processing in the Doppler domain [10; 11; 12; 13; 14]. However, Doppler diversity has limited ability in processing frequency offset. Some studies have reported that the Doppler diversity is effective in the kHz level of mobile communication field. Doppler diversity might have poor adaptation to MHz large Doppler when it is applied to the communication of hypersonic vehicles directly. Therefore, it is still necessary to decrease Doppler frequency offset within the processing capacity range of diversity through compensation before using the Doppler diversity reception method. The main idea and specific implementation of the proposed communication scheme is shown in Figure 1. Large Doppler frequency offsets and Doppler rate-of-change are estimated roughly through fast coarse acquisition. Meanwhile, the received signal is compensated and Doppler diversity treatment is performed to signals with residual Doppler frequency offset. The advantage of this communication scheme avoids the complicated refine acquisition and phase-locked tracking loop ingeniously, and it does not design complicated reliable mechanisms to guarantee secondary acquisition and tracking upon unlocking fault [15; 16].  
At the sending end, IFFT (Inverse Fast Fourier Transformation) is conducted to the modulated sending signal and the sending symbol \(d(t)\) is formed. \(d(t)\) is viewed as the frequency domain signals and pilot signals are inserted to form \(s(t)\) for sending. \(h(t)\) is the fading coefficient of time-varying channel. The received signal model at the reception end can be expressed as:  
\[y(t)=h(t)s(t)e^{j2\pi(f_{d}t+\frac{1}{2}f_{d}t^{2})}+v(t) \tag{1}\]  
where \(f_{d}\) and \(f_{a}\) are the Doppler frequency offset and Doppler rate-of-change. \(v(t)\) denotes the additive white Gaussian noise.  
Fast coarse acquisition of the received signals with large dynamic Doppler frequency offset is implemented. Suppose the rough estimations of Doppler frequency offset gained from coarse acquisition and its change rate are \(F^{{}^{\prime}}_{d}\) and\(F^{{}^{\prime}}_{a}\), respectively. The received signals were compensated by the rough estimations, getting signals with residual Doppler frequency offset \(y_{r}(t)\). \(y_{r}(t)\) is shift by different frequency obtains branches and each branch channel state information is gained through channel estimation. All branches are combined by using channel state information in all branches, thus getting diversity gains. Finally, information is demodulated.