Method and Apparatus for CRC Rate Matching in Communications Systems

TECHNICAL FIELD

The present invention relates generally to a system and method for variable length CRC rate matching in communications systems, and more particularly in a radio frequency wireless communications system.

BACKGROUND

As wireless communication systems such as cellular, satellite, and microwave communication systems become widely deployed and continue to attract a growing number of users, there is a continuing need to improve efficiency of such systems. Presently, work is progressing on advancing the standards for wireless communications systems to support present and future enhanced services, such as providing capacity to replace wired telephony systems, support for video, audio or data (for example software updates) broadcasts simultaneously to many users, support for data file communications, and support for internet services over an air interface, as well as support for existing voice, email, text, photographs, and SMS messaging services. The group of standards presently in development comprises the International Mobile Telecommunications ("IMT") Advanced ("IMT-A") project and the 3G long term evolution project, for example.

The third generation partnership project long term evolution ("3GPP LTE") is the name generally used to describe an ongoing effort across the industry to improve the universal mobile telecommunications system ("UMTS") for mobile communications. The improvements are being made to cope with continuing new requirements and the growing base of users. Goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards and backwards compatibility with some existing infrastructure that is compliant with earlier standards. The project envisions a packet switched communications environment with support for such services as Voice over Internet Protocol ("VoIP") and Multimedia Broadcast/Multicast Services ("MBMS"). MBMS may support services where base stations transmit to multiple user equipment simultaneously such as mobile television or radio broadcasts, for example. The 3GPP LTE project is not itself a standard-generating effort, but will result in new recommendations for standards for the UMTS.

The wireless communication systems as described herein are applicable to, for instance, IMT-A and 3GPP LTE compatible wireless communication systems and of interest is an aspect of LTE referred to as "evolved UMTS Terrestrial Radio Access Network," or e-UTRAN. In general, e-UTRAN resources are assigned more or less temporarily by the network to one or more UEs by use of allocation tables, or more generally by use of a downlink resource assignment channel or physical downlink control channel (PDCCH). LTE is a packet-based system and, therefore, there may not be a dedicated connection reserved for communication between a UE and the network. Users are generally scheduled on a shared channel every transmission time interval (TTI) by a Node B or an evolved Node B (e-Node B). A Node B or an e-Node B controls the communications between user equipment terminals in a cell served by the Node B or e-Node B. In general, one Node B or e-Node B serves each cell. A Node B or an e-Node B may be referred to as a "base station." Resources needed for data transfer are assigned either as one time assignments or in a persistent/semi-static way. The LTE, also referred to as 3.9G, generally supports a large number of users per cell with quasi-instantaneous access to radio resources in the active state. It is a design requirement that at least 200 users per cell should be supported in the active state for spectrum allocations up to 5 megahertz (MHz), and at least 400 users for a higher spectrum allocation.

The UTRAN comprises multiple Radio Network Subsystems (RNSs), each of which contains at least one Radio Network Controller (RNC). However, it should be noted that the RNC may not be present in the actual implemented systems incorporating E-UTRAN. LTE may include a centralized or decentralized entity for control information. In UTRAN operation, each RNC may be connected to multiple Node Bs which are the UMTS counterparts to Global System for Mobile Communications (GSM) base stations. In E-UTRAN systems, the e-Node B may be, or is, connected directly to the access gateway ("aGW," sometimes referred to as the services gateway "sGW"). Each Node B may be in radio contact with multiple UEs (generally, user equipment including mobile transceivers or cellphones, although other devices such as fixed cellular phones, mobile web browsers, laptops, PDAs, MP3 players, gaming devices with transceivers may also be UEs) via the radio over the air or Uu interface.

In order to facilitate scheduling on the shared channel, the e-Node B transmits a resource allocation to a particular UE in a downlink-shared control channel (PDCCH) to the UE. The allocation information may be related to both uplink and downlink channels. The allocation information may include information about which resource blocks in the frequency domain are allocated to the scheduled user(s), the modulation and coding schemes to use, what the size of the transport block is, and the like.

The lowest level of communication in the E-UTRAN system, Level 1 , is implemented by the Physical Layer ("PHY") in the UE and in the e-Node B. The PHY performs the physical transport of the packets between them over the air interface using radio frequency signals. In order to ensure a transmitted packet was received, an automatic retransmit request ("ARQ") and a hybrid automatic retransmit request ("HARQ") approach is provided. Thus, whenever the UE receives packets through one of several downlink channels, including command channels and shared channels, the UE performs a communications error check on the received packets, typically a Cyclic Redundancy Check (CRC), and in a later sub frame following the reception of the packets, transmits a response on the uplink to the e-Node B or base station. The response is either an Acknowledged (ACK) or a Not Acknowledged (NACK) message. If the response is a NACK, the e-Node B may automatically retransmit the packets in a later sub frame on the downlink or DL. In other cases, the e-Node B may choose to drop this current packet and schedule a new packet for transmission. In the same manner, any UL transmission from the UE to the e-Node B is responded to, at a specific sub frame later in time, by a NACK/ ACK message on the DL channel to complete the HARQ. In this manner, the packet communications system remains robust with a low latency and short turnaround time.

In the known and presently planned wireless communication systems, data frames or packets are formed and various error correction and detection schemes are used. Error detection involves providing additional information with a data payload that enables the receiver to determine, without additional transmissions from the transmitter, whether a valid data packet has been received and decoded. In addition, forward error correction (FEC) is often applied to such packet based communications systems. In FEC, an operation is performed on the data packet that forms a longer message including redundancy information. This operation could be performed by using, as non limiting examples, convolutional codes, turbo codes, LPDC or similar codes. By transmitting the extra information which is then decoded at the receiver, erroneous bits that are received may be corrected by the receiver and thus the message conveyed by the data packet may be correctly received even over a noisy communications channel or in the presence of interference from other transmitters, reflections or physical interference and the like.

The standards presently in use for such communications systems and particularly for air interface radio frequency signaling such as GSM, 3G, 3GPP LTE, UTRAN, e-UTRAN and the like often specify a fixed length field for error detection using a CRC approach. CRC is an error detection technique that applies a polynomial to the packet being transmitted and determines a number of bits that are also transmitted. The receiver can easily check, usually by a division or

checksum operation, whether the packet data was received correctly (including the CRC bits) by a calculation at the receiver end.

The effectiveness of CRC error detection is greatly determined by the number of bits used. The present standards specify a fixed number of bits for CRC. This approach limits the effectiveness of the CRC error detection and thereby increases the possibility that erroneous messages will be determined as valid. Further, the fixed length CRC field prevents adaptive improvements in error detection for critical messages, or to overcome errors in particularly noisy or weak signals.

Wireless communications systems have an ongoing need for increased efficiency. Errors in transmission and reception directly impact efficiency of such systems. An ongoing need exists for methods and apparatuses to provide a system that will efficiently provide improved error detection to prevent erroneous operations and improve the reliability and throughput of these systems.

SUMMARY OF THE EMBODIMENTS

The following summary description briefly describes exemplary embodiments, but does not limit the invention or the appended claims to these example embodiments. Additional embodiments are provided in the detailed description. In a first exemplary embodiment, a method is provided comprising: receiving transmitted code words corresponding to packets of length P ₁ as radio frequency signals with a code word of length n bits at a code rate r; determining the maximum positive integer 'a' so that the sum of P ₁ and 'a' is less than or equal to the product of r and n; de-rate matching and decoding the received n bits code word to obtain a packet P ₁ + a; calculating extended cyclic redundancy check (CRC) bits for the packet; comparing the calculated CRC check bits to the received extended CRC bits in packet P ₁ + a; if the compare is true, indicating the packet is valid.

In yet another exemplary embodiment, the above described method is provided wherein determining the maximum positive integer 'a' comprises determining 'a' that satisfies the inequality (P ₁ + a )/r < n. In yet another exemplary embodiment, the above described method is provided wherein receiving transmitted code words further comprises receiving packets on a dedicated downlink channel.

In yet another exemplary embodiment, the above described method is provided wherein receiving transmitted code words further comprises receiving radio frequency signals transmitted over an air interface. In yet another exemplary embodiment, the radio frequency signals may be spread spectrum signals, or OFDM signals.

In yet another exemplary embodiment, the above described method is provided, wherein a must be less than or equal to a predetermined maximum value a _max.

In yet another exemplary embodiment, the above described method is provided wherein a must be greater than a minimum a _min ,

In a further exemplary embodiment, the above described method is provided wherein receiving transmitted code words further comprises receiving transmitted code words with a variable CRC length.

In another exemplary embodiment, a method is provided comprising: forming a packet of length P ₁ having a fixed length data field and a default length cyclic redundancy check (CRC) field for transmission, using a code word of n bits at a code rate r; selecting the largest positive

integer 'a' so that the sum of P ₁ and 'a' is less than or equal to the product of r and n; extending the default length CRC field by 'a' bits to form an extended packet ; calculating the check bits for the extended packet; forward error correction (FEC) encoding the extended packet with code rate r; and rate matching the encoded code word to obtain a code word of length n. In yet another exemplary embodiment, the above described method is provided further comprising transmitting the code word of length n over an air interface using radio frequency signals. In yet another exemplary embodiment, the radio frequency signals may be selected from spread spectrum signals, or OFDM signals.

In yet another exemplary embodiment, the above described method is provided wherein determining the maximum positive integer 'a' comprises determining 'a' that satisfies the inequality (pi + a )/r < n.

In yet another exemplary embodiment, the above described method is provided wherein transmitting the code word further comprises transmitting on a dedicated downlink channel.

In yet another exemplary embodiment, the above described method is provided wherein transmitting the code word further comprises transmitting from a base station in a wireless communications system.

In another exemplary embodiment, a method is provided comprising: receiving a data packet having a fixed length data field and a default length cyclic redundancy check (CRC) field for transmission at a code rate r; selecting a packet size P ₁ from a set of predetermined permitted packet sizes, the packet P ₁ including the data packet and the default cyclic redundancy check (CRC) field; selecting a code word length Ti ₁ from a set of predetermined permitted code word lengths; selecting the largest positive integer 'a' so that the sum of P ₁ and 'a' is less than or equal to the product of r and n; forming an extended packet by extending the default CRC field by 'a' bits; calculating the CRC check bits for the extended packet; forward error correction (FEC) encoding the packet with code rate r; and rate matching the FEC encoded packet to obtain a code word of length n^

In yet another exemplary embodiment, the above described method is provided wherein determining the maximum positive integer 'a' comprises determining 'a' that satisfies the inequality (P ₁ + a )/r < n. In yet another exemplary embodiment, the above described method is provided, and further comprising: transmitting the code word using radio frequency signals over an air

interface. In yet another exemplary embodiment the radio frequency signals may be selected from spread spectrum signals, or OFDM signals, or other similar signals.

In yet another exemplary embodiment, the above described method is provided wherein transmitting the code word further comprises transmitting on a downlink shared control channel. In a further exemplary embodiment, the above described method is provided wherein transmitting further comprises transmitting from a base station in a wireless communications system.

In another exemplary embodiment, a method is provided comprising: receiving a code word having an undetermined packet size P ₁ + 'a' comprising a fixed length data field and a default length CRC field of length P ₁ and an extended CRC field of length 'a' unknown to the receiver, and having an undetermined code word length Ti ₁ at a rate r; for each member of a set of predetermined values of packet size P ₁ and for each member of a set of predetermined values of code word length n _l5 performing the method of: selecting the largest positive integer a so that the sum of P ₁ and 'a' is less than or equal to the product of r and n; de- rate matching and decoding the received Ti ₁ bits to obtain an extended packet of bits P ₁ + a; calculating the extended cyclic redundancy check (CRC) bits for the extended packet P ₁ ; comparing the calculated CRC bits with the extended packet received CRC bits; and if the comparison is true, accepting the packet as a valid packet.

In another exemplary embodiment, the above described method is provided wherein receiving the code word further comprises receiving a code word on a downlink dedicated control channel.

In another exemplary embodiment the above described method is provided wherein receiving the code word further comprises receiving a radio frequency signal over an air interface. In another exemplary embodiment the above described method is provided wherein the radio frequency signals may be selected from spread spectrum signals, or OFDM signals, or other similar signals.

In another exemplary embodiment, the above described method is provided wherein receiving the code word further comprises receiving a code word in a wireless communications device. In another exemplary embodiment, the above described method is provided wherein receiving the code word in a wireless communications device further comprises receiving a code word in a cellular telephone.

In another exemplary embodiment, an apparatus is provided comprising: a transmitter adapted to transmit a data packet of length P ₁ having a fixed length data field and a default length cyclic redundancy check (CRC) field using code words of length n at a code rate r; a processor adapted to select a positive integer 'a' so that the sum of P ₁ and 'a' is less than or equal to the product of r and n; the processor adapted to extend the default CRC field by 'a' bits and calculate the CRC check bits for the extended packet; the processor adapted to forward error correction (FEC) encode the extended packet including the extended CRC field with code rate r; the processor further adapted to perform rate matching to obtain a code word of length n.

In a further exemplary embodiment, the above described apparatus is provided further comprising a transmitter adapted to transmit the code word over an air interface using radio frequency signals.

In yet another exemplary embodiment, the above described apparatus is provided wherein the processor and transmitter comprise a portion of a base station in a wireless communications system. In yet another exemplary embodiment, the above described apparatus is provided wherein the transmitter is further adapted to transmit the code word on a dedicated downlink channel.

In another exemplary embodiment an apparatus is provided, comprising: a receiver adapted to receive code words of length n at a code rate r, the code words including a packet of size P ₁ comprising a fixed length data field and a default length CRC field and an extended CRC field of an unknown length 'a'; a processor adapted to determine the largest positive integer a so that the sum of P ₁ and 'a' is less than or equal to the product of r and n ; the processor further adapted to de-rate match and decode the received n bits of the code word to obtain the extended packet of size P ₁ + a; the processor further adapted to calculate the cyclic redundancy check (CRC) for the extended packet; the processor further adapted to compare the calculated CRC to the received CRC for the extended packet; and if the comparison is true, to accept the packet as valid.

In yet another exemplary embodiment, the above described apparatus is provided wherein the receiver further comprises a receiver adapted to receive signals over a radio frequency air interface. In yet another exemplary embodiment, the above described apparatus is provided wherein the processor forms a portion of a wireless communications device.

In yet another exemplary embodiment, the above described apparatus is provided wherein the wireless communications device further comprises a cellphone.

In still another exemplary embodiment, the above described apparatus is provided and further comprises a transmitter adapted to transmit radio frequency signals over the air interface. In another exemplary embodiment, a computer readable medium is provided containing instructions that, when executed by a programmable transmitter, perform: forming a packet of length P ₁ having a fixed length data field and a default length cyclic redundancy check (CRC) field for transmission using a code word of n bits at a code rate r; selecting the largest positive integer 'a' so that the sum of P ₁ and 'a' is less than the product of r and n; forming an extended packet by extending the default length CRC field by 'a' bits to form an extended CRC field; calculating the CRC check bits for the extended packet; forward error correction (FEC) encoding the extended packet with code rate r; and rate matching the FEC encoded code word to obtain a code word of length n.

In yet another exemplary embodiment, the above provided computer readable medium is provided further comprising additional instructions that, when executed by a programmable transmitter, perform transmitting the code word using radio frequency signals over an air interface.

In another exemplary embodiment, a method is provided, comprising: determining that a packet of length P ₁ having a fixed length data field and a default length cyclic redundancy check (CRC) field is to be transmitted over a channel using code words of length n at a code rate r, the channel having a bit error rate above a predetermined threshold; selecting the largest positive integer 'a' so that the sum of P ₁ and 'a' is less than or equal to the product of r and n; extending the default length CRC field by 'a' bits to form an extended packet; calculating the CRC check bits for the extended packet; forward error correction (FEC) encoding the extended packet with code rate r; and rate matching the encoded code word to obtain a code word of length n.

In yet another exemplary embodiment, the above described method is provided and further comprising transmitting the code word of length n over an air interface using radio frequency signals.

In still another exemplary embodiment, the above described method is provided wherein transmitting the code word further comprises transmitting on a dedicated downlink channel.

In another exemplary embodiment, a method is provided comprising, determining that a packet of length P ₁ having a fixed length data field and a default length cyclic redundancy check (CRC)

field is to be transmitted over a critical channel using code words of length n at a code rate r; selecting the largest positive integer 'a' so that the sum of P ₁ and 'a' is less than or equal to the product of r and n; extending the default length CRC field by 'a bits to form an extended packet; calculating the check bits for the extended packet; forward error correction (FEC) encoding the

extended packet with code rate r; rate matching the FEC encoded code word to obtain a code word of length n; and transmitting the code word over the channel using radio frequency signals on an air interface.

In another exemplary embodiment, the above described method is provided wherein the critical channel is a shared control channel. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates in a simplified system level diagram a radio frequency interface communication system including a wireless communication system;

Figure 2 illustrates in a system level diagram a communication system including a wireless communication system;

Figure 3 is a diagram illustrating time domain communications in a system with base stations and mobile stations;

Figure 4 is a diagram illustrating time domain communications in a system with base stations and mobile stations; Figure 5 is a diagram illustrating time domain communications in a system with base stations and mobile stations;

Figure 6 is a diagram illustrating a data packet and CRC field of the prior art; Figure 7 is a diagram illustrating a data packet embodiment of the present invention;

Figure 8 is a simplified block diagram of an embodiment of a wireless communication element incorporating features of the invention; and

Figure 9 is a simplified flow diagram depicting a method embodiment of the invention.

DETAILED DESCRIPTION

Example embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides inventive concepts that can be embodied in a wide variety of contexts. Even though the majority of embodiments describe the invention as applied to wireless communications systems, the embodiments may also be advantageously applied to other communications systems where error detection is used to ensure reliable communications between communications terminals.

Prior to presenting embodiments of the invention, some additional background information will be provided to increase the readers' understanding of the characteristics and problems of some systems.

Referring initially to Figure 1 , illustrated is a system level diagram of a radio frequency interface communication system including a wireless communication system that provides an environment for the application of the principles of the present invention. The wireless communication system may be configured to provide features included in the UMTS terrestrial radio access network ("UTRAN") or the evolved UMTS terrestrial radio access network ("e- UTRAN") services. Mobile management entities ("MMEs") and user plane entities ("UPEs") designated by reference 1 provide control functionality for one or more e-UTRAN node B (designated "eNB," an "evolved node B," also commonly referred to as a "base station") 3 via an Sl interface or communication link. The base stations communicate via an X2 interface or communication link. The various communication links are typically fiber, microwave, or other high-frequency metallic communication paths such as coaxial links, or combinations thereof. Other related systems will have similar topologies.

The base stations 3 further communicate over an air interface with user equipments 5 (designated "UE"), typically a mobile transceiver carried by a user. Alternatively, the user equipments 5 may be a mobile web browser, text messaging appliance, a laptop with a mobile PC modem, or other user device configured for cellular or mobile services. Thus, communication links (designated "Uu" communication links) coupling the base stations to the user equipment are air links employing a wireless communication signal. For example, the devices may communicate using a known signaling approach such as a 1.8 GHz orthogonal frequency division multiplex ("OFDM") signal. Other radio frequency signals may be used.

Figure 2 illustrates in a system level diagram a communication system including a wireless communication system that provides an environment for the application of the principles of the present invention. The wireless communication system provides an e-UTRAN architecture including base stations 3 or eNBs providing e-UTRAN user plane (packet data convergence protocol/radio link control/media access control/physical transport) and control plane (radio resource control) protocol terminations directed towards user equipment 5 (designated as "UE"). The base stations 3 are interconnected with an X2 interface or communication link. The base stations 3 are also connected by an S 1 interface or communication link to an evolved packet core ("EPC") including, for instance, a mobility management entity 1 ("MME") and a user plane entity ("UPE"), which may form an access gateway ("aGW," a system architecture evolution gateway). The Sl interface supports a multiple entity relationship between the mobility management entities/user plane entities and the base stations and supports a functional split between the mobility management entities and the user plane entities. The base stations 3 in Figures 1 or 2 may host functions such as radio resource management (e.g., internet protocol ("IP"), header compression and encryption of user data streams, ciphering of user data streams, radio bearer control, radio admission control, connection mobility control, dynamic allocation of resources to user equipment in both the uplink and the downlink), selection of a mobility management entity at the user equipment attachment, routing of user plane data towards the user plane entity, scheduling and transmission of paging messages (originated from the mobility management entity), scheduling and transmission of broadcast information (originated from the mobility management entity or operations and maintenance), and measurement and reporting configuration for mobility and scheduling. The mobility management entity/user plane entity 1 may host functions such as distribution of paging messages to the base stations, security control, terminating U-plane packets for paging reasons, switching of U-plane for support of the user equipment mobility, idle state mobility control, and system architecture evolution bearer control. The user equipment receives an allocation of a group of information blocks from the base stations.

Some standards comprise frequency division duplex (FDD) spectrum allocated in paired channels with uplink (from a mobile user equipment to a base station) and downlink (from the base to the mobile station) channels. Another approach is based on time division duplex (TDD) channels where uplink and downlink communications are separated into time slots. In a more general manner, radio frequency resources can be divided in both time and frequency, combining aspects of both TDD and FDD.

In many TDD wireless communications systems, synchronization is used in the same frequency band. In a synchronized TDD network it is possible to reuse the same radio resource (frequency spectrum) with manageable interference. Figure 3 illustrates one typical scenario.

In Figure 3, a basestation 31 is communicating with mobile 33. Basestation 37 is communicating with mobile 35 using the same radio resources, e.g. at the same frequency. The timeline depicts time slots in a TDD protocol used for these communications. As shown in Figure 3, timeslot 1 is a downlink slot, and timeslot 2 is an uplink timeslot. In timeslot 1, the base stations 31 and 37 are both transmitting. Because of the spatial separation between the two areas where the signals are strong enough to be received, there is no interference and both basestations may transmit at the same frequency in timeslot 1.

Figure 3 depicts a scenario where base stations are transmitting information to mobile devices or receivers. This communication is typically referred to as a "downlink" communication. Communications from the receivers to the Node Bs or base stations are "uplink" communications. In order to support full duplex traffic, in some standards uplink messages and downlink messages are modulated differently so that both can be transmitted simultaneously.

In Figure 4, a similar situation is shown as in Figure 3 for the same system at timeslot 2, which is an uplink timeslot. Mobile 33 is transmitting to basestation 31 in the uplink direction. At the same time, mobile 35 is transmitting to basestation 37 in the uplink direction. These transmissions can use the same frequency due to the spatial separation (illustrated by the dotted lines.)

In present standards, downlink and uplink communications are usually divided into channels. An important downlink channel is the shared control channel or PDSCCH in 3GPP terminology; in other standards a similar abbreviation may be used. Receivers monitor the shared command channel for system information. Another channel of importance is the dedicated control channel or PDCCH. The packets on the dedicated command channel include some form of device ID. In this manner, a receiver determines when and if it is to respond to a message. Because the devices are addressed over this dedicated channel, errors made in decoding these messages are particularly problematic. If a receiver incorrectly decides a message allocating an uplink resource has been received with its device identifier, it will start transmitting. If at the same time another receiver correctly determines it has been allocated an uplink resource for that frame or timeslot, the two devices may interfere with one another. Even

an uplink acknowledge message, if received from the wrong receiver (one that incorrectly decoded a device ID) can cause interference and result in erroneous operations.

Further the shared control channel includes important parametric information that tell the receivers what modulation, power levels, time intervals and the like the base station expects the receivers (or transceivers, typically) to use. If the receiver cannot correctly decode this information, it is at least very important that the receiver be aware of the error; that the error be detected. If the receiver somehow decides it has correct information and the receiver acts on erroneously received information, the receiver will not be able to communicate with the base station. Figure 5 illustrates another typical situation. The conditions a receiver or UE is experiencing at a given time are very dependent on distance from the base, the presence of other active transmitters, the number of UEs in a cell, how many UEs are active, and other changing environmental conditions. In Figure 5, mobile 35 is not able to receive, or not able to receive very well, the downlink communication from the base station 37. The interference is from an unsynchronized UE 33, which is communicating uplink information to base station 31. In this "near - far" scenario, the UE 33 is closer to the UE 35 than the base station 37 and so the receiver is in difficult conditions for reception.

Figure 6 depicts a typical prior art data packet and default CRC frame used in such communications systems. The data packet is a fixed length, P ₁ . The CRC frame is a fixed number of bits. A selected CRC polynomial is applied to the data bits of the packet to generate the CRC bits prior to transmission. Typical fixed lengths for CRC frames might be 2, 8, 16, or 24 bits as non limiting examples.

In addition, after the data bits with the CRC forms the packet, FEC is applied to the packet. In most wireless communications systems, FEC is a 1/2, 1/3, or other convolutional encoding scheme, including without limitation turbo codes, LPDC, and other forms of FEC coding. The denominator indicates how efficient the scheme is; a 1/2 convolution will double the length of the resulting packet. FEC therefore adds bits, which of course adds redundancy, sometimes making error correction to repair erroneously received bits possible in the receiver.

The use of FEC c also results in the need for rate matching in most known systems. Rate matching is necessary because the length of the FEC processed packets may not fit the frame length designed for the system. Two forms of bit manipulation may be used to rate match. In puncturing, selected bits known to the transmitter and receivers are removed from the frames to be transmitted. If these bits are selected carefully, the receiver can either insert zero bits, or

replicate bits, to replace the punctured bits at the receiver end, and the FEC algorithm will still yield correct results - so long as the bit error rate (BER) for the receiver is not too high.

In another manipulation, the FEC packet can be extended to fill a frame length before transmission; in this manner, bits may be inserted or replicated in a way that fills out a frame to the proper word length, the receiver can then combine the received duplicate bits and recover the correct information.

The use of FEC is important, but if the bit error rate is high, FEC will still not allow the message to be correctly received. So in poor signaling conditions, an important consideration is whether a corrupted message will be incorrectly determined to be valid. This function is determined by the CRC. The bit error rate as a function of code bits is fairly flat, which means that adding FEC bits will not greatly improve the bit error rate.

In contrast, the probability of a false positive is greatly dependent on the number of CRC bits, it can be approximated as:

2 ⁿ , where n is the number of CRC bits; and the bit error rate approaches 0.5, a low SNR case.

The false positive probability is the probability by which a received packet is erroneously accepted as being correct. Thus, the use of additional bits for CRC will greatly decrease the false positive probability; adding just one CRC bit will improve the false positive probability by a factor of 2. This is in contrast to the slight improvement achieved by extending error correction bits.

A very important downlink message that must be received correctly for proper system operation is the dedicated downlink control channel. These channels are directed at particular UEs and provide downlink and uplink resource allocation information. If a UE incorrectly receives one of these channels, it may respond incorrectly and create serious problems and erroneous system behavior. These channels are also "blindly" decoded. The receivers have to locate these channels through a procedure of identifying their particular device ID in a message payload. It is very important then that a UE does not incorrectly decode a device ID (fail to detect an error, creating a false positive detection) and thereby mistakenly determine that a dedicated downlink channel message is directed to it, when in fact it is directed to a different UE. Thus, adding CRC bits to a fixed length packet for these conditions can greatly improve the probability that an error will be detected in an erroneous received packet.

An illustrative example is presented to emphasize the importance of this phenomenon. This example is taken from a 3GPP Technical Support Group RAN document WG1#47, submitted November 6-10,2006. The document proposes increasing a fixed CRC field to improve error detection on a shared control channel, but the illustration shows how likely a false positive can be.

To illustrate the problem of the false positive indication, let's assume that we have a very weak signal on the shared control channel (causing the bit error probability of each received bit to be 0.5). This will happen when applying power control or power balancing to the shared control channel, and addressing a user close to the Node B, while having more distant users also listening for the same allocation information. Following this assumption ofp _e =0.5, the probability of accepting a resource scheduling information that is not intended for a given UE can be written as:

P FP, I = 2 e, 16).

2 n , wh 1, k ere k is the number of CRC bits (in this example, cas

Now, as the UE will be listening for a number of different resource assignments (m), and that we will have n UEs listening for the resource allocations, the probability that a UE will erroneously accept a resource allocation as correct will become:

p _FP and n could have a value in the order of 100 (meaning that we have 100 users actively listening for their resource allocations).

With the indicated values, we have a false positive probability of 1.5%, meaning that in

15 out of 1000 cases, we will have a resource allocation erroneously accepted as designated for this given UE.

So it is clear from this example that for appropriate conditions, increasing CRC bits will definitely improve system operation and reduce erroneous behavior. Figure 7 illustrates in an example embodiment of the present invention an extended data packet 75 to be used in embodiments where the length of a CRC field may be varied for a fixed length data packet.

In Figure 7, the packet including the fixed length data field 71 plus the original default length CRC field 72 and the extended CRC field 73 will be subject to FEC and rate matching.

So the rate matching will still form extended packets that are then rate matched to be of proper length for the frame. By extending the CRC length by extra bits 73, the system is emphasizing (from a resource point of view) increased probability of error detection over the likelihood that forward error correction will occur. Since each additional CRC bit has a major impact on proper error detection, for appropriate cases such as dedicated and shared control channels carrying critical information; or for cases where reception is poor, where interference is occurring, or where a receiver frequently requests retransmission, or other cases not listed here, the use of additional CRC bits can be of significant benefit in reducing errors.

In one exemplary embodiment, for a simple downlink communication case, an algorithm for the transmitter to determine the value 'a' is:

1. Determine the largest integer 'a' that satisfies:

( _Pl +a)/r< n;

2. Extend the original CRC field length by 'a' bits and calculate the check bits for the extended packet; 3. FEC encode the extended packet with a code rate r;

4. Rate match to get a code word or words of length n;

The algorithm for the transmitter side assumes that there are certain time-frequency resources corresponding to a code word of n bits at code rate r, and that the packet length can have a known length P ₁ that includes the fixed length data field and the original default CRC length.

Step 1 above may be restated simply as finding, for a packet of length P ₁ that includes a fixed length data field and a default length CRC field, the largest integer 'a' so that the sum of P ₁ and 'a' is less than, or equal to, the product of the code word length n and the code rate r.

In another exemplary embodiment algorithm for the transmitter, the above steps are modified by limiting 'a' to < some a _max . Then the first step above becomes:

1. Select the largest integer a < a _max so that it satisfies (P ₁ +a)/r< n.

Again this step may be restated as finding, for a packet of length P ₁ that includes a fixed length data field and a default length CRC field, the largest integer 'a' less than a specified maximum length a _max so that the sum of P ₁ and 'a' is less than, or equal to, the product of the code word length n and the code rate r.

In a corresponding exemplary method or algorithm to be performed at the receiver, the receiver performs these steps:

1. For a received code word corresponding to a packet of length P ₁ that includes a fixed data length and a default CRC field length extended by a CRC field of length 'a', where 'a' is unknown to the receiver,, determine the largest integer a < a _max that satisfies:

2. De-rate match and decode the received n bits to get extended packet bits P ₁ +a.;

3. Calculate the extended CRC check bits for the extended packet and compare with the received CRC for the extended packet;

4. If the CRC matches, accept the packet as valid.

For an exemplary embodiment that addresses a control channel, the embodiment must also account for the fact that for control channels, not all of the transmit parameters are known to the receiver, so that the actual code word must be blindly detected. Assuming the mother code rate r is known, but that the packet size P ₁ and the code word length Ti ₁ are selected from some finite set of values, the following algorithm may be used for the transmitter:

1. For a packet size P ₁ that includes a fixed length data field and a default length CRC field, and a code word length n _l5

Determine the largest integer 'a' that satisfies: (P ₁ +a)/r< n _t

2. Extend the original CRC field by 'a' bits and calculate the check bits for the extended packet

3. FEC encode the extended packet with a code rate r

4. Rate match the FEC encoded word to get a code word or words of length H ₁ Step 1 in the examples above may be simply restated as, for receiving a transmitted packet of size P ₁ that includes a fixed length data field and a default length CRC field that is extended by 'a' bits where 'a' is unknown to the receiver, determine the largest integer 'a' so that the sum of the original packet size P ₁ and 'a' is less than, or equal to, the product of the code word length n or H ₁ and the code rate r.

Importantly for the command channel case, the receiver does not know the values P ₁ and Xi ₁ . So a "blind decoding" may be performed. In an exemplary method, the receiver performs the steps of:

For all P ₁ and n _l5 do; these may be a small set of permitted values 1. For a received code word that corresponds to an extended packet of length

P ₁ + a comprising a fixed length data field and a default length CRC field and an extended CRC field of length a, where 'a' is unknown to the receiver, select the largest integer a < a _ma χ that satisfies: 2. De-rate match and decode the received n bits to obtain the extended packet bits P ₁ + a;

3. Calculate the extended CRC check bits for the extended packet and compare with the received CRC for the extended packet ;

4. If the CRC check matches, accept the packet. End for.

Step 1 in the blind decoding example may be restated as: for all permitted values of a packet size P ₁ and a code word length n _l5 where a received code word of length P ₁ + a corresponds to an extended packet that includes a fixed length data field and a default length CRC field extended by an extended CRC field of 'a' bits, find the largest integer 'a' so that the sum of a and P ₁ is less than or equal to the product of the code word length Ti ₁ and the code rate r.

So the receiver will loop through a set of P ₁ and H ₁ values looking for any combination with extended CRC bits 'a' that satisfies the inequality and then de-rate match, decode, and calculate the CRC check to see if the word is valid, if so, it is properly received.

As is known to those skilled in the art, a variety of possible CRC methods are available for calculating the extended CRC. In a particular system with a default CRC size, the extended CRC bits might be obtained by replicating check bits. In another exemplary system that requires increased CRC error protection for all bit error probabilities, a separate CRC polynomial might be used for each extended CRC length.

In an embodiment of a base station transmitter for a wireless communication system, the transmitter can dynamically allocate extra error protection by allocating rate matching resources

more to detection (by increasing CRC length) than correction (FEC length) and thereby insure extra error detection probability.

In another embodiment, a transmitter preparing to transmit code words on a shared control channel may allocate additional CRC bits before rate matching to ensure proper error detection on these critical communications.

Figure 8 depicts in block diagram form a possible implementation of a wireless communication element 7 that incorporates the CRC rate matching of the present invention. Wireless communication element 7 includes, without limitation, an RF transceiver 4 for transmitting and receiving radio frequency signals via an antenna. These signals include downlink control channels and dedicated downlink channels, and uplink channels. Processor 2 may be any processing device including without limitation a microcontroller, microprocessor, DSP, RISC or CISC computer, state machine, sequential machine and the like. Memory 6 may include any storage memory such as, without limitation, EPROM, ROM, RAM, DRAM, SRAM, EEPROM, flash, volatile or nonvolatile memory. A controller 10 labeled CRC rate match controller may be, in one embodiment, dedicated hardware to implement the algorithms for the receiver and transmitter described above. In an alternative embodiment, the CRC controller may be program instructions to be executed by the transceiver 4 and the processor 2 to perform the algorithms above.

As shown, wireless communication element is coupled to a network control element 9 and a telecommunication network. This would be the case if the wireless communication element were, for example, a base station. Wireless communication element 7 may also be a mobile device, such as a UE, in which case the connection to the network control element would not be present.

Wireless communication element 7 may be provided as a single integrated circuit, a plurality of integrated circuits, and one or more circuit boards. Memory 6 may be embedded with other components in an integrated circuit, or provided as off the shelf DRAM, flash, EPROM or other volatile or non- volatile memory. Similarly, processor 2 may be an off the shelf device such as a programmable DSP.

Embodiments of the present invention comprise methods and apparatus that provide solutions to the problem of improved error detection by using rate matching to increase detection resources in relation to correction resources and extending CRC bits in packets to be communicated in a packet based communications system. The embodiments of the invention are

particularly advantageous when applied to a shared or dedicated control channel, or to a channel that is particularly noisy or has a high bit error ratio.

Figure 9 depicts in a simple state diagram a method embodiment for a transmitter incorporating the CRC rate matching. In Figure 9, a data packet and the default CRC bits are provided as a packet for transmission in state 50. In state 52, the channel is evaluated. If the channel condition is poor or the message is particularly important, the channel will be designated a "critical channel". If the channel is a critical channel, the CRC rate matching approach will be used. A critical channel may include, without limitation, shared control channels, dedicated control channels, or any other downlink channel that is known to have a high bit error rate, a low signal to noise ratio SNR, or be directed to a mobile or UE receiver that makes repeated retransmission requests (HARQ in 3GPP terminology) or otherwise has a high rate of errors. Other channel conditions could be defined that are determined to be indicative of a critical channel.

If, in state 52, the channel for transmission is not a critical channel, the transmission follows the prior art approach of using the data packet and default CRC bits, performing forward error correction convolution at a rate r in state 57, performing rate matching on the FEC output at state 56, and transmitting the code word at state 58. If instead the channel is determined to be a critical channel in state 52, the transmitter then performs the algorithms described above for the CRC rate matching approach by extending the CRC field in the packet. In state 53, the value for the integer 'a' is selected. In state 54, the extended packet CRC bits are calculated and appended to the packet P ₁ . In state 57, FEC encoding is performed on the extended packet. In state 56, rate matching is again performed to obtain a code word of length n. Finally, in state 58, the word is transmitted.

In the above described manner a transmitter, such as a base station, can adaptively decide to increase the CRC bits used in a transmitted message to improve the reliable reception of a transmitted message over a critical channel.

In an embodiment for a programmable transceiver, a computer readable medium may be provided that contains stored instructions that, when executed perform the CRC rate matching algorithms for the transmitter, and/or the receiver, as described above. Alternative embodiments include a computer readable medium storing instructions for a programmable transmitter that, when executed, perform the algorithms for the transmitter described above. Alternative embodiments further provide a computer readable medium storing instructions that when executed by a programmable receiver, perform the receiver algorithms described above.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.