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Wireless operators constantly aim to increase the capacity of their cellular networks. With the exception of MIMO, all the technologies put into practice so far have resulted in an incremental accumulative improvement of, say, 10%, 20%, or 30%. Multipoint broadcasting, adopted into LTE-advanced under the name of CoMP (Coordinated Multi-Point transmission), promises a multi-fold capacity improvement that ranges 300% ~ 1000% and beyond.

Multipoint broadcasting boosts the capacity by having base stations collaborate to transmit to mobile users. This requires the base stations to have the knowledge of downlink channels. Since only the mobile users have the downlink channel information, the base stations have to rely on the feedback from mobile users via uplink channels. The channel feedback overhead, however, can be so large that it may exceed the entire uplink capacity.

One may expect that a TDD network can escape the uplink capacity crunch by channel feedback. In theory, a base station can acquire the downlink information from the uplink, which it can estimate, by the channel reciprocity from the downlink/uplink sharing of the same frequency band in TDD. A closer look, however, reveals that the channel reciprocity does not hold if the base station and the mobile user have different RF characteristics in the TX/RX chains, as they generally do.

We have developed innovative channel feedback mechanisms for both TDD and FDD networks that have near-zero or zero feedback overhead.  The innovations also eliminate the need for base stations to exchange the downlink channel information via high-speed backbone.

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无线运营业界一直致力于扩充无线移动网络的容量。至今为止，除了MIMO，所有付诸实践的技术所带来的改进幅度都是增量式的，比如10%，20%，或30%。多点传播技术则可以使网络容量成倍式地提高，比如300% ~ 1000%， 或更多。多点传播技术已为LTE- advanced所采纳，并被称为CoMP（Coordinated Multi-Point，协作多点传输）。

在多点传播中，几个基站通过协作的方式向移动用户传播信号，从而提升网络容量。这需要基站掌握下行信道的信息。由于只有移动用户才有下行信道的信息，基站必须依赖于移动用户通过上行信道的反馈。然而信道反馈的开销往往很大，甚至会超过上行信道容量。

时分双工网络是否可以避免信道反馈？理论上，时分双工网络中的基站可以应用信道互易原理，从上行信道信息导出下行信道信息，从而不需要信道反馈。然而，进一步的观察就会发现，通常情况下基站和移动用户的收发链包括射频链并不匹配，因此包含收发链的整体上下行信道并不互易。

青山无线开发了独特新颖的信道反馈方案。这些方案在频分双工和时分双工网络中具有零或近乎零的反馈开销。基站之间也不再需要通过高速主干回路来交换下行信道信息。