System and method for embedding OFDM in CDMA systems

• US 20050063345A1
• Filed: 08/11/2004
• Published: 03/24/2005
• Est. Priority Date: 08/11/2003
• Status: Active Grant

• Current Assignee: Ericsson AB (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Wu, Jianming, Boudreau, Daniel, Xu, Hua, Tong, Wen, Wu, Shiquan, Zhu, Peiying, Jia, Ming, Fong, Mo-Han, Edwards, Keith R.
• Granted Patent: US 7,471,932 B2
• Time in Patent Office: Days
• US Class Current: 370/335
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 2201/70701 featuring pilot assisted reception
• CPC Class Codes: H04L 5/026 using code division

Abstract

Systems and methods of combining OFDM and CDMA signals are provided. An OFDM packet data channel is overlaid over CDMA transmissions. The channel is scheduled slotwise between multiple users. In some embodiments, there is a CDMA packet data channel which is scheduled together with the OFDM packet data channel.

Claims

1. A transmitter adapted to transmit a downlink signal comprising:

at least one code separated CDMA (code division multiple access) channel(s) that are scrambled with a common scrambling code;

an OFDM packet channel overlaid over the CMDA channels, the OFDM packet channel being divided into OFDM slots during which OFDM signals are transmitted with capacity allocated on a per OFDM slot basis, the OFDM packet channel not being scrambled by the common scrambling code.

2. A transmitter according to claim 1 wherein the at least one code separated CDMA channel(s) comprises at least one continuous CDMA channels and a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.

3. A transmitter according to claim 1 wherein the at least one code separated CDMA channel(s) comprises a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.

4. The transmitter according to claim 2 adapted to transmit packet data on a sequence of slots, each slot being either an OFDM slot or a CDMA slot, but not both.

5. The transmitter according to claim 2 adapted to transmit packet data on a sequence of slots, each slot being an OFDM slot and/or a CDMA slot.

6. The transmitter according to claim 1 further adapted to transmit periodically at least one CDMA signalling channel that is not spread by the scrambling code.

7. The transmitter according to claim 6 wherein during periods of overlap between the transmission of the at least one signalling channel and the transmission of an OFDM slot, the OFDM slot transmits zeros or known data so that those periods can be treated as a prefix for the OFDM slots upon reception.

8. The transmitter according to claim 1 wherein each OFDM slot comprises:

a prefix followed by an IFFT period, followed by a suffix.

9. The transmitter according to claim 8 wherein each OFDM slot is 10/15 ms in duration, and wherein the prefix is 128 chips, the IFFT period is 2304 chips, and the suffix is 128 chips.

10. The transmitter according to claim 8 wherein the IFFT period comprises:

a plurality of IFFTs separated by zero insertions.

11. The transmitter according to claim 10 wherein the IFFT period comprises 18 128-chip IFFTs.

12. The transmitter according to claim 8 wherein the prefix is a designed training sequence.

13. The transmitter according to claim 6 wherein the at least one signalling channel is transmitted during the prefix of the OFDM slots, and wherein the OFDM slots transmit zeros during the prefix so that the OFDM channel does not interfere with the at least one signalling channel.

14. The transmitter according to claim 2 further adapted to transmit a control channel that identifies to receivers which OFDM slots and/or which CDMA slots are for a given receiver.

15. The transmitter according to claim 8 adapted to transmit a control channel during the suffix that identifies to receivers which OFDM slots and/or which CDMA slots are for a given receiver.

16. The transmitter according to claim 11 wherein the control channel contains a receiver specific mask and/or a receiver specific CRC code.

17. The transmitter according to claim 1 wherein each OFDM slot comprises in sequence:

a first prefix;

a first IFFT period, a first suffix;

a second prefix, a second IFFT period, and a second suffix.

18. The transmitter according to claim 17 wherein each OFDM slot is 10/15 ms in duration, and each prefix is 128 chips, each IFFT period comprises 16 64 chip IFFTs, and each suffix is 128 chips.

19. A system comprising the transmitter of claim 1 in combination with a plurality of receivers at least one of which is an OFDM-capable receiver, wherein each OFDM-capable receiver demodulates the CDMA channels on an ongoing basis, and each OFDM-capable receiver demodulates a given OFDM slot only if scheduled during the OFDM slot.

20. A system comprising the transmitter of claim 2 in combination with a plurality of receivers at least one of which is an OFDM-capable receiver, and each OFDM-capable receiver demodulates a given OFDM slot only if scheduled during the OFDM slot, and each receiver demodulates a given CDMA slot only if scheduled during the CDMA slot.

21. The transmitter of claim 1 further comprising a scheduler adapted to schedule receivers on the downlink by:

for each of a plurality of OFDM-capable receivers, obtaining a respective channel quality indicator for CDMA slots and/or for OFDM slots;

scheduling transmission to OFDM-capable receivers on CDMA and/or OFDM slots according to the channel quality indicators.

22. The transmitter of claim 21 wherein for a given receiver, the channel quality indicator for CDMA slots and/or for OFDM slots comprises a better of the CDMA channel quality indicator and the OFDM channel quality indicator as determined by the receiver.

23. The transmitter of claim 15 wherein the scheduler is further adapted to:

for CDMA-only capable receivers, obtain a channel quality indicator for the CDMA slots only, and schedule transmission to CDMA-only capable receivers on the basis of the CDMA channel quality indicator.

24. The transmitter of claim 1 further adapted to:

for at least some of the OFDM slots, partition OFDM sub-carriers transmitted during at least some of the OFDM slots between a plurality of receivers on a partitioning period basis.

25. The transmitter of claim 24 wherein a partitioning period comprises one OFDM slot.

26. A transmitter according to claim 25 further adapted to transmit a control channel identifying how each OFDM slot is partitioned between receivers such that each receiver can obtain its content.

27. A transmitter according to claim 26 wherein the control channel comprises:

identifiers for each receiver scheduled during the slot and an indication of which sub-carriers are for which receiver.

28. The transmitter according to claim 27 wherein the control channel comprises:

a first sub-channel and a second sub-channel;

the first sub-channel containing an identifier of a first receiver and an identifier of a last OFDM sub-carrier for the first receiver;

the second sub-channel containing an identifier of a second receiver, and an identifier of a last OFDM sub-carrier for the second receiver, the second sub-channel being only transmitted when there is a second receiver scheduled during a given slot.

29. The transmitter according to claim 24 further adapted to perform sub-carrier and power allocation.

30. The transmitter according to claim 29 adapted to perform sub-carrier and power allocation dynamically as a function of mobility and/or frequency selectivity.

31. The transmitter according to claim 29 further adapted to perform sub-carrier and power allocation dynamically as a function of mobility and frequency selectivity by:

defining conditions in which CDMA is used instead of OFDM as a function of channel quality and/or mobility and/or frequency selectivity;

when conditions indicate OFDM is to be used;

a) using a narrow frequency band and high power for high mobility, high frequency selectivity users;

b) using a wide frequency band and low power for mid-mobility and mid-frequency selectivity users.

32. The transmitter according to claim 29 further adapted to perform OFDM allocation subject to an overall power budget for an OFDM slot.

33. The transmitter according to claim 1 adapted for use in a multiple carrier CDMA system.

34. The transmitter according to claim 33 comprising:

an OFDM modulator adapted to generate a single set OFDM sub-carriers that spans the multiple carriers, and in which a single wideband IFFT is performed to do OFDM modulation.

35. The transmitter according to claim 33 wherein each carrier is allocated to only one of CDMA or OFDM on a per allocation period basis, and for any carrier that is allocated to CDMA during a given allocation period, zeros are inserted in a portion of the OFDM IFFT that overlaps the carrier during that allocation period.

36. The transmitter according to claim 35 wherein the OFDM modulator is adapted to insert guardband sub-carriers between groups of sub-carriers for each carrier.

37. The transmitter according to claim 1 wherein each OFDM slot comprises:

a control portion which is non-OFDM followed by a plurality of IFFTs each separated by a respective prefix.

38. The transmitter according to claim 37 wherein each IFFT is a 128 chip IFFT, and each prefix is 22 zero chips.

39. The transmitter according to claim 1 further adapted to:

adjust at least one power control parameter prior to transmitting OFDM slots so as to avoid/mitigate power control loop panic.

40. The transmitter according to claim 39 wherein the at least one power control parameter comprises an outer loop power control threshold.

41. The transmitter of claim 2 adapted to preferentially transmit CDMA slots in flat fading conditions.

42. The transmitter of claim 2 adapted to preferentially transmit OFDM slots in dispersive conditions.

43. A receiver comprising:

an A/D converter adapted to produce a sequence of samples of received signal;

CDMA demodulator for performing CDMA demodulation upon the sequence of samples;

OFDM demodulator for performing OFDM demodulation upon the sequence of samples.

44. The receiver according to claim 43 further adapted to receive a control channel identifying for a given slot whether or not the receiver is scheduled during the slot;

wherein the receiver demodulates a given OFDM slot only if scheduled during the slot.

45. The receiver according to claim 43, further adapted to:

determine a CDMA channel quality indicator for CDMA and an OFDM channel quality indicator for OFDM;

feed back a better of the CDMA channel quality indicator and the OFDM channel quality indicator;

if the CDMA channel quality is better, use CDMA mode to demodulate a next slot for the receiver;

if the OFDM channel quality is better, use OFDM mode to demodulate a next slot for the receiver.

46. The receiver according to claim 43 adapted to:

perform CDMA demodulation by descrambling at least one common channel, de-spread and perform soft-de-mapping and FEC decoding during which the OFDM content will be substantially converted to AWGN;

demodulate the OFDM slots by generating an interference term due to a CDMA component of the received signal, subtract the interference term from the received signal, perform OFDM demodulation on a remaining signal.

47. The receiver according to claim 46 adapted to generate the interference term by:

de-scrambling, de-spreading, soft de-mapping and FEC decoding (or hard decision), re-encoding, re-spreading, re-scrambling, convolving with an estimated channel response.

48. The receiver according to claim 43 adapted to use OFDM pilots to generate channel estimates for both OFDM and CDMA.

49. The receiver according to claim 43 adapted to perform channel estimation by:

performing an FFT on the sequence of samples;

performing a first transformation on a set of samples collectively output by the FFT;

performing a per sub-carrier transformation on an output of the first transformation;

performing a second transformation on outputs of the per sub-carrier transformations collectively to produce an OFDM channel estimate; and

performing an IFFT on the OFDM channel estimate to produce a time domain channel estimate for CDMA finger searching.

50. The receiver according to claim 43 adapted to use CDMA pilots for channel estimation for both CDMA and OFDM.

51. The receiver according to claim 43 adapted to perform channel estimation by:

performing CDMA searching and correlation to produce a time-domain channel response;

performing an FFT on the time domain channel response;

performing a first transformation on a set of samples collectively output by the FFT;

performing a per sub-carrier transformation on an output of the first transformation;

performing a second transformation on outputs of the per sub-carrier transformations collectively to produce an OFDM channel estimate; and

performing an IFFT on the OFDM channel estimate to produce a time domain channel estimate for CDMA finger searching.

52. The receiver according to claim 43 adapted to receive a signal on multiple carriers, the receiver comprising an FFT function adapted to perform an FFT for sub-carriers spanning the multiple carriers.

53. A method comprising:

generating at least one code separated CDMA (code division multiple access) channel(s) that are scrambled with a common scrambling code;

generating an OFDM packet channel, the OFDM packet channel being divided into OFDM slots during which OFDM signals are transmitted with capacity allocated on a per OFDM slot basis, the OFDM packet channel not being scrambled by the common scrambling code;

combining the code separated CDMA channel(s) and the OFDM channel into a combined signal and transmitting the combined signal.

54. The method of claim 53 wherein the at least one code separated CDMA channel(s) comprises at least one continuous CDMA channels and a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.

55. The method of claim 53 wherein the at least one code separated CDMA channel(s) comprises a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.

56. The method of claim 53 further comprising for at least some of the OFDM slots, partitioning OFDM sub-carriers transmitted during at least some of the OFDM slots between a plurality of receivers on a partitioning period basis.

57. The method of claim 53 further comprising performing sub-carrier and power allocation dynamically as a function of mobility and/or frequency selectivity.

58. The method of claim 53 adapted for use in a multiple carrier CDMA system, the method further comprising generating a a single set OFDM sub-carriers that spans the multiple carriers, and in which a single wideband IFFT is performed to do OFDM modulation.