ADAPTIVE TIME DIVERSITY AND SPATIAL
DIVERSITY FOR OFDM
CROSS-REFERENCE TO RELATED PATENT
This application is a continuation application of U.S. patent application Ser. No. 09/750,804 to Shiquan Wu et al., filed Dec. 29, 2000 now U.S. Pat. No. 6,985,434, and incorporates its subject matter in its entirety herein by 10 reference. U.S. patent application Ser. No. 09/750,804 claims priority to U.S. Provisional Patent Application No. 60/229,972, filed Sep. 1, 2000, the contents of which are also incorporated in their entirety herein by reference.
Reference is made to co-pending patent application 15 entitled "Channels Estimation For Multiple Input-Multiple Output, Orthogonal Frequency Division Multiplexing (OFDM) System", and incorporates its subject matter by reference with respect to channels estimation.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to adapting time diversity and spatial diversity for use in an orthogonal frequency-division 25 multiplexing (OFDM) environment, using a multiple input and multiple output (MIMO) structure.
2. Discussion of Related Art
A multiple input, multiple output (MIMO) structure has multiple communication channels that are used between 30 transmitters and receivers. A space time transmitter diversity (STTD) system may be used on a MIMO structure, but it will not increase the data throughput. Indeed, for a high level configuration, the data rate may even reduce. In an STTD system, the transmitters deliver the same information con- 35 tent within consecutive symbol duration so that time diversity may be exploited. To efficiently use the multiple transmitters of the MIMO structure, however, the transmission data rate needs to be increased.
The most straightforward solution to increase the trans- 40 mission data rate is to in forward error correction (FEC) dump independent data to each transmitter. A forward error correction (FEC) encoder produces in-phase and quadraturephase data streams for the digital QAM modulator in accordance with a predetermined QAM constellation. The QAM 45 modulator may perform baseband filtering, digital interpolation and quadrature amplitude modulation. The output of the QAM modulator is a digital intermediate frequency signal. A digital to analog (D/A) converter transforms the digital IF signal to analog for transmission. 50
The problem arises, however, as to how to safely recover the transmitted data. For a 2x2 system (two transmitters, two receivers) for example, after the channel information is obtained, the recovery process entails formulating two equations with two unknowns that need to be solved. The two 55 unknowns may be determined only if the 2x2 channel is invertible. In practice, however, two situations may be encountered, i.e., the channel matrix is rank deficient so the unknowns cannot be determined or the frequency response channel matrix is invertible but has a very small eigen value. 60
The first situation arises when the channels are highly correlated, which may be caused either by not enough separation of the transmitters or by homology of the surroundings. For the second situation, although the equations are solvable, the solution can cause a high bit error rate 65 (BER), because a scale up of the noise can result in an incorrect constellation point.
Orthogonal frequency-domain multiplexing (OFDM) systems were designed conventionally for either time diversity or for space diversity, but not both. The former will provide a robust system that combats signal fading but cannot increase the data rate capacity, while the latter can increase the data rate capacity but loses the system robustness. An OFDM signal contains OFDM symbols, which are constituted by a set of sub-carriers and transmitted for a fixed duration.
The MIMO structure may be used for carrying out time diversity for an OFDM system. For instance, when one transmitter transmits an OFDM signal, another transmitter will transmit a fully correlated OFDM signal to that transmitted by the one transmitter. The same OFDM signal is transmitted with, for instance, a fixed OFDM duration.
On the other hand, spatial diversity entails transmitting independent signals from different transmitters. Thus, transmitting two independent OFDM signals from two transmitters, respectively, results in a double data rate capacity from the parallel transmission that occurs.
When the signal to noise ratio (SNR) is low, the frame error rate (FER) is large, so that a data packet transmission will be decoded incorrectly and will need to be retransmitted. The quality of service (QoS) defines the number of times that the same packet can be retransmitted, e.g., within an OFDM architecture. The OFDM system on a MIMO structure, therefore, should be adaptable to ensure that the QoS is maintained.
For any given modulation and code rate, the SNR must exceed a certain threshold to ensure that a data packet will be decoded correctly. When the SNR is less than that certain threshold, the bit error rate (BER) is larger, which results in a larger FER. The larger the FER, the more retransmissions of the same packet will be required until the packet is decoded correctly. Thus, steps may need to be taken to provide the OFDM system with a higher gain. If the SNR is at or above the threshold, then there is no need to increase the gain of the architecture to decode the data packets correctly. One challenge is to adapt the OFDM system to use time diversity when signal fading is detected as problematic and to use spatial diversity at other time to increase the data rate transfer.
In a conventional OFDM system, there are many OFDM modes, for examples are the 1 k mode (1024 tones) and the half k mode (512 tones). For 1 k mode, the number of sub-carriers is 1024 and for the half k mode, the number of sub-carriers is 512. The 1 k mode is suitable for a channel with long delay and slow temporal fading, while the 512 mode is suitable for the channel with a short delay and fast temporal fading. But which mode will be used is really depending on the real environment.
A transaction unit of a conventional OFDM signal is an OFDM frame that lasts 10 ms. Each OFDM frame consists of 8 OFDM slots and each slot lasts 1.25 ms. Each OFDM slot consists of 8 OFDM symbols and some of the OFDM symbols will be the known preambles for access and channels estimation purposes. An OFDM super frame is made up of 8 OFDM frames and lasts 80 ms.
In addition to transmitted data, an OFDM frame contains a preamble, continual pilot sub-carriers, and transmission parameter sub-carriers/scattered sub-carriers. The preamble contains OFDM symbols that all used for training to realize timing, frequency and sampling clock synchronization acquisitions, channel estimation and a C/I calculation for different access points. The continual pilot sub-carriers contain training symbols that are constant for all OFDM