# Patent

## Title
Simulcasting MIMO communication system

## Abstract

A wireless multiple-input multiple-output (MIMO) communication system includes signal simulcasting. Base stations include a plurality of transmit antennas and terminals include a plurality of receive antennas to form MIMO channels. In one embodiment, a simulcasting MIMO wireless communication system includes orthogonal frequency division multiplexing (OFDM). This arrangement achieves the spectral efficiency advantages of OFDM and simulcasting.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
The present invention relates generally to communication systems and, more particularly, to wireless communication systems.
BACKGROUND OF THE INVENTION
A wide variety of wireless communication techniques can be used to transmit and receive data between a transceiver, e.g., a base station, and a terminal, e.g., a mobile phone or station. Exemplary network types include time division multiplexing (TDM), frequency division (FDM), and code division. Each of these systems have concomitant advantages and disadvantages. For example, single carrier systems, such as TDM and FDM, suffer problems from signal delay spread, which can degrade system performance and impact the overall efficiency.
Simulcasting techniques for wireless communication are well known in the art. In general, a plurality of transmitting stations each simultaneously transmit a given signal from the same frequency (FDM) and/or time (TDM) slots. Users within areas covered by the simulcasting transmitters receive the simulcast signals. When near cell boundaries, a user will receive a simulcast signal from each base station serving a neighboring cell. Simulcasting enhances coverage and spectrum efficiency as compared to systems that broadcast a given signal on different channels for each user requesting the signal when the same signal is requested by multiple users.
However, simulcasting systems suffer some of the same disadvantages as non-simulcasting technologies. For example, single carrier simulcasting systems typically suffer problems from signal delay spread and co-channel interference and limited frequency re-use for non-simulcast signals. Furthermore, delay spread can be even longer in simulcast systems since the signal is transmitted by many base stations which may be located at a range of distances from a mobile receiver.
Multiple-input multiple-output (MIMO) is another technique that increases spectral efficiency. In MIMO systems, multiple transmit antennas transmit different signals, all of which are separated and detected by multiple receive antennas. In general, with M receive antennas, up to M signals, either MIMO or co-channel interfering signals, or a combination thereof, can be separated and detected and/or suppressed at the receiver. Thus, when co-channel interference is not present, the use of N transmit and M receive antennas results in an increase in link capacity of the minimum of N and M, i.e., if N less than or equal to M, an N-fold increase in capacity, theoretically without any increase in total transmit power. However, N-fold MIMO increases the number of co-channel interferers N-fold, requiring an N-fold increase in the number of receive antennas to suppress the co-channel interference. Alternatively, for a given number of receive antennas, the degree of MIMO permitted in a system is reduced with aggressive frequency re-use, if MIMO is permitted at all.
It would, therefore, be desirable to provide a wireless simulcasting communication system that overcomes the aforesaid and other disadvantages.
SUMMARY OF THE INVENTION
The present invention provides a wireless multiple-input multiple-output (MIMO) communication system having simulcasting capability. This arrangement provides a spectrally efficient system that combines the advantages of MIMO and simulcasting techniques since simulcasting has limited co-channel interference (CCI), MIMO can be used to its fullest capability. While the invention is primarily shown and described in conjunction with a wireless cellular system, it is understood that the invention is applicable to wireless systems in general, in which spectral efficiency is desired.
In one aspect of the invention, a wireless communication system includes a plurality of base stations, each having a plurality of transmit antennas, and a plurality of mobile stations, each having a plurality of receive antennas. In one embodiment, each of the plurality of base stations serves a respective cell or sector. The base stations can simulcast one or more signals to the mobile stations located throughout the wireless system.
In a further aspect of the invention, a wireless orthogonal frequency division multiplexing (OFDM) communication system includes a plurality of simulcasting MIMO base stations for communicating with a plurality of mobile stations. With this arrangement, the system receives the advantages of OFDM systems (e.g., mitigation of signal delay spread) the advantages of simulcasting systems (e.g., relatively high spectral efficiency without co-channel interference) and the full use of MIMO because of the lack of co-channel interference.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic representation of a wireless MIMO communication system having simulcasting capability in accordance with the present invention;
FIG. 2 is a schematic representation showing an exemplary embodiment of a wireless MIMO communication system having simulcasting capability in accordance with the present invention;
FIG. 3 is a schematic representation of a wireless MIMO OFDM system having simulcasting capability in accordance with the present invention;
FIG. 4 is an exemplary OFDM MIMO station that can form a part of a simulcasting MIMO communication system in accordance with the present invention;
FIG. 5 is a graphical depiction of a subcarrier that can be used in the OFDM MIMO system of FIG. 4;
FIG. 6 is a graphical depiction showing the orthogonal nature of the subcarriers of FIG. 5; and
FIG. 7 is an exemplary OFDM MIMO system that can form a part of a simulcasting MIMO communication system in accordance with the present invention.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a wireless communication system 100 providing multiple-input multiple-output (MIMO) data communication between simulcasting base stations BS1-BS7, each of which covers a respective cell or sector C1-C7, and mobile stations MS1-MSX and/or fixed-location terminals FT1-FTY. As described in detail below, the base stations BS include a plurality of transmit antennas that form respective MIMO channels with receive antennas located on the mobile stations MS and fixed terminals FT. The system achieves the advantages of simulcasting and MIMO systems. In one particular embodiment, the communication system utilizes orthogonal frequency division multiplexing (OFDM) to minimize the effects of signal delay spread, further enhancing the system spectral efficiency.
Simulcasting is well known to one of ordinary skill in the art. In general, simulcasting refers to the broadcast of a given signal by a plurality of base stations BS or transmitters. Mobile stations MS and/or fixed terminals FT located within the cells covered by the base stations BS receive the signal. Near cell boundaries, a user can receive multiple versions of simulcast signals, which can improve the system performance by providing better coverage with a stronger signal, as well as better performance because of the lack of co-channel interference. Simulcasting is useful in a variety of network configurations. For example, a high demand channel, such as real-time stock quotes, can be broadcast by each base station serving at least one user that desires to receive the channel. In another embodiment, a network can simulcast a signal from a series of low power transmitters, which can be located in various buildings, to provide coverage for users anywhere within the buildings. It is understood that the network can selectively simulcast based on user channel demand or can constantly simulcast in predetermined areas without regard for user location.
As is also known in the art, conventional cellular networks have a predetermined re-use factor, such as seven for single carrier systems, for non-simulcast channels. The re-use factor defines the number of cells in a pattern that minimizes co-channel interference. Each base station utilizes a subset of channels to avoid use of the same channels within a predetermined distance. That is, base stations using the same channels should be sufficiently spaced apart so as to minimize co-channel interference. It is understood that simulcast channels do not generate co-channel interference with each other since the same signal is broadcast from different locations in the same frequency/time slots. In addition, since a mobile station located near a cell boundary can receive two relatively weak versions of the same signal, overall system performance can be boosted, e.g., a 3 dB power improvement, coverage can be more uniform, and handoffs are avoided.
FIG. 2 shows an exemplary MIMO system 200 that can form a part of a wireless MIMO simulcasting communication system in accordance with the present invention. The MIMO system 200 can be provided from a variety of wireless network types including time division multiple access (TDMA), code division multiple access (CDMA), frequency division multiple access (FDMA), and orthogonal frequency division multiplexing (OFDM). It is understood that other known and now unknown wireless network technologies can be used to provide a MIMO simulcasting system without departing from the present invention.
In general, the MIMO system 200 includes a plurality of transmit antennas TA1-TAN, each of which broadcasts a respective signal S1-SN. A data stream, which can comprise one or more signals, is encoded by an encoding system 202, which generates the respective signals S1-SN for broadcast by the transmit antennas TA1-TAN. The transmitted signals are received by a plurality of receive antennas RA1-RAM associated with a terminal, such as a mobile station. It is understood that the number of receive antennas is not necessarily equal to the number of transmit antennas. The receive antennas RA1-RAM receive the transmitted signals and provide the signals to a decoding system 204 for signal detection and decoding.
Exemplary wireless MIMO systems are shown and described in Chevreuil, A., Vandendorpe, L., “MIMO MMSE-DFE: a General Framework,” Statistical Signal and Array Processing, 1998. Proceedings., Ninth IEEE SP Workshop on, 1998, pages: 368-371, Ruly Lai-U Choi; Letaief, K. B.; Murch, R. D., “MIMO CDMA Antenna Systems,” 2000 IEEE International Conference on Communications, Volume: 2, 2000 Pages: 990-994 vol. 2, and Jian Yang; Roy, S., “On Joint Transmitter and Receiver Optimization for Multiple-Input-Mulitple-Output (MIMO) Transmission Systems,” IEEE Transactions on Communications, Volume: 42 Issue: 12, December 1994 Pages: 3221-3231, all of which are incorporated herein by reference.
FIG. 3 shows an exemplary MIMO OFDM system 300 with simulcasting in accordance with the present invention. The system includes a plurality of MIMO OFDM base stations BS1-BSN simulcasting a signal to a MIMO terminal, such as a mobile station MS1. With this arrangement, the advantages of OFDM systems and simulcasting systems are realized. More particularly, the system 300 obtains the benefits of minimal co-channel interference and increased signal power provided by simulcasting and lack of signal delay spread problems provided by OFDM systems so as to enhance the overall spectral efficiency of the system.
Referring briefly to FIGS. 4-6, an exemplary MIMO OFDM system 400, which can form a part of a simulcasting MIMO system in accordance with the present invention, includes subsystems for transmission and reception of data. A coding subsystem 402 encodes binary data from a data source. The coded data is interleaved by an interleaving subsystem 404 and then mapped onto multi-amplitude multi-phase constellation symbols by a mapping subsystem 406. In one particular embodiment, the multi-amplitude multi-phase constellation symbols include quadrature phase shift keying (QPSK) symbols. Pilot signals can then be inserted by a pilot insertion subsystem 408 to estimate the channel at the remote subscriber unit receivers. A serial-to-parallel conversion subsystem 410 converts the serial data stream to a parallel data stream that is provided to an inverse fast Fourier transform (IFFT) subsystem 412.
The transformed data is converted to serial data stream by a parallel-to-serial converter 414. Cyclic extension and windowing can be added by a subsystem 416 prior to digital-to-analog conversion by a DAC 418 and transmission by an antenna system 420 including a plurality of transmit antennas TA. The receive portion 422 of the OFDM system includes similar corresponding components for extracting the data from the received OFDM signal.
As shown in FIG. 5, the OFDM system utilizes an overlapping orthogonal multicarrier modulation technique having a plurality of subcarriers 450. FIG. 6 shows the orthogonal nature of the subcarriers. More particularly, each of four subcarriers 460 of one OFDM data symbol has an integral number of cycles in the interval T. The number of cycles between adjacent subcarriers differs by one.
FIG. 7 shows an illustrative MIMO-OFDM system 500 having multiple (here shown as four) transmit antennas TA1-4 and a plurality of receive antennas RA1-P. A data stream is split into first and second signals that are transmitted by respective pairs of transmit antennas TA1,TA2:TA3,TA4. Although the MIMO-OFDM system is shown having four transmit antennas, it is understood that any number of transmit antennas can be used. In addition, the number of receive antennas can be different from the number of transmit antennas.
The MIMO-OFDM system 500 includes a first space time encoder STEL that receives a first data block b1[n,k] and a second space-time encoder STE2 that receives a second data block b2[n,k]. At time n at tone k, each of the two data blocks, {bi[n,k]: k=0, 1, . . . } for i=1 and 2, is transformed into two signals, {t2i+j[n,k]: k=0, 1, . . . , & j=1, 2} for i=1 and 2, respectively, through the first and second space-time encoders STE1,STE2. Each of the coded signals forms an OFDM block. The transmit antennas TA1-4 transmit the OFDM signals after respective inverse fast Fourier transform IFFT1-4 modulation by respective signals tmi[n,k] for i=1, . . . , 4.
The signals sent by the transmit antennas TA1-4 are received by the receive antennas RA1-RAP. The received signals r1[n,k], r2[n,k], . . . , rP[n,k] are transformed by respective fast Fourier transform (FFT) subsystems FFT1-FFTP to generate signals that are provided to a space-time processor STP, which provides detected signal information to respective first and second space-time decoders STD1,STD2. A channel parameter estimator CPE receives the transformed signals from which channel parameter information is determined and then provided to the space-time processor STP for use in decoding the signals.
To achieve transmit diversity gain and detection of the transmitted signals, the space-time processor STP extracts the required signals for decoding by the first and second space-time decoders STD1, STD2. The space-time processor and space-time decoders each require channel state information. In one embodiment, the CPE utilizes conventional training sequences to exploit time and frequency domain correlations of the channel parameters. Further details of the MIMO-OFDM system 500 of FIG. 7 are provided in U.S. patent application Ser. No. 09/791,523, filed on Feb. 23, 2001, now issued as U.S. Pat. No. 7,068,628, which is incorporated hereon by reference.
By combining simulcasting with MIMO OFDM, the wireless communication system benefits from the spectral efficiency and minimal co-channel interference of simulcasting to enhance MIMO and the mitigation of signal delay spread of the multicarrier OFDM signals.
In an alternative embodiment, a simulcasting MIMO system includes TDM and FDM. The system can transmit both simulcast and non-simulcast signals based upon whether users in the coverage area demand the same signals. Under certain conditions, it may be desirable to reduce the number of MIMO channels to reduce co-channel interference of non-simulcast signals. More particularly, when simulcasting is used over a limited area, with other cells re-using the frequencies, then the level of co-channel interference into the simulcasting system can be higher (particularly near the simulcasting area boundary) and a reduced degree of MIMO may be used.
One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

## Claims

1. A method for wireless communication comprising:
simulcasting at least one signal from a plurality of multiple-input multiple-output base stations;
non-simulcasting at least one other signal from the plurality of multiple-input multiple-output base stations, wherein the simulcasting comprises transmitting the at least one signal in a same frequency or in a same timeslot; and
adjusting a number of multiple-input multiple-output signals comprised of the at least one signal that is simulcast and the at least one other signal that is non-simulcast by adjusting a number of simulcasted signals from the plurality of multiple-input multiple-output base stations based upon user requests for respective signals.

2. The method according to claim 1, further comprising:
utilizing time division multiplexing to transmit the at least one simulcast signal and the at least one non-simulcast signal.

3. The method according to claim 1, further comprising:
utilizing frequency division multiplexing to transmit the at least one simulcast signal and the at least one non-simulcast signal.

4. A method for wireless communication comprising:
simulcasting at least one signal from a plurality of multiple-input multiple-output base stations;
non-simulcasting at least one other signal from the plurality of multiple-input multiple-output base stations, wherein the simulcasting comprises transmitting the at least one signal in a same frequency or in a same timeslot; and
adjusting a number of multiple-input multiple-output signals comprised of the at least one signal that is simulcast and the at least one other signal that is non-simulcast by adjusting a number of transmitted multiple-input multiple-output signals based upon a level of co-channel interference.

5. The method according to claim 4, further comprising:
utilizing time division multiplexing to transmit the at least one simulcast signal and the at least one non-simulcast signal.

6. The method according to claim 4, further comprising:
utilizing frequency division multiplexing to transmit the at least one simulcast signal and the at least one non-simulcast signal.