FORM -2
THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003
PROVISIONAL
Specification
(See Section 10 and rule 13)
CELLULAR NETWORKS

TATA CONSULTANCY SERVICES LTD.,
an Indian Company of Nirmal Building, 9th floor, Nariman Point, Mumbai 400 021, Maharashtra,
India
THE FOLLOWING SPECIFICATION DESCRIBES THE INVENTION


Field of the Invention:
This invention relates to cellular networks.
Particularly this invention relates to error checking and error correction methods in cellular networks.
Background of the Invention:
Hybrid Automatic Repeat Request (HARQ) is a variation of the Automatic Repeat Request error control and checking method. It is a technique that enables faster recovery from errors in cellular networks by storing corrupted packets in the receiving device rather than discarding them. By this method a good packet can be obtained from a combination of bad packets.
HARQ method includes transmission of forward error correction bits along with the existing error detection.. In the basic form HARQ adds both error detection bits and forward error correction bits to each message prior to transmission. When the coded data block is received, the receiver first decodes the error-correction code. If the channel quality is good enough, all transmission errors are correctable, and the receiver can obtain the correct data block, sends a positive-acknowledge (ACK) to the transmitter. If the channel quality is bad, and not all transmission errors can be corrected, the receiver detects this using the error-detection code, then the received coded data block is discarded and a retransmission is requested by the receiver through a negative-acknowledgement (NACK) notification.
HARQ mechanism has a signaling overhead while sending its feedback (ACK/NACK) from the receiver to transmitter both in uplink and downlink. Current mobile system supports different traffic parameters like fixed-size data
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packets on a periodic basis, variable size packets with variable data rate. It is the scheduler that takes the decision how to provide the grant for these resources, based on the priority and the QOS (quality of the service) requirement. Choice of persistent resource allocation is one of mechanisms by which resource allocator can reduce its overhead. At the same time HARQ provides the reliable data transmission at the cost of signaling overhead.. Thus making combination of HARQ with persistent resource allocation enhances the throughput. Assigning more responsibilities such as assisting of resource allocation method by carrying special control bits to the HARQ feedback channel, besides carrying the ACK/NACK information only, reduces the signaling overhead of maintaining a separate control channel related to HARQ resource allocation. Reducing computational overhead of bit rate processing block of physical layer enhances the throughput also. The present invention is an attempt towards this direction.
Summary of the Invention:
This invention relates to the feedback channel in HARQ - Hybrid Automatic Repeat Request. This invention envisages a novel way of taking the decision whether to perform synchronous and asynchronous HARQ, depending on the traffic parameters and the buffered data length of the incoming data.
This invention envisages reduction of signaling overhead of the HARQ mechanism, carrying extra information in the HARQ-feedback channel apart from the HARQ-ACK and NACK bit (indication of the error detection of the transmitted data) as well as reduction of computational overhead inside the bit rate processing block of physical layer.
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In accordance with the present invention, a novel mechanism for signaling overhead reduction w.r.t scheduling , usage of HARQ-feedback channel in a dual role (feedback and control channel) and computational overhead reduction of the bit rate processing block of physical layer particularly the rate matching block for HARQ, is presented.
For development of this overhead reduction mechanism of signaling as well as bit rate computation of physical layer for HARQ some assumptions are made regarding the transmitter and receiver node of downlink (Dl) and uplink (Ul). Downlink transmitter node has a resource allocator/scheduler that determines availability of resources for uplink and downlink data transfer. It also has a data admission control module. Each node supports multiple type of data traffic, HARQ. Both downlink and uplink nodes negotiate their capabilities before transferring data traffic to each other by exchanging some control messages (MAC layer). The nodes support OFDMA as access mechanism. Downlink (DL)-transmitter will send an indication to its scheduler after the end of every frame transmission. DL-transmitter maintains a downlink control channel for carrying resource allocation information both for downlink and uplink data transfer.
The invention involves a scheme of detection of HARQ-mode depending on the traffic parameters and the accumulated buffer length. Example - data traffic consisting of fixed-size packets, arriving at fixed time intervals is the best candidate for synchronous HARQ, because of their predictive nature, retransmission time and time duration of getting the HARQ- feedback are constant. Synchronous HARQ, in combination with non adaptive HARQ (i.e. constant modulation and code rates) reduces the signaling overhead further
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particularly in case of retransmission. The accumulated data length helps
scheduler determine the allocation period of the synchronous HARQ.
At the beginning of this allocation period, scheduler determines best possible
modulation and coding rates and maintains this for the complete allocation
period.
Scheduler of DL-transmitter configures HARQ-feedback channel and uses this for passing control information if any change happens to the allocation mechanism for synchronous HARQ transmission or retransmission. UL-transmitter uses this HARQ-Feedback channel to send the indication whether it has received the synchronous HARQ resource allocation information at the beginning of the allocation time period or not, thus any other extra control channel for this purpose is not required. It can also be applicable for a-synchronous HARQ.
Rate matching block can use the same size of the code-block (input bit-stream from MAC to PHY after channel coding) for the whole allocation time period for the synchronous HARQ. Thus the computation of determining the code-block size for this period can be skipped.
The resource allocation of variable size packets with variable data rate demands varying resource allocation depending on incoming data rate hence occurrence of HARQ-feedback and HARQ retransmission will vary, therefore a-synchronous HARQ with choice of adaptive HARQ (varying modulation and coding rate) can enhance throughput in this case.
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The invention comprises three procedures which are invoked sequentially. These procedures are:
1) Determination of synchronous or asynchronous HARQ mode based on traffic parameter and accumulated data buffer length and taking decision whether to use constant bit rate and modulation or variable bit rate and modulation. 2) Processing HARQ-feedback differently according to HARQ mode selection i.e. synchronous or asynchronous.
3) Using HARQ Feedback channel as a control indicator channel to indicate non receipt of the synchronous HARQ allocation information by the uplink (UL) -transmitter.
The invention will now be described with respect to the accompanying drawings.
Brief description of the accompanying drawings:
Figure 1 shows how to select the HARQ mode and its flow based on the traffic
parameters and accumulated buffer length
Figure 2 shows the allocation time period and MCS (modulation code rate) for
different HARQ mode.
Detailed Description of Invention:
Procedures as mentioned above are described in detalis:
1) HARQ-mode detection and controlling code-Word-length determination of
the rate-matching block:
Scheduler uses the traffic parameters of the incomjng packet as determined by
the admission control module. Incoming data packets are accumulated in the
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data-buffer, accumulation of certain amount of data, is termed as accumulated data length (al) (al < total buffer length). Based on its available bandwidth and link quality DL-transmitter determines the modulation and coding rate (MCS), and also determines the resources required to be mapped inside the physical layer. It determines the allocation period (AT), which is a function of buffer length, modulation and coding rate. AT (Allocation period) = f(al,MCS). AT gives the time duration while same resource allocation will be used by the physical layer.
Data packet of fixed-size and arriving at fixed time intervals does not require any variable MCS. Retransmission and HARQ ACK feedback time duration are also predictive and fixed in this case, thus the synchronous-HARQ becomes the best choice.
So in case of synchronous-HARQ scheduler keeps MCS constant throughout AT. Scheduler also calculates the next allocation start-time. Thus only at the beginning of every AT scheduler sends the indication to the physical layer about the resource requirement and MCS and that information is reused by the PHY through out that time period (AT).
Rate-matching block of physical layer is used to calculate the size of the code-block by using the above mentioned inputs given by the scheduler. Code-block-size determination of the rate-matching block: Number bits used in modulation = M (based on modulation type) Code rate used - 1/R Allocated resources in bits (AB) = AB Code-block-size = (AB*R)/M
The same code-block size can be used throughout AT as the inputs are constant during this time.
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Data packets of variable size with variable data rate need the varying resource allocation every time for every new transmission and even for retransmission. So getting of HARQ-feedback and retransmission time will vary accordingly. Thus a-synchronous HARQ with adaptive HARQ i.e. having varying modulation and coding rate becomes the best choice in this case.
2) Processing of HARQ-Feedback channel as HARQ-Feedback and Control
channel (HFC-CH)
HARQ-Feedback processing can be made effective as follows:
• Using of two bits in the HARQ feedback.
• Using of QPSK modulation in general.
• LSB will be used for HARQ-ACK and NACK indication.
• MSB will be used to indicate any change in the resource allocation mechanism for synchronous HARQ in case of DL transmitter.
• MSB will be used in case of uplink if UL transmitter does not receive any resource allocation Information for synchronous-HARQ.
• One bit can be punctured during the synchronous-HARQ allocation period from the DL- side to use it only for HARQ-feed back channel and that time the modulation BPSK can be used.
3) Using HARQ Feedback channel as a control indicator channel by UL-
transmitter.
• HARQ-Feedback channel comprises two bits in.this case.
• LSB will be used for HARQ-ACK and NACK indication
• UL-transmitter will use MSB (value = 1) to indicate if the resource allocation information throughout AT for the synchronous HARQ is not received or not decoded properly.
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• After decoding this bit, scheduler will retransmit this persistent resource allocation information for the synchronous HARQ.
• UL-transmitter can also use this bit if does not receive the resource allocation information for a- synchronous HARQ.
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Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the chained invention. Accordingly, it is to be understood that the drawings and descriptions herein are offered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.