Estimation of two propagation channels in OFDM

Details Estimation of two propagation channels in OFDM Estimation of two propagation channels in OFDM

1999-12-21

2001-10-02

Hsu, Alpus H. (Department: 2662)

Multiplex communications

Generalized orthogonal or special mathematical techniques

Quadrature carriers

C370S208000, C375S260000, C375S349000, C455S060000, C455S063300

Reexamination Certificate

active

06298035

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to channel estimation, and more particularly to a method and apparatus for estimating two propagation channels in an Orthogonal Frequency Division Multiplexing (OFDM) system with two transmitter antennas using specifically selected training information.
BACKGROUND OF THE INVENTION
The growing area of personal communications systems is providing individuals with personal terminals capable of supporting various services such as multimedia. These services require the use of increased bit rates due to the large amount of data required to be transferred. The use of increased bit rates generates problems in conventional single carrier systems due to inter-symbol interference (ISI) and deep frequency selective fading problems.
One solution to these problems utilizes orthogonal frequency division multiplexing (OFDM) within the radio mobile environment to minimize the above-mentioned problems. Within OFDM, a signal is transmitted on multi-orthogonal carriers having less bandwidth than the coherence bandwidth of the channel in order to combat frequency selective fading of the transmitted signal. The inter-symbol interference is mitigated by the use of guard intervals. OFDM systems are presently adopted in Europe for digital audio broadcasting and have been proposed for use in digital TV broadcasting systems. It is used also in asymmetric digital subscriber lines (ADSL) to transmit high rate data. OFDM has also been selected as the modulation method for wireless local area network (WLAN) standards in United States, Europe and Japan.
Transmitter diversity is an effective technique to mitigate multipath fading. One significant advantage of transmitter diversity is that the receiver needs only one antenna with Radio Frequency (RF) receiving chain. Since RF components are quite expensive the cost of the receiver can be reduced with transmitter diversity compared to a system using receiver diversity, that needs two or more antennas and corresponding receiving RF chains. Recently Space-Time Codes (STC) have been introduced as a method to achieve transmitter diversity system. Space-Time codes encode information over multiple antennas to achieve diversity advantage, however decoding of STC needs an estimate of the propagation path from each transmitter antenna to the receiver antenna.
Since radio channels often are subjected to multipath propagation, the receiver needs to comprise some sort of equalizer to eliminate this phenomenon. The equalizer requires an estimated frequency response of the transmission channel, i.e., a channel estimation. Existing channel estimation methods are based on adaptive signal processing wherein the channels are assumed to vary slowly. The estimated channel parameters at a particular time depend on the received data and channel parameters at a previous time. In the case of fast varying channels, such as in high data rate mobile systems, these methods must be modified to reduce the estimation time.
Single channel estimation is a well known problem and numerous methods exist to solve that problem. However, their extension to estimating multiple channels in an OFDM system has not been discussed. For example, Space-Time coded communication systems use multiple transmit antennas to achieve transmitter diversity gain, but require each propagation channel to be separately estimated. A trivial way to use existing single channel estimation algorithms is to separate transmission of the training information in time for each transmit antenna. Then, the existing algorithms can be used for each antenna as each antenna is transmitting training information.
A drawback of separating the training information in time is that it reduces the amount of information used to estimate each channel, provided that a fixed amount of training data is available. Time divisioning the training data between two antennas decreases the quality of the estimate of each channel. Another option is to double the amount of training data, which in turn increases the system overhead.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the deficiencies described above by providing a method and apparatus for estimating separate channel frequency responses in a communication system with two transmitters. The channel frequency responses are estimated using specifically selected training symbols that are broadcast from the two transmitters. The invention has the advantage of retaining the same amount of training symbols as required in a single channel estimation case while improving the channel estimate for each channel.
According to one embodiment of the present invention, a method and apparatus for estimating separate channel frequency responses for channels in an orthogonal frequency division multiplexing system with two transmitters is disclosed. First and second training symbols (A
1
, A
2
) and data from a first transmitter are transmitted to a receiver using a first channel. Third and fourth training symbols (B
1
, B
2
) and data from a second transmitter are transmitted to the receiver using a second channel. First and second received symbols are received at the receiver. The first and second received symbols are then combined. A first channel estimate and a second channel estimate are then derived from the combined received symbols, wherein the first and second received symbols comprise the training symbols, wherein the first and third training symbols form a first symbol pair and the second and fourth training symbols form a second symbol pair.


REFERENCES:
patent: 5005188 (1991-04-01), Clark
patent: 5127051 (1992-06-01), Chan et al.
patent: 5732113 (1998-03-01), Schmidl et al.
patent: 5828650 (1998-10-01), Malkamäki et al.
patent: 5867478 (1999-02-01), Baum et al.
patent: 5912876 (1999-06-01), H'mimy
patent: 5923666 (1999-07-01), Gledhill et al.
patent: 5933768 (1999-08-01), Skold et al.
patent: 5953311 (1999-09-01), Davies et al.
patent: 5956318 (1999-09-01), Saeki
patent: 5973642 (1999-10-01), Li et al.
patent: 6141393 (2000-10-01), Thomas et al.
patent: 6144711 (2000-11-01), Raleigh et al.

Affiliated with

Also associated with

Rating

Estimation of two propagation channels in OFDM does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Estimation of two propagation channels in OFDM, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Estimation of two propagation channels in OFDM will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2580889