Radio access networks
Among other things, a communication system comprising remote units, a reference timing source, a controller, a controller clock, and a remote unit clock. The remote units exchange radio frequency (RF) signals with mobile devices. The reference timing source is synchronized with a coordinated universal time (UTC) or a Global Positioning System (GPS). The controller clock is synchronized with the reference timing source and provides timing information to the controller. The remote unit clock is synchronized with the controller clock and provides timing information to a remote unit. The controller and the remote unit are configured to transmit time stamp messages to synchronize the controller clock and the remote unit clock, by avoiding contention between time stamp transmissions and baseband data transmissions or between time stamp transmissions of different remote units to the controller.
1. A communication system comprising:
remote units to exchange radio frequency (RF) signals with mobile devices, at least some of the RF signals comprising information destined for, or originating from, at least one mobile device;
a controller comprising one or more baseband modems, the controller being configured to transmit data corresponding to the information, at least one of the baseband modems being configured to perform real-time scheduling of transmission of data to a remote unit;
a reference timing source that is synchronized to a stable reference clock;
a controller clock synchronized with the reference timing source, wherein a controller modem frame clock signal is based on an output of the controller clock, the controller modem frame clock signal for providing frame timing to the at least one baseband modem; and
a remote unit clock synchronized with the controller clock, wherein a remote unit modem frame clock signal is based on an output of the remote unit clock, the remote unit modem frame clock signal for providing frame timing to the remote unit;
wherein the controller is configured to send a message to a mobile device to advance frame timing for transmitting relative to frame timing for receiving by an amount greater than a time delay associated with round trip transmission of the RF signals between a remote unit and the mobile device, and wherein the amount of the advance is based on a processing time available to the controller to process baseband data transmissions;
wherein the remote units are located remotely from the controller and at least one of the remote units is located remotely from at least one of the other remote units; and
wherein each of the remote units is communicatively coupled to the controller.
2. The communication system of claim 1 , wherein the amount of the advance corresponds to an increase, relative to a reference, in an amount of processing time available to the controller to process baseband data transmissions.
3. The communication system of claim 1 , wherein the controller is configured to advance frame timing in the controller by a fixed amount relative to frame timing in the remote unit.
4. The communication system of claim 1 , wherein the message comprises a long term evolution (LTE) timing advance message.
5. The communication system of claim 1 , wherein the message indicates the amount of frame timing advance.
6. The communication system of claim 1 , wherein the controller is configured to affect processing time available to the controller to process the baseband data transmissions by choosing the amount of the frame timing to advance to be greater than the time delay associated with the round trip transmission of the RF signals between the remote unit and the mobile device.
7. The communication system of claim 1 , wherein the controller is configured to advance, in time, a subframe of data to be delivered to the remote unit in order to compensate for the time delay.
8. The communication system of claim 1 , wherein the remote units are configured to communicate with the controller on a communication channel, and a frequency of the communication channel is based on the output of the controller clock.
9. The communication system of claim 1 , wherein the controller clock comprises a crystal oscillator configured to generate clock signals for baseband processing in the controller.
10. The communication system of claim 1 , wherein the remote unit clock comprises a crystal oscillator configured to generate clock signals for at least one of analog-digital-analog converters (A/D/As), RF synthesizers, or baseband processing in remote units.
11. The communication system of claim 1 , wherein the controller and the remote unit are configured to transmit time stamp messages multiple times between the controller and the remote unit;
wherein the remote unit is configured to adjust the remote unit clock based on one or more of the time stamp messages that are deemed to be most reliable among the time stamp messages, the remote unit being configured to adjust the remote unit clock to correct an offset between the controller clock and the remote unit clock; and
wherein the one or more time stamp messages that are deemed to be most reliable correspond to one or more offsets between the controller clock and the remote unit clock, the one or more offsets being less than at least one other offset between the controller clock and the remote unit.
12. The communication system of claim 1 , wherein the controller and the remote unit are configured to transmit time stamp messages multiple times between the controller and the remote unit; and
wherein the remote unit is configured not to adjust the remote unit clock when an estimate of an offset between the controller clock and the remote unit clock is deemed to be unreliable based on transmissions of the time stamp messages multiple times, wherein the estimate of the offset is deemed to be unreliable when the estimate exceeds a predefined threshold.
13. The communication system of claim 1 , wherein the controller clock is directly coupled to the reference timing source and the remote unit clock is not directly coupled to the reference timing source.
14. The communication system of claim 1 , wherein each of the remote units is communicatively coupled to the controller over a switched Ethernet network.