The invention relates to communication networks, and, more particularly, to wireless communication networks utilizing time division duplex techniques.
Time division duplex (TDD) and frequency division duplex (FDD) represent two different strategies for allocating the spectrum resource between a pair of stations for two-way communication. TDD is a well-known technique that divides a single communication channel into uplink and downlink time slots; this is in contrast to FDD which separate uplink and downlink communication into different frequencies/channels. Many existing cordless telephones utilize TDD-based techniques. Although cellular telephones today primarily rely on FDD techniques, many recent third generation (3G) and fourth generation (4G) wireless proposals introduce schemes based on time division duplex. See, e.g., G. J. R. Povey and Masao Nakagawa, “A Review of Time Division Duplex—CDMA Techniques,” Proceedings, 1998 IEEE 5th International Symposium on Spread Spectrum Techniques and Applications, vol. 2, pp. 630-33, Sep. 2-4, 1998. TDD offers numerous potential advantages over FDD, including the ability to exploit the reciprocity/symmetry of the downlink and uplink radio frequency channels, which facilitates adaptive modulation, coding, spatial domain processing, and scheduling at the transmit side of a base station. In short, TDD can potentially make more efficient use of the wireless spectrum resource.
On the other hand, a drawback of conventional time division duplex is that it requires a guard time interval to separate the downlink and uplink time slots. Since the guard time reflects the round trip propagation delay to the farthest mobile station in a cell, the guard time must increase as cell size increases in order to take into account the longer propagation delays. In existing time division duplex systems, the guard time interval accounts for both inefficiency (since no information is transmitted within a guard time interval) and a restriction on maximum cell size. Moreover, the disadvantage may increase when the system further employs time-division multiple access (TDMA) technology, because the time resource has to be sliced between the uplink and downlink as well as among the multiple users.
Accordingly, there is a need for improvements in the efficiency of a time division duplex communication system.
In U.S. Pat. Nos. 5,959,980 and 6,388,997, the contents of which are incorporated by reference herein, a technique was disclosed which reduces the guard time based on an estimate of the actual propagation time between the base station and a particular mobile station. By measuring the distance between the base station and the mobile station, the base station can order the mobile station to decrease the guard time for the mobile station's time slots based on the estimated distance rather than the cell radius. This results in a guard time that is, on average, shorter than available with conventional time division duplex techniques.
The present invention is directed to mechanisms that further improve the efficiency of a time division duplex communication system. In accordance with an aspect of the invention, the uplink transmission slots of a plurality of mobile stations are arranged in an order so as to take advantage of the short propagation delay to the closest mobile stations. Where the propagation delay between the base station and a first mobile station is less than the propagation delay between the base station and a second mobile station, the first mobile station is permitted to transmit in a time slot before the time slot of the second mobile station in the frame. The guard time between transmissions from the base station to the mobile stations and transmissions received by the base station from the mobile stations is set to a value based on the propagation delay between the base station and the first mobile station, which is preferably the closest mobile station. The time resource of the time division duplex frame is preferably divided into a downlink superframe and an uplink superframe, with a guard time interval there between. The uplink superframe begins with the uplink time slot of a mobile station close to the base station, with a corresponding smaller propagation delay to the base station, followed by the remaining mobile stations. The overall guard time, thereby, is only restricted by the propagation delay to the closest mobile station.
The present invention results in a time slot structure in a time division duplex communication system that has a higher efficiency than existing systems and, moreover, that results in a guard time design that is potentially independent of the cell size.
These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings.
The mobile stations 110, 120, 130, 140 and the base station 100 are assumed to have appropriate interfaces for establishing a wireless communication link. It should be noted that the present invention is not limited to any particular wireless link technology, although, as mentioned above, the present invention has particular relevance to next generation (3G/4G) wireless systems.
It is assumed that some form of time division duplex (TDD) is utilized to establish two-way communication between the base station 100 and the mobile stations 110, 120, 130, 140. The communication link from the base station to the mobile stations is referred to as downlink communication, while the link from a mobile station to the base station is referred to as an uplink. Uplink communication and downlink communication are allocated in separate time slots of a TDD frame, as depicted in
As depicted in
This represents the round-trip propagation time (RTT) between the base station and a mobile station positioned at the edge of the cell (depicted as the time between t0 to t2 in
Since the distance from any mobile station to the base station is always less than the cell radius, the guard time interval would be on average shorter than in conventional TDD, thereby improving the efficiency of the system.
Nevertheless, the efficiency of the system can be further improved by allocating uplink transmissions from different mobile stations in a particular order. This advantageously results in a guard time interval that can be independent of cell size radius.
Thus, the invention results in a guard time interval that is only restricted by the distance of the mobile station closest to the base station, rather than the distance of the mobile station farthest from the base station (i.e., the cell radius) as in conventional TDD systems. With this approach, the cell size is no longer a limiting factor in TDD systems. With proper ordering of the collective uplink transmission, the proposed mechanism only requires a single guard time interval whose duration is determined by the mobile station closest to the base station in supporting communication between the base station and the given set of mobile stations and, hence, significantly improves the efficiency of TDD systems. Consider, for example, a typical TDD/TDMA system such as the digital European cordless telephone (DECT) system. A DECT frame consists of 24 time slots, the first 12 time slots for downlink transmission of 12 users (i.e., the downlink superframe) and the next 12 time slots for uplink transmission of 12 users (i.e., the uplink superframe). Each time slots has 480 bits, including 60 bits for guard time. It is assumed that 12 users are uniformly located within the cell; hence, the probability density functions (pdf's) of the distance from the base station to an arbitrary mobile station and the distance from the base station to the closest mobile station are, respectively, given by:
Pany({tilde over (r)})=2{tilde over (r)},
Pnear({tilde over (r)})=2K{tilde over (r)}(1−{tilde over (r)}2)K−1,
where {tilde over (r)} is the distance from the base station normalized by the cell radius; K is the number of mobile stations in the cell, which is 12 in the DECT system. η is herein defined as the TDD efficiency, which is the percentage of data transmission in each frame (i.e., the total frame interval minus the guard time interval, neglecting other overheads for simplicity). Therefore, in the DECT system, the TDD efficiencies of the above embodiment of the present invention would be:
where 0.2482 is the mean value of the normalized distance from the base station to the closest mobile station among 12 mobile stations and {fraction (1/24)} is a factor due to the collective uplink transmission. The above efficiency computation is in contrast to the efficiency of conventional TDD and the above-mentioned enhanced prior art scheme:
where ⅔ is the mean value of the normalized distance from the base station to an arbitrary mobile station. From this numerical example, the above embodiment can potentially provide 14.14% greater TDD efficiency than traditional TDD and 8.95% greater than the above-mentioned enhanced scheme. In another form, the TDD inefficiency is merely 0.13%, as opposed to 12.50% and 8.33% for the prior art mechanisms.
Note that any desired asymmetry between downlink and uplink traffic can be readily adjusted by varying the durations of the uplink/downlink superframes. The order of the downlink time slots can be arbitrary. The order of the remaining uplink Time slots after the time slot fo the closest mobile station can also be arbitrary, although it can be advantageous in certain circumstances to arrange the remaining time slots in an order of increasing progpagation delays, so as to permit outer lying mobile stations sufficient time to commence uplink transmissions. Alternatively, the different propagation delays of the different mobile stations can be split into zones of uplink time slots in the uplink superframe. Those mobile stations in the zone closest to the base station are placed in the first group of uplink time slots in the uplink superframe, those mobile stations in the zone next closest to the base station are allocated the next group of uplink time slots, etc.
It will be appreciated that those skilled in the art will be able to devise numerous arrangements and variations which, although not explicitly shown or described herein, embody the principles of the invention and are within their spirit and scope.