Compensation Of Reference Frequency Drift In System Requiring Critical Upstream Timing

Abstract

Systems and methods are disclosed for to compensating reference frequency drift in a communications system having a plurality of modems and a headend, where the system requires critical upstream timing. One embodiment of the method includes learning or determining the relative delay of each modem and reporting each modem's unique delay (relative to the closest modem) to the headend. The method further includes the headend monitoring its own reference for frequency drift, the modem broadcasting pertinent frequency drift information to the modems and adjusting the modems' upstream timing to account for each modem's unique distance (i.e., delay) combined with the broadcast stream of frequency drift information.

1 . A method for mitigating timing offset in a system having critical timing synchronization of return transmissions, comprising: 
 determining relative delay in the system;    reporting said relative delay;    monitoring frequency drift;    broadcasting information on said frequency drift; and    adjusting upstream timing.    
   
   
       2 . The method of  claim 1 , wherein determining said relative delay further comprises determining relative delay of each modem in the system.  
   
   
       3 . The method of  claim 2 , wherein determining said relative delay further comprises setting each said modem to an unitialized mode.  
   
   
       4 . The method of  claim 3 , wherein determining said relative delay further comprises determining when each said modem responses to an initial maintenance interval.  
   
   
       5 . The method of  claim 1 , wherein reporting said relative delay comprises reporting each modem's delay to a headend.  
   
   
       6 . The method of  claim 5 , wherein reporting each modem's delay further comprises reporting said delay relative to a closest modem to said headend.  
   
   
       7 . The method of  claim 5 , wherein reporting said relative delay further comprises using proprietary messaging.  
   
   
       8 . The method of  claim 1 , wherein monitoring said frequency drift comprises a headend monitoring its own frequency drift.  
   
   
       9 . The method of  claim 8 , wherein monitoring said frequency drift further comprises said headend monitoring its own frequency drift relative to a stable reference.  
   
   
       10 . The method of  claim 1 , wherein monitoring said frequency drift comprises each modem in the system monitoring its own downstream symbol clock frequency drift.  
   
   
       11 . The method of  claim 1 , wherein monitoring said frequency drift comprises building a delay at a headend in the system and measuring a frequency a delayed reference.  
   
   
       12 . The method of  claim 1 , wherein monitoring said frequency drift comprises using a frequency discriminator.  
   
   
       13 . The method of  claim 1 , wherein broadcasting said information on said frequency drift comprises broadcasting said information to each modem.  
   
   
       14 . The method of  claim 13 , wherein broadcasting said information on said frequency drift further comprises using actual frequency normalized by nominal frequency.  
   
   
       15 . The method of  claim 1 , wherein adjusting said upstream timing comprises adjusting each modem's upstream timing to account for distance combined with said information on said frequency drift.  
   
   
       16 . The method of  claim 1 , further comprising using proprietary messaging to accommodate adding additional modems to the system.  
   
   
       17 . A method for mitigating timing offset in a communications system having at least one headend and a plurality of modems, comprising: 
 determining a relative delay of each modem in the communications system;    reporting each modem's delay to the headend;    monitoring the headend's frequency drift;    broadcasting said frequency drift information to each modem; and    adjusting each modem's upstream timing.    
   
   
       18 . A communication system adapted to mitigate timing offset in return transmissions, comprising: 
 at least one modem; and    a headend adapted to determine relative delay of said at least one modem and monitor its own reference for frequency drift,    wherein said at least one modem is adapted to adjust its upstream timing to account for said delay.