Method and apparatus to improve SCDMA headroom

• US 20030067883A1
• Filed: 10/09/2001
• Published: 04/10/2003
• Est. Priority Date: 10/09/2001
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: ATI International SRL (Advanced Micro Devices, Inc.), ATI Technologies Incorporated (Advanced Micro Devices, Inc.)
• Inventors: Azenkot, Yehuda, Rakib, Selim Shlomo
• Granted Patent: US 7,002,899 B2
• Time in Patent Office: Days
• US Class Current: 370/252
• CPC Class Codes: H04J 13/16 Code allocation
• CPC Class Codes: H04L 25/0226 sounding signals per se
• CPC Class Codes: H04L 5/0016 Time-frequency-code
• CPC Class Codes: H04L 5/0048 Allocation of pilot signals, i.e. of signals known to the receiver
• CPC Class Codes: H04N 7/17309 Transmission or handling of upstream communications

Abstract

A method of determining when cable modems in a distributed digital data delivery service over cable TV hybrid fiber coaxial cable network have a headroom problem and resolving said problem. The method involves measuring the burst power from each cable modem, and if the burst power is too low, requesting the cable modem whose burst power is too low to increase its transmit power, and keeping track of which modems have been requested to increase their power. If a predetermined number of requests to increase power have not resulted in the cable modem transmitting with sufficient power for reliable reception, the cable modem is listed as having a headroom problem. Subsequent requests for upstream bandwidth from all modems with headroom problems are analyzed to determine if the requested burst size is too large and will result in a headroom problem. If so, a calculation as to the maximum number of spreading codes that each modem with a headroom problem can simultaneously transmit on without a headroom problem. The requested burst is then broken down into smaller burst fragments, and appropriate upstream minislot assignments adequate to transmit the burst fragments are made and sent to the cable modem.

Claims

1. A process comprising the steps of:

(1) determining in any way that a particular remote transmitter or cable modem in a distributed code division multiplexed or orthogonal frequency division multiplexed or other digital data communication system in which individual remote transmitters or cable modems simultaneously transmit on a plurality of logical channels has a headroom problem; and

(2) resolving the problem in any way by restricting the number of spreading codes in the case of a code division multiplexed system or the number of frequency carriers in the case of an orthogonal frequency division multiplexed system that are allocated to any remote transceiver that has a headroom problem for use in transmitting a burst, said restricted number being a number which will cause the total transmit power of the remote transmitter or cable modem to be divided among fewer spreading codes or fewer carriers or fewer logical channels so as to transmit on each spreading code or carrier or logical channel simultaneously with sufficient power to overcome said headroom problem.

2. The process of claim 1 wherein step 1 comprises measuring the signal-to-noise ratio or bit error rate or both of a burst from each cable modem or remote transmitter and comparing said measurement to a reference, and, if the measured quantity is unfavorable compared to said reference, requesting the cable modem or remote transmitter to increase the power of transmission of a subsequent burst, and then measuring the signal-to-noise ratio or bit error rate or both of said subsequent burst and comparing the measured quantity to said reference, and, if said measured quantity compares unfavorably to said reference, either concluding the cable modem or remote transmitter which sent said burst has a headroom problem, or, for one or more additional times, repeating the process of requesting an increase in power of a subsequent burst and measuring the signal-to-noise ratio or bit error rate or both of said subsequent burst and comparing the measured quantity to said reference, and if said comparison is still unfavorable, concluding that said cable modem or remote transmitter has a headroom problem.

3. The process of claim 1 wherein step 1 is comprised of the steps of:

measuring the power of a burst from each cable modem or remote transmitter and comparing said measured power to a reference power;

if the measured power is lower than said reference power, requesting the cable modem or remote transmitter to increase the power of transmission of a subsequent burst;

measuring the power of said subsequent burst and comparing the measured power to said reference power;

if the measured power is lower than said reference, either concluding the cable modem or remote transmitter which sent said burst has a headroom problem, or, for one or more additional times, repeating the process of requesting an increase in power of a subsequent burst and measuring the power of said subsequent burst and comparing the measured power to said reference power, and if said measured power is still lower than said reference power, concluding that said cable modem or remote transmitter has a headroom problem.

4. The process of claim 1 wherein step 2 comprises the steps of:

(A) for each cable modem or remote transmitter that has a headroom problem, determining the amount of power shortfall a cable modem or remote transmitter has for a transmitted burst assuming that all available logical channels were assigned to the cable modem or remote transmitter which transmitted said burst;

(B) for each cable modem or remote transmitter that has a headroom problem, calculating the amount of restriction in assigned logical channels that will cause the total available transmitted power to be divided by the restricted number of assigned logical channels such that each logical channel will be transmitted with enough power to overcome the headroom problem; and

(C) sending a downstream message to each cable modem or remote transmitter having a headroom problem allocating to said cable modem or remote transmitter the restricted number of logical channels calculated in step (B) that will solve the headroom problem.

5. The process of claim 2 wherein step 2 comprises the steps of:

(A) for each cable modem or remote transmitter that has a headroom problem, determining the amount of power shortfall a cable modem or remote transmitter has for a transmitted burst assuming that all available logical channels were assigned to the cable modem or remote transmitter which transmitted said burst;

(B) for each cable modem or remote transmitter that has a headroom problem, determining the amount of restriction in assigned logical channels that will cause the total available transmitted power to be divided by the restricted number of assigned logical channels such that each logical channel will be transmitted with enough power to overcome the headroom problem; and

(C) sending a downstream message to each cable modem or remote transmitter having a headroom problem allocating to said cable modem or remote transmitter the restricted number of logical channels calculated in step (B) that will solve the headroom problem.

6. The process of claim 3 wherein step 2 comprises the steps of:

(A) for each cable modem or remote transmitter that has a headroom problem, determining the amount of power shortfall a cable modem or remote transmitter has for a transmitted burst assuming that all available logical channels were assigned to the cable modem or remote transmitter which transmitted said burst;

(B) for each cable modem or remote transmitter that has a headroom problem, determining the amount of restriction in assigned logical channels that will cause the total available transmitted power to be divided by the restricted number of assigned logical channels such that each logical channel will be transmitted with enough power to overcome the headroom problem; and

(C) sending a downstream message to each cable modem or remote transmitter having a headroom problem allocating to said cable modem or remote transmitter the restricted number of logical channels calculated in step (B) that will solve the headroom problem.

7. The process of claim 6 wherein the step of determining the amount of restriction in assigned logical channel comprises the step of using the measured power of a burst from a cable modem or remote transmitter which has a headroom problem to look up in a lookup table the amount of restriction necessary to solve the headroom problem.

8. A process comprising the steps of:

(1) determining in any way that a particular remote transmitter or cable modem in a distributed code division multiplexed digital data transmission system has a headroom problem; and

(2) resolving the problem by restricting in any way the number of spreading codes that are allocated to any remote transceiver that has a headroom problem for simultaneous use in transmitting a burst, said restricted number of spreading codes being a number which will cause the total available transmit power of the remote transmitter to be divided among fewer spreading codes such that transmissions on each spreading code occur with sufficient power to overcome said headroom problem.

9. The process of claim 8 wherein step (1) is performed by performing the steps of:

measuring the power of a ranging or training burst transmitted by each remote transmitter or cable modem;

if the power is too low, sending a message requesting an increase in power in transmission of the next ranging or training burst;

recording the fact that such a request was made to a particular remote transmitter or cable modem; and

measuring the power of the next ranging or training burst sent by that remote transmitter or cable modem;

if the power is too low, requesting that the power of transmission of the next ranging or training burst transmitted by that remote transmitter or cable modem be increased again;

measuring the power of the next ranging or training burst transmitted by the remote transmitter which has been requested to increase its transmit power at least twice, and, if the power is still too low, determining that said remote transmitter or cable modem has a headroom problem.

10. The process of claim 8 wherein step (2) is performed by performing the steps of:

(A) determining the power shortfall of a remote transmitter or cable modem that has been determined to have a headroom problem, and, using said power shortfall to calculate the maximum burst size that the remote transmitter or cable modem can transmit which will result in mapping to a sufficiently restricted number of spreading codes and spreading intervals such that transmissions from this remote transmitter or cable modem on each spreading code will arrive at the CMTS or headend receiver with the proper nominal power level or at least a power level that is adequate to allow the CMTS or headend receiver to receive the transmissions with a sufficiently low bit error rate for reliable reception;

(B) receiving upstream bandwidth requests from a remote transmitter or cable modem that has a headroom problem which gives the burst size the remote transmitter or cable modem would like to transmit;

(C) examining the burst size of the requested burst, and if the requested burst size is too big for this remote transmitter or cable modem given its headroom problem, determining how many pieces the requested burst needs to be broken into so that the maximum burst size of any burst fragment is the same as the maximum burst size calculated in step (A) and maps to a number of minislots that maps to a number of spreading codes and spreading intervals which is sufficiently small to solve the headroom problem;

(D) sending one or more downstream messages to each remote transmitter or cable modem which has a headroom problem which requested bandwidth for an upstream burst, each said downstream message(s) allocating for a burst fragment calculated in step (C) a number of minislots during which said burst fragment may be transmitted, said number of minislots for each burst fragment mapping to the number of spreading codes and spreading intervals needed to transmit a burst fragment of that size.

11. The process of claim 9 wherein step (2) is performed by performing the steps of:

(A) determining the power shortfall of a remote transmitter or cable modem that has been determined to have a headroom problem, and, using said power shortfall to calculate the maximum burst size that the remote transmitter or cable modem can transmit which will result in mapping to a sufficiently restricted number of spreading codes and spreading intervals such that transmissions from this remote transmitter or cable modem on each spreading code will arrive at the CMTS or headend receiver with the proper nominal power level or at least a power level that is adequate to allow the CMTS or headend receiver to receive the transmissions with a sufficiently low bit error rate for reliable reception;

(B) receiving upstream bandwidth requests from a remote transmitter or cable modem that has a headroom problem which gives the burst size the remote transmitter or cable modem would like to transmit;

(C) examining the burst size of the requested burst, and if the requested burst size is too big for this remote transmitter or cable modem given its headroom problem, determining how many pieces the requested burst needs to be broken into so that the maximum burst size of any burst fragment is the same as the maximum burst size calculated in step (A) and maps to a number of minislots that maps to a number of spreading codes and spreading intervals which is sufficiently small to solve the headroom problem;

(D) sending one or more downstream messages to each remote transmitter or cable modem which has a headroom problem which requested bandwidth for an upstream burst, each said downstream message(s) allocating for a burst fragment calculated in step (C) a number of minislots during which said burst fragment may be transmitted, said number of minislots for each burst fragment mapping to the number of spreading codes and spreading intervals needed to transmit a burst fragment of that size.

12. A process carried out in a headend transceiver of any code division multiplexed distributed digital data communication system where distributed remote transceivers send spread spectrum upstream digital data to said headend transceiver for solving a headroom problem, comprising the steps of:

recognizing in any way at said headend transceiver that one or more remote transceivers has a headroom problem;

determining the amount of power shortfall for each remote transceiver which has a headroom problem;

receiving an upstream bandwidth request that includes the size of burst or amount of data a remote transceiver wishes to send;

determining if said upstream bandwidth request came from a remote transceiver with a headroom problem, and, if so, determining if the requested burst size exceeds a size which will cause a headroom problem to exist for that transmission;

if a headroom problem will exist for the burst, determining how many pieces into which the burst must be divided to make each burst fragment small enough that it can be transmitted with a small enough number of spreading codes to solve the headroom problem; and

sending one or more downstream messages authorizing transmission of one or more burst fragments in corresponding designated windows of time, each burst fragment being small enough that the number of codes that are mapped to the burst fragment is small enough to solve the headroom problem.

13. The process of claim 12 wherein the step of recognizing comprises the steps of:

1) measuring the burst power of a training burst sent by a remote transceiver;

2) comparing the burst power to a nominal power level;

3) if the measured power is less than said nominal power, recording this fact and sending a message to said remote transceiver that sent said training burst requesting said remote transceiver to increase the power level of the next training burst it sends;

repeating steps

     1) through

     3) a predetermined number of times, and, if after said predetermined number of iterations, the measured power of a training burst still is not at or above said nominal power, concluding that said remote transceiver has a headroom problem.

14. The process of claim 12 wherein the step of recognizing comprises the steps of:

receiving an initial training burst from a remote transceiver and measuring the bit error rate thereof;

if after doing whatever is possible to reduce the bit error rate of said initial training burst, said bit error rate is too high, sending a downstream message to the remote transceiver which sent said initial training burst requesting that said remote transceiver increase the power level of a subsequent training burst;

15. An improved training system which can detect and resolve power headroom problems of cable modems in a DOCSIS bidirectional digital data delivery system comprising the steps of:

1) sending invitations for training bursts during an initial training window from a CMTS to a plurality of cable modems;

2) transmitting an initial QPSK training burst from each cable modem that needs to be trained during the time said cable modem (hereafter CM) thinks said initial training window is, said initial training burst being at an initial low power and including an ID code which identifies said cable modem and known preamble data;

3) at the CMTS using the initial training burst transmitted by each CM, measuring the power of the initial training burst and measuring a ranging offset for said CM and sending said ranging offset to said CM, and calculating upstream equalization coefficients and sending said equalization coefficients down to said CM, and calculating and storing phase and amplitude error correction factors for said CM from said known preamble data for use in receiving subsequent burst symbols transmitted by the CM;

4) if the initial training burst was too low in power, sending a request to the CM that transmitted said initial training burst asking said CM to increase the power level on subsequent bursts and recording the fact that a request to increase power was made for each CM to which such a request was sent;

5) if a CM sent an initial training burst and did not receive any message back from said CMTS, performing in the CM a random collision backoff delay process and increasing the power level at which the next training burst will be sent and sending another training burst in response to the next invitation to send initial training bursts;

6) repeating steps 1 through 5 multiple times until an initial training burst is successfully received from each CM;

7) allocating upstream minislots to a particular CM for periodic station maintenance bursts and sending a downstream message to said CM inviting it to send a periodic station maintenance burst during said allocated minislots;

8) at each CM which has received an invitation to send a periodic station maintenance burst, sending a periodic station maintenance burst during the allocated minislots, and increasing power if possible if said CMTS has previously instructed the CM to increase power;

9) CMTS receives periodic station maintenance burst from each CM and measures power again and calculates refined equalization coefficients and ranging offset values;

10) determining whether the power of said periodic station maintenance burst from each CM is adequate, and, if the power is too low, sending another message to the CM whose power is too low asking the CM to increase its transmit power level, and recording the fact that such a request was made to a specific CM, and sending said refined equalization coefficients and ranging offset to said CM;

11) in each CM which received refined equalization coefficients, convolving said refined equalization coefficients with the upstream equalization coefficients currently in use by said CM, and adjusting a ranging offset used by said CM using said refined ranging offset;

12) allocating more upstream minislots for each CM and sending a message to each CM requesting that it send another periodic station maintenance burst;

13) in each CM that received an invitation to send another periodic station maintenance burst, sending another periodic station maintenance burst and increasing the transmit power level if possible and if a request to increase the power level has been received from said CMTS;

14) CMTS again measures the power of the periodic station maintenance burst, and, if the power is still too low, determines if the CM which sent the burst has been instructed at least a predetermined number of times before to increase its transmit power level, and, if it has, the CMTS notes the CM'"'"'s ID on a list of CMs that have headroom problems;

15) CMs receive data to be transmitted upstream and send upstream bandwidth requests to the CMTS which include the size of the requested burst and the ID of the requesting CM;

16) CMTS receives the bandwidth requests and looks up the CM on its list of CMs that have headroom problems to determine if the CM is on the list;

17) if the CM is on the list of CMs that have headroom problems, the CMTS examines the requested burst size to be sent by said CM and determines whether the burst size is too large to solve the headroom problem;

18) if the burst size is too large for a CM with a headroom problem, the CMTS calculates the maximum number of codes that the CM can use simultaneously such that the received power on each code would meet any standard of measure used by the CMTS as a power level which would cause reliable reception;

19) after calculating the maximum number of codes which the CM can use simultaneously, the CMTS determines the number of burst fragments that the requested burst from a CM that has a headroom problem must be divided into so that each burst fragment maps to no more than the maximum number of codes calculated in step 18;

20) the CMTS then allocates a number of minislots that is adequate to transmit one of said burst fragments and sends a message to the CM with the headroom problem telling it the minislots to use to transmit;

21) the CM then receives this message and uses the allocated minislots to send only that portion of the burst that will fit in the allocated minislots;

22) steps 20 and 21 are repeated until all burst fragments are sent; and

23) steps 17 through 22 are repeated until all CMs that have headroom problems have all their data sent.

16. A process comprising steps for:

determining all the cable modems in a cable TV digital data communication system which have headroom problems by, for each cable modem, measuring their signal to noise ratio and/or bit error rate in one or more bursts and comparing the measured values to a reference and, if the comparison is unfavorable, requesting the next burst be transmitted at a higher power level, and repeating the process of measuring, comparing and requesting more power a predetermined number of times before concluding that said cable modem has a headroom problem;

receiving upstream burst requests from one or more of said cable modems and determining if each cable modem has a headroom problem, and, if so, if the requested burst size is too big;

if the requested burst size from a cable modem with a headroom problem is too big, determining how many fragments into which the cable modem must be broken so as to restrict the number of codes used to send the burst fragments to a small enough number to solve the headroom problem; and

allocating upstream bandwidth to said cable modem in terms of a number of assigned minislots which are adequate only to send a burst fragment which is small enough to solve the headroom problem, and sending a message to said cable modem assigning the allocated bandwidth for the burst fragment to said cable modem.

17. A process carried out in a headend transceiver of any code division multiplexed distributed digital data communication system where distributed remote transceivers send spread spectrum upstream digital data to said headend transceiver for solving a headroom problem, comprising the steps of:

recognizing in any way at said headend transceiver or CMTS that one or more remote transceivers or cable modems has a headroom problem;

determining the amount of power shortfall for each remote transceiver or cable modem which has a headroom problem;

receiving an upstream bandwidth request from a remote transceiver or cable modem;

for each such upstream bandwidth request received, determining if it came from a remote transceiver or cable modem with a headroom problem, and, if not, sending a downstream message authorizing upstream transmission in a normal way;

if a headroom problem will exist for the remote transceiver or cable modem that made the upstream bandwidth request, for each such upstream bandwidth request, calculating a restricted number of DMT carriers which can be assigned to that remote transceiver or cable modem which will solve the headroom problem; and

sending a downstream message to each remote transceiver or cable modem with a headroom problem authorizing transmission of upstream data by a remote transceiver or cable modem using the restricted number of DMT carriers which was calculated to be small enough to solve the headroom problem.

18. A CMTS apparatus comprising:

any downstream transmitter;

a code division multiplexed receiver with circuitry capable of measuring the power of received bursts;

a computer coupled to said transmitter and receiver and programmed to control said transmitter and receiver to implement the following steps;

determining all the cable modems in a cable TV digital data communication system which have headroom problems;

receiving upstream burst requests from one or more of said cable modems and determining if each cable modem has a headroom problem, and, if so, if the requested burst size is too big;

if the requested burst size from a cable modem with a headroom problem is too big, determining how many fragments into which the cable modem must be broken so as to restrict the number of codes used to send the burst fragments to a small enough number to solve the headroom problem; and

allocating upstream bandwidth to said cable modem which is adequate only to send a burst fragment which is small enough to solve the headroom problem, and sending a message to said cable modem assigning the allocated bandwidth for the burst fragment to said cable modem.

19. A CMTS apparatus comprising:

any downstream transmitter;

a orthogonal frequency division multiplexing receiver with circuitry capable of measuring the power of received bursts or circuitry capable of measuring the signal-to-noise ratio of received bursts or circuitry capable of measuring the bit error rate of received bursts;

a computer coupled to said transmitter and receiver and programmed to control said transmitter and receiver to implement the following steps;

determining in any way all the cable modems or remote transmitters in a digital data communication system which have a headroom problem;

receiving an upstream burst request from a cable modem or remote transmitter which has a headroom problem, and determining a restricted number of upstream logical channels which can be assigned to said cable modem or remote transmitter which will eliminate said headroom problem;

sending a message to said cable modem or remote transmitter having said headroom problem and which sent said upstream burst request assigning to said cable modem or remote transmitter said restricted number of upstream logical channels which will solve said headroom problem.