Some wireless telecommunications systems employ a time division multiplexing scheme. The transmission time available is divided into slots. By way of example, in GSM (Global System for Mobile Communications) time is divided into sets of eight slots. Each set of eight slots is collectively referred to as a frame.
In this description, assignment refers to signalling used to identify slots that are made available to a given mobile station. The set of slots assigned to a unidirectional data flow is referred to as a TBF (temporary block flow). The TBF is a unidirectional entity: an uplink TBF relates to uplink assignment/allocation and a downlink TBF relates to downlink assignment/allocation.
In this description, an allocation refers to the actual reception/transmission of data on specific slots. An allocation will necessarily be a subset or all of the available assignment. Multiple mobile stations can have the same or overlapping assignments, and allocation will be used to avoid collisions.
With the GSM frame definition, the slot numbering for the uplink is offset from the slot numbering for the downlink such that a downlink slot and an uplink slot with the same number can be assigned and allocated on both the downlink and the uplink without requiring the mobile station to receive and transmit at the same time. For a given mobile station, the same physical time slot in a given frame can be assigned and/or allocated for either the uplink or the downlink, but not both. However, due to the offset numbering scheme described above, slots in a given frame having the same slot number can be assigned and allocated on both the uplink and downlink.
Multiple mobile stations in a given area share these time slots. Whenever each mobile station has data, it will, based on an uplink allocation mechanism, send data in the uplink direction. The network will also send data in the downlink direction on these slots to multiple mobile stations. For example, in a first frame slot 0 may contain data for a first mobile station, while in a next frame, the same slot may contain data for a second mobile station. Since a slot is a very small time unit, a slot may be allocated to a mobile station over multiple consecutive frames. For example, a BTTI (Basic Transmit Time Interval) block consists of a slot allocated over four consecutive frames. For example, frame 1 slot 1, frame 2 slot 1, frame 3 slot 1 and frame 4 slot 1 make up a BTTI block. In some implementations, a frame is approximately 5 ms in duration, such that a BTTI block will span over four frames, or a 20 ms interval. A BTTI TBF is a TBF which uses BTTI blocks.
An RTTI (Reduced Transmit Time Interval) block uses the same frame structure introduced above, but an RTTI block consists of a pair of slots during a first frame, and a pair of slots during the next frame such that an RTTI block will span over two frames or a 10 ms interval. An RTTI TBF is a TBF which uses RTTI blocks. The transmission interval for an RTTI block compared to a BTTI block is reduced by half.
A radio block is the collection of 4 bursts that is used sent to send an RLC/MAC data block, PACCH block, etc. All transmissions referred to in this description are sent as 4-burst radio blocks. For BTTI (basic transmission time interval), a radio block is sent using the same timeslot number in four frames; for RTTI (reduced transmission time interval) it is sent using two timeslots in two frames. A radio block period is then the duration of 4 or 2 TDMA frames in which a radio block is sent. An allocation for an uplink transmission allocates a BTTI block to the same timeslot in each of four TDMA frames or allocates an RTTI block to two timeslots in each of two TDMA frames.
An example of a downlink frame 30 and an uplink frame 32 each divided into 8 timeslots is shown in FIG. 1A. The downlink frame is offset in time from the uplink frame such that a mobile station can receive on a timeslot #n in a downlink frame and transmit a response on a timeslot having the same timeslot #n in an uplink frame without having to transmit and receive simultaneously.
To perform uplink BTTI allocation, the network transmits a USF (uplink state flag) during a downlink BTTI block in a downlink slot of a preceding block period. The mobile station is thereby allocated a timeslot for uplink transmission of an uplink BTTI block that has the same number as that of the downlink slot used to transmit the USF. FIG. 1A shows an example of downlink transmission of a single BTTI block, indicated at 40 which includes the first slot of each of four consecutive downlink frames and a BTTI uplink allocation at 41 which includes the first slot of each of four consecutive uplink frames. In the illustrated example, the four downlink slots #1 of the previous block period (not shown) also contain a USF for the mobile station that allocates BTTI uplink block 41 to the mobile station. USFs in BTTI are sent with BTTI blocks and allocate an uplink block in the BTTI radio block period after the USF is sent. FIG. 1B shows an example of an RTTI downlink transmission generally indicated at 50 and an RTTI uplink transmission generally indicated at 51. In this example, an RTTI block is transmitted to the mobile station in the downlink on timeslots #1,#2, and, by means of USF signalling on timeslots #1,#2 in the previous radio block period (not shown), the mobile station is allocated uplink timeslots #1,#2 for transmission of an uplink RTTI block, these slots being defined as the “corresponding slot pair” or “corresponding PDCH (packet data channel)-pair” to the downlink pair consisting of timeslots #1,#2. While in this example, the uplink slots are the same as for the downlink slots used to transmit USFs for the purpose of uplink allocation, this is not always the case with RTTI allocations. USFs in RTTI USF mode are sent like RTTI blocks (i.e. they occupy a pair of slots over two consecutive frames) and point to the RTTI block on the corresponding uplink timeslots in the two frames after the USF is sent. There is also a hybrid version of RTTI allocation where two BTTI USFs are used to allocate two RTTI blocks. Specifically, a first BTTI USF is used to allocate an RTTI radio block in the first two frames of the four frames that follow the two BTTI USFs, and a second BTTI USF is used to allocate an RRTI block in the second two frames of the four frames the follow the two BTTI USFs.
Historically, more specifically up to and including 3GPP Release 6, a poll by the network using the RRBP (Reserved Radio Block Period) or ES/P (EGPRS Supplementary/Polling) fields in the header of a downlink block performed two functions:
a) it allocates a particular uplink block in the future for the mobile station to transmit;
b) it indicates to the mobile station the contents of that block.
In early specifications (i.e. up to and including 3GPP Release 6), the uplink blocks to be sent by the mobile station in response to a poll are always control blocks sent on the PACCH (Packet Associated Control Channel), typically the response being a downlink ACK/NACK (acknowledgement/negative acknowledgement) message such as an EGPRS Packet Downlink ACK/NACK message. When polled by the network for a PACCH block, it is clear from the specification, as per 3GPP TS 44.060 v7.15.0 section 10.4.5, that the response message must be sent on the same timeslot number as that on which the poll was received. An example of this is shown in FIG. 2. In FIG. 2, a network is indicated at 10, and a mobile station is indicated at 12. The network 10 is shown transmitting a poll for PACCH block at 14 in Frame #x, and Timeslot #n. In the early specifications, the poll is indicated by the contents of the RRBP, ES/P fields. The Frame #x and Timeslot #n simply represent the frame number and timeslot number, as selected by the network, in which the first burst of the radio block containing the poll is transmitted. In response, the mobile station 12 transmits a PACCH block (e.g. EGPRS Packet Downlink ACK/NACK) as indicated at 16 starting in Frame #y, Timeslot #n. The Frame #y and Timeslot #n represent the frame number and timeslot number used by the mobile station to transmit the first burst of the radio block containing the response to the poll. For clarity, the transmission of the subsequent bursts of these radio blocks is not shown. The Timeslot #n is the same as that used by the network to transmit the poll. In addition, the relationship between frame numbers x and y is explicitly specified by the poll message (see e.g. 10.4.4b, 10.4.5 in 3GPP TS 44.060).
In 3GPP Release 7, the possibility was added for a poll to indicate that the mobile shall transmit an RLC/MAC data block with a Piggy-backed ACK/NACK bitmap field (PAN). This is requested by the appropriate setting of bits within the newly defined CES/P (Combined EGPRS Supplementary/Polling) field. Such a poll is included in a downlink data block and points to a frame where the poll response starts. A poll can be sent in BTTI mode, meaning in the same slot for four frames, or RTTI mode, meaning on a pair of slots for two frames. The mobile station knows whether the uplink reserved block uses RTTI or not and can figure out where to send the response.
Normally, allocations for uplink radio resources for the transmission of data blocks are signaled by means of the USF which are sent in the radio block period immediately preceding the radio block period in which the uplink allocations are valid, as detailed above.
In the absence of the need to transmit an RLC/MAC block with PAN, in spite of the relatively short time between the mobile station determining that it has an allocation for an uplink data transfer and the allocated uplink radio block transmission time, this is not a problem for processing/encoding, since it is possible for the mobile station to encode the radio block in advance, since the coding does not depend on exactly when the data block will be transmitted, in particular, in which timeslot number the data block will be transmitted.
Note that polls that request an uplink transmission in a given radio block period are sent much earlier than USFs which allocate resources in the same radio block period. It is possible that a poll and a USF may refer to the same uplink transmission opportunity. This is taken into account by the network when performing scheduling.