Docomo has talked previously about their "Super 3G" system....
Message 20340138
The radio access method is described as using OFCDM in the downlink...which would seem to leave an enhanced form of W-CDMA for the uplink. In their timeline, they seem to indicate that this would be the final upgrade for the network in the 3G spectrum (3.9G <g>). There next upgrade after this would be in new 4G spectrum.
FWIW, I did a quick patent search for the term OFCDM. I came up with 12 hits with 9 coming from Docomo, one from KDDI, one from NEC and one from Samsung.
appft1.uspto.gov
The following patent seems to provide the best description of OFCDM. Anybody want to take a guess on whether Qualcomm's post 1995 patents would apply to this type of radio interface?
appft1.uspto.gov
snip................
SUMMARY OF THE INVENTION
[0009] For the next-generation mobile telecommunications (the fourth generation mobile telecommunications) after IMT-2000, it is necessary to substantiate cellular systems that permit wide communication coverage at much higher information transmission rates (throughput), i.e., specifically, the maximum throughput of 100 Mbps or higher for downlinks and the maximum throughput of 20 Mbps or higher for uplinks in consideration of the asymmetry of up and down links of data traffic in the current cellular systems. However, approaches based on the aforementioned expansion of existing radio interfaces (HDR and HSPDA) have their limits to increase in the information transmission rates and it is hard to realize the maximum information transmission rate of about 100 Mbps. For example, supposing the radio bandwidth of 5 MHz allocated to W-CDMA (DS-CDMA base) is broadbandized to approximately 50-100 MHz, the broadbandization, i.e., higher chip rates will permit improvement in resolution of paths and result in separation into an extremely large number of paths in a small signal power per path. Therefore, this will cause increase of Multi-path Interference (MPI) and degradation of channel estimation accuracy, so as to cancel out the Rake time diversity effect, which will end in increasing the transmission power for realizing the required reception quality at the required information transmission rates and decreasing link capacity. Accordingly, the radio access schemes based on DS-CDMA are not suitable for high-speed and large-capacity packet transmission in the broadband of 50-100 MHz.
[0010] The orthogonal frequency division multiplexing (OFDM) scheme used in digital terrestrial broadcasting, wireless LAN, and others, can decrease the influence of MPI in such a way that the symbol period of each sub-carrier is set adequately long within the range where it is sufficiently smaller than delay times of multiple paths, i.e., the symbol rate is lowered and that a guard interval is inserted into each symbol. Therefore, in comparison with the aforementioned radio access scheme using DS-CDMA, OFDM is able to keep down the degradation of characteristics due to MPI in connection with the broadbandization and is thus suitable for high-speed signal transmission in the bandwidth of 50-100 MHz or higher.
[0011] In the OFDM, however, co-channel interference does not allow use of a common carrier frequency in adjacent cells and frequency reuse of cells is necessary. In the OFDM system, therefore, a frequency band that can be used per cell is a bandwidth obtained by dividing the entire frequency band of the system by the cell frequency reuse (cluster size), and this decreases efficiency of utilization of frequency. This OFDM system requires sophisticated Dynamic Channel Allocation (DCA) for realization of the frequency reuse of one cell and control becomes very complicated. In addition, the cell frequency reuse is essential for common control channels such as broadcast channels, paging channels, etc., which are constantly transmitted to communicating users in cells.
[0012] On the other hand, in the case of OFCDM (Orthogonal Frequency and Code Division Multiplexing) based on multi-carrier CDMA for multi-carrier transmission of signals spread on the frequency axis, the lower symbol rate is achieved using a number of sub-carriers, so that the influence of MPI is reduced. For this reason, it can implement larger capacity than the radio access schemes based on DS-CDMA, as is reported in Document 1 [S. Abeta, et al., IEEE VTC2000-Spring, pp.1918-1922] and Document 2 [Hiroyuki Atarashi, Sadayuki Abeta, and Mamoru Sawahashi, IEICE Technical Reports RCS-2000-136, October 2000]. However, this OFCDM permitted increase in system capacity in the case of multi-cell systems like cellular systems, as compared with OFDM, but had the problem that it was not feasible to implement higher capacity in the case of isolated cell systems like wireless LAN and office environments, as compared with OFDM involving no spreading.
[0013] A first object of the present invention is thus to provide a radio transmission system configured to vary spreading factors for transmitted information through use of OFCDM and thereby enable broadband packet transmission in wide cell coverage.
[0014] A second object of the present invention is to provide transmitter apparatus used in such a radio transmission system.
[0015] A third object of the present invention is to provide receiver apparatus used in such a radio transmission system.
[0016] A radio transmission system according to the present invention is a radio transmission system configured to, on the occasion of radio transmission of information between a transmitter and a receiver, perform the radio transmission of information using an orthogonal frequency and code division multiplexing transmission scheme of parallelly transmitting identical information by a plurality of sub-carriers, wherein the transmitter comprises converting means for parallelly converting channel-coded information in accordance with symbols transmitted simultaneously; and spreading means for spreading a sequence of parallelized symbols in at least one of a frequency direction and a time direction by a spreading code sequence of a spreading factor determined based on a plurality of changeable spreading factors.
[0017] This radio transmission system can be made to act as an OFCDM or OFDM system by changing the radio parameter of the transmitter's and receiver's spreading factor in the same radio access scheme. For this reason, it becomes feasible to provide flexible use of the two schemes of OFCDM and OFDM and to provide a radio access scheme capable of realizing large capacity with high frequency utilization efficiency (the number of communicating users capable of meeting the required reception quality per cell), independent of cell configurations and propagation environments.
[0018] In the radio transmission system of the present invention, the spreading means is preferably configured to spreading a sequence of parallelized symbols in both a frequency direction and a time direction by a spreading code sequence of a spreading factor determined based on a plurality of changeable spreading factors.
[0019] In the radio transmission system of the present invention, the transmitter is preferably configured to comprise a spreading factor determining means for obtaining a propagation environment indicating a condition of a propagation path between the transmitter and the receiver and determining the spreading factor according to the propagation environment.
[0020] In the radio transmission system of this configuration, the propagation environment, which indicates the condition of the propagation path between the transmitter and the receiver, is first obtained and the spreading factor is then varied according to the propagation environment. For example, the variation of the spreading factor is carried out in such a way that the spreading factor is set at a value not less than 1 for a propagation environment preferring operation in OFCDM or that the spreading factor is set at 1 for a propagation environment preferring operation in OFDM. As a result, the radio transmission system of the present invention permits selection (changeover) of the access scheme (OFCDM scheme or OFDM scheme) suitable for a propagation environment.
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