Synchronization in Digital Communication Systems by Fuyun Ling

Author:Fuyun Ling [Ling, Fuyun]
Language: eng
Format: azw3, pdf
Publisher: Cambridge University Press
Published: 2017-05-31T04:00:00+00:00

4.6.2.2 Mobile/Wireless Device Transmitter Frequency Synthesis

In wireless communication systems, the carrier frequencies generated by the BTS are usually very accurate. This is because BTSs can afford to use high-precision (yet high-cost) oscillators. Moreover, BTSs often synchronize their clocks to the clock of GPS or other similar systems to achieve high-frequency accuracy.

However, to save cost and power, mobile device manufacturers prefer to use low-cost XOs. When free running, such XOs can generate only frequency references with low precision, e.g., with an inaccuracy of 2 ppm to 10 ppm or worse. However, due to standard and local regulatory requirements, mobile devices must maintain necessary precision of the carrier frequency of their transmitted signals. During a normal communication session, a device receiver acquires the carrier frequency of the remote BTS through carrier synchronization. The acquired carrier frequency can be used as the local frequency reference by the device. Therefore, the synthesized local reference frequency in the device can approach the accuracy of the remote transmitter’s carrier frequency.

The carrier synchronization of receivers is achieved through a phase-locked loop as discussed in Section 4.5.2 and as will be discussed in the next chapter. Carrier synchronization of mobile and wireless devices may also be implemented by using FLLs, which has been mentioned in Section 4.6.1.2 and will be described in Section 5.4.7. If the phase or frequency controller in the PLL or FLL is implemented by using a type of VCXOs, such as VC-TCXO, the output of the local oscillator is synchronous to the remote transmitter carrier after carrier synchronization is achieved. Thus, using the output of the VC-TCXO as the reference for the frequency synthesizer can ensure that the carrier frequency of the device’s transmitter attain the required accuracy.

Another possibility is that the receiver’s PLL or FLL for carrier synchronization is implemented by using a free-running XO together with a software phase controller implemented in DSP or ASIC. While the frequency of the XO’s output is not synchronous to the remote carrier frequency, accurate local transmitter’s carrier frequency can be generated based on knowledge of the estimated frequency offset from the PLL or FLL. For example, the local transmitter carrier frequency can be generated as illustrated in Figure 4.27.

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