With the rapid development of wireless services, the bandwidth requirement of wireless backhaul (link between base station and wireless switching equipment) is increasing rapidly. In the past, because wireless voice and low-speed wireless data were the main services of wireless, wireless base stations needed less bandwidth, so renting E1/T1 lines could meet the demand. Nowadays, with the advent of the 3G era, the rapid development of wireless data and video services makes the demand for coverage and bandwidth of wireless networks continue to grow.
However, because the growth rate of bandwidth is much faster than that of revenue, operators are facing the contradiction of increasing revenue. Return lease line accounts for about 45% of the total OPEX cost of operators. For traditional voice services, 2G base stations can transmit 1-2 E1/T1 basically. At this time, the contradiction between bandwidth and revenue is not prominent. But in the era of 3G, base stations usually need 4-5 E1 interfaces, and base stations supporting HSDPA may even need 8-16 E1/T1. Therefore, if the transmission mode of renting E1/T1 is still adopted, the OPEX of its network will continue to rise because of the growth of bandwidth demand, which forces operators to find low-cost wireless return solutions with business security and quality assurance.
PON, as one of the network technologies that can meet the needs of the above operators, has been paid more and more attention by operators and standard organizations. GPON/10G GPON time synchronization solution has been completed and announced in ITU-T related standard series. IEEE is also working on the relevant standards of EPON/10G EPON time synchronization solution and will be released soon.
Time Synchronization Requirements for Wireless Return Networks
For WCDMA, because of the use of asynchronous base station technology, there is no need for wireless return network to provide time synchronization. For CDMA2000 and TD-SCDMA, the synchronous base station technology is adopted, so the wireless return network is needed to provide time synchronization. The LTE of 4G also tends to adopt synchronous base station technology. According to the requirement of 3GPP2 standard (cdma2000 international standard), the accuracy of time synchronization is better than (+3?S), so the accuracy of time synchronization between base station and reference clock is better than (+1.5?S). According to the requirement of 3GPP standard (international standard of TD-SCDMA and WCDMA) for TD-SCDMA, the accuracy of time synchronization is better than (+3?S); similarly, the accuracy of time synchronization between base station and reference clock is better than (+1.5?S). Therefore, the accuracy requirement of phase synchronization between base station and reference clock is better than (+1.5?S).
From the above analysis, it can be seen that the time synchronization requirement is the basic requirement of cdma2000, TD-SCDMA, LTE and other wireless systems for the return network. If PON does not support high-precision time synchronization, it will not be able to serve as these wireless return networks.