Patent Application
20060291429
The present invention relates generally to 3G communications systems, and more specifically to transport formats in Universal Mobile Telecommunications Systems (UMTS).
In UMTS, frames exchanged over the radio interface include transport blocks (TBs). A transport format (TF) specifies the number and size of TBs to be transmitted in a frame, and transport format combinations (TFCs) are combinations of TFs specifying the number and size of TBs to be transmitted in a frame for each of multiple channels. A device communicating in a UMTS environment selects a TFC from a set of available TFCs. This set is referred to as the transport format combination set (TFCS).
In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views.
RNC 144 and Node-b 142 are included within UMTS terrestrial radio access network (UTRAN) 140. Node-b 142 is a base station that provides physical access to the radio interface 120 for RNC 144. RNC 144 provides a medium access control (MAC) layer that communicates with a MAC layer in ME 112.
Embodiments of the various blocks shown in
In UMTS, frames exchanged over the radio interface (Uu) are processed by the radio interface protocol stack, which exists in ME 112 and in RNC 144. The radio interface protocol stack processes data from upper layers and creates transport blocks (TBs) of a certain length, ready to send as data over the radio interface after some more operations performed in the physical layer (e.g. scrambling, interleaving, data rate matching, modulation and coding). During each radio frame, a number of transport blocks (TBs) can be transmitted between ME 112 and RNC 144.
According to the amount of data to send to/from a certain ME, the medium access control (MAC) scheduler, responsible for scheduling TBs to send over the radio interface, chooses one of the available transport format combinations. These combinations specify several variations of the number of transport blocks that can be sent in one radio frame on all ME channels; they are chosen by the radio resource control (RRC) layer in such a way that the traffic on all ME channels does not exceed bandwidth allocated for this ME.
In UMTS, variances in signal quality (and hence transmission quality) may be counteracted by a power control mechanism. Additionally, an automatic repeat request (ARQ) in the radio link control (RLC) layer operating in acknowledged mode (AM) enables retransmission of erroneous TBs (for which the attached CRC—cyclic redundancy checksum—indicates that there were bit errors on the radio interface). In case of a higher bit error rate (BER), the block error rate (BLER) is also higher. Power control may not always be able to compensate for lower signal quality (as measured by a lower S/N signal-to-noise ratio) by increasing transmission power. So if BLER increases, more TBs contain errors and have to be retransmitted. For a certain value of BER, the longer is a TB the greater is the possibility that it will contain an error. This is described more fully below with reference to
Various embodiments of the present invention provide radio link control (RLC) and medium access control (MAC) implementations that allow the TB size to be changed dynamically from one radio frame to the next. Dynamic modification of TB size allows the use of longer TBs if signal quality is high (and there are few errors on the radio interface) and shorter TBs if the signal quality is lower (and there are more errors on the radio interface). This mechanism enables UMTS to operate more efficiently, be more robust and decreases the amount of data that has to be retransmitted.
Three bit errors are shown in
Power control procedures try to keep transmission quality at certain levels (which may be imposed by channel QoS requirements), but sometimes it is hard because transmitted signal strength cannot be too high (to avoid causing too much signal interference). Various embodiments of the present invention may be used to improve transmission quality by decreasing BLER, either if power control cannot correctly adjust power or to decrease the amount of retransmitted data. Decreasing TB length may have a small negative impact on UTMS performance because more bandwidth is used for TB headers, but this may be traded off with the bandwidth savings due to fewer TB retransmissions.
UMTS radio access bearers (RABs), i.e. channels, can operate in three modes: transparent mode (TM), unacknowledged mode (UM) and acknowledged mode (AM). In TM, which is used mainly for voice traffic, TB length can be different on a single channel, but TM is the simplest mode and does not offer reordering and sequence checking. UM enables TB (or, precisely, RLC PDU) sequence ordering and detection of missing RLC PDUs, but retransmissions have to be done by higher layers. Only AM provides an automatic repeat request (ARQ) mechanism to retransmit erroneous or lost TBs at the UMTS RLC level.
For each radio frame, the MAC scheduler chooses, for a certain ME, one of the available transport format combinations (TFC) from a transport format combination set (TFCS). Each TFC contains transport formats (TF) for each channel configured for this ME. Each TF describes, among other things, the number and length of TBs that can be sent. An example of a TFCS for an ME with 2 channels is shown in
In the example TFCS of
Various embodiments of the present invention provide transport format combinations (TFCs) with varying TB sizes for one or more channels in a single TFCS. To dynamically change the TB size, the MAC scheduler need only select a different TFC from within the TFCS. Example transport format combination sets (TFCSs) having varying TB sizes are shown in
In various embodiments of the present invention, a MAC scheduler in a ME or RNC may choose a TFC according to current BER (in order to maintain a similar BLER all the time). For example, if signal quality decreases, the scheduler may choose to send 2 TBs, each containing 168 bits (TFC3 in
In some embodiments, TB size is modified in response to BER, and in other embodiments, TB size is modified in response to BLER. Further, in some embodiments, TB size is modified in response to a combination of one or both of BER and BLER along with information regarding current transmit power levels. For example, errors may be managed by modifying power levels until the power level reaches a certain point at which errors are managed by dynamically adjusting TB size. There are other methods to improve transmission quality in case of changing radio conditions, and these other methods may be combined with dynamic TB size adjustment without departing from the scope of the present invention.
Method 700 is shown beginning with block 710 in which a transport format combination (TFC) is selected form a transport format combination set (TFCS). By selecting a TFC, one or more transport block (TB) sizes may be set. For example, a TFC may be selected from TFCS 500 (
One or more radio frames may be transmitted in one or more channels using parameters specified by the transport formats (TFs) according to the TFC selected at 710. For example, referring now back to
At 720, one or more desired TB sizes may be determined in response to various criteria. For example, a channel error indication (e.g., BER or BLER) may be received and a desired TB size may be determined in response thereto. If channel errors have increased, or if it is determined that channel errors are too high, the actions of 720 may determine that a smaller TB size should be used. Further, the actions of 720 may determine TB sizes in response to power levels. For example, a TB size may be held constant while the power level is low enough to allow for an increase in transmit power to compensate for increased channel errors. In some embodiments, it may be desirable to not increase a transmit power level, and TB sizes may be decreased in response to increased channel errors.
At 730, a different TFC is selected from the TFCS. The different TFC selected may correspond to a TFC having TB sizes determined by the actions of 720. For example, if TB sizes are to be reduced, the TFC having TB sizes reduced for one or more channels may be selected.
Utilizing method 700, TB sizes may be dynamically adjusted from one radio frame to the next radio frame. For example, the actions of 710 may be performed for a first radio frame, and the actions of 730 may be performed for a subsequent radio frame, and the actions of 720 may performed in the interim period.
Radio frequency circuit 860 communicates with antenna 870 and processor 820. In some embodiments, RF circuit 860 includes a physical interface (PHY) corresponding to a communications protocol. For example, RF circuit 860 may include modulators, demodulators, mixers, frequency synthesizers, low noise amplifiers, power amplifiers, and the like. In some embodiments, RF circuit 860 may include a heterodyne receiver, and in other embodiments, RF circuit 860 may include a direct conversion receiver. In some embodiments, RF circuit 860 may include multiple receivers. For example, in embodiments with multiple antennas 870, each antenna may be coupled to a corresponding receiver. In operation, RF circuit 860 receives communications signals from antenna 870, and provides analog or digital signals to processor 820. Further, processor 820 may provide signals to RF circuit 860, which operates on the signals and then transmits them to antenna 870.
Memory 850 represents an article that includes a machine readable medium. For example, memory 850 represents a random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), read only memory (ROM), flash memory, magnetic disk, CD-ROM, or any other type of article that includes a medium readable by processor 820. Memory 850 may store instructions for performing various method embodiments of the present invention. For example, memory 850 may include instructions that when accessed by processor 820 cause processor 820 to perform methods represented by
Example systems represented by
Although the present invention has been described in conjunction with certain embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims.
