The present invention generally relates to the field of wireless communications. More particularly, the invention relates to a backward compatible transmitter diversity scheme for use in an OFDM system.
Wireless communication systems commonly include information carrying modulated carrier signals that are wirelessly transmitted from a transmission source to one or more receivers within an area or region. A major design challenge in wireless communication systems is to maximize system capacity and performance in the presence of interference, and a time-varying multipath channel. Multipath propagation is caused by the transmitted signal reflecting off objects near the transmitter and receiver and arriving at the receiver over multiple paths where each received signal varies from each other received signal in both amplitude and phase over time. Multipath fading makes reliable reception more difficult than in an additive white Gaussian noise (AWGN) channel. The presence of multipath can severely distort the received signal. In a multipath environment, the multiple copies of the transmitted signal can interfere constructively in some portions of the occupied bandwidth. In other portions of the occupied bandwidth, the multiple copies can interfere destructively at the receiver. This signal duplication causes unwanted variations in the received signal strength over the bandwidth. Diversity is an effective way to combat this problem. Currently, most diversity schemes are implemented at the receiver side, which combines the signals received from multiple antenna elements in the hope that the signals received from the different antennae do not experience fading at the same time. The signals obtained from the different antenna are combined at the receiver through techniques such as switch diversity and maximum ratio combining. For example, the current IEEE 802.11a standard refers to the use of a receiver side switch diversity scheme which provides a low cost solution. As is well known, switch diversity requires the use of multiple antenna elements at the receiver.
One drawback associated with employing switch diversity on the receiver side is that it is not cost effective in that each mobile station in the network must employ multiple antenna elements. A more cost effective solution is to implement transmitter diversity at the base station to combat multi-path fading at a low cost to mobile users. Transmitter diversity is a technique whereby a transmitter is provided with two or more (N) antennas. These N antennas imply N channels that suffer from fading in a statistically independent manner. Therefore, when one channel is fading due to the destructive effects of multi-path interference, another of the channels is unlikely to be suffering from fading simultaneously. By virtue of the redundancy provided by these independent channels, a receiver can often reduce the detrimental effects of fading.
A basic transmitter diversity system with two transmitter antennas 10 and 11 and one receiver antenna 12 is illustrated in
To this end, the present invention is directed to a transmitter diversity scheme preferably for use in an IEEE 802.11a wireless communication system that is backward compatible with the existing OFDM systems. It is noted that the present invention finds primary, but not limiting, application in an 802.11a wireless communication system.
The present invention is directed to a method and system for providing backward compatible transmitter diversity in an orthogonal frequency division modulated (OFDM) communication system.
According to one aspect of the invention, a method is provided for providing backward compatible transmitter diversity. The method generally includes the steps of: receiving an input data bit stream; transforming the received input data bit stream into an OFDM symbol stream comprised of even and odd symbols; dividing said OFDM symbol stream into a first symbol sub-stream including only even symbols from said OFDM symbol stream and a second symbol sub-stream including only odd symbols from said OFDM symbol stream; processing said first symbol sub-stream by a first processing block to output a first processed symbol sub-stream; processing said second symbol sub-stream by a second processing block to output a second processed symbol sub-stream; transmitting said first processed symbol sub-stream from a first diversity antenna; and transmitting said second processed symbol sub-stream from a second diversity antenna; wherein said first and second OFDM symbol sub-streams are transmitted over non-overlapping frequencies.
According to another aspect of the present invention, there is provided a backward compatible transmitter diversity system. The system includes a first processing circuitry module for transforming an input data bit stream bi into an OFDM symbol stream; and dividing said OFDM symbol stream into a first and a second OFDM symbol sub-stream wherein said first OFDM symbol sub-stream is comprised of only even symbols from said OFDM symbol stream and said second OFDM symbol sub-stream is comprised of only odd symbols from said OFDM symbol stream; a second processing circuitry module for further processing said first OFDM symbol sub-stream; a third processing circuitry module for further processing said second OFDM symbol sub-stream; a first antenna for transmitting said further processed first OFDM symbol sub-stream; and a second antenna for transmitting said further processed second OFDM symbol sub-stream wherein said first and second OFDM symbol streams are transmitted over non-overlapping frequencies.
The invention provides a cost savings advantage by only requiring a modification to a transmitting node in the communication system without having to modify a plurality of receiving nodes. A further advantage of the invention is that it is backward compatible with existing OFDM systems.
The foregoing features of the present invention will become more readily apparent and may be understood by referring to the following detailed description of an illustrative embodiment of the present invention, taken in conjunction with the accompanying drawings, where:
In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.
It is noted that the embodiment of
Diversity Transmitter
In the present embodiment, the first processing circuitry module 22 for processing the initial data bit stream, bi, includes a scrambler 253, an FEC coding unit 255 and an interleaving and mapping unit 257, all of which are conventional. Of particular significance is the recognition that the interleaving and mapping unit 257 outputs two separate OFDM symbol streams, OFDM-odd 105 and OFDM-even 106 as described above. The even symbol stream 105 comprised of only even OFDM symbols and the odd symbol stream 106 comprised of only odd OFDM symbols.
In the embodiment, both processing circuitry modules 24 and 26 include identical processing circuitry. That is, both modules 24 and 26 include a serial-to-parallel converters 260a and 280a; inverse fast-fourier transform devices 260b and 280b; GI addition modules 260c and 280c; symbol wave shaping modules 260d and 280d; and IQ modules 260e and 280e.
The second and third processing circuitry modules 24, 26 are shown to be connected to respective transmission antenna 110 and 112.
Non-Diversity Receiver
The foregoing is to be constructed as only being an illustrative embodiment of this invention. Persons skilled in the art can easily conceive of alternative arrangements providing a functionality similar to this embodiment without any deviation from the fundamental principles or the scope of this invention.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB03/05808 | 12/8/2003 | WO | 6/8/2005 |
Number | Date | Country | |
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60432887 | Dec 2002 | US |