DYNAMIC ALLOCATION OF OFDM SYMBOLS FOR PDCCH IN WIRELESS COMMUNICATION SYSTEM

The disclosure is directed to a transmitter, a receiver, and method of operating a wireless communications network. BACKGROUND

Universal Mobile Telecommunications System (UMTS) is a third-generation (3G) cell phone technology. Evolved-UMTS Terrestrial Radio Access (E-UTRA), also known as UMTS Long Term Evolution (3GPP LTE), is a key 3G technology to ensure the competitiveness of UMTS and provide a high-data-rate, low-latency and packet-optimized system. Unlike UMTS, which uses code division multiple access (CDMA) as the air interface, E-UTRA uses orthogonal frequency division multiplexing (OFDM) as the air interface.

In an OFDM cellular network, each cell employs a base station (BS) that communicates with any user equipment (UE), such as cell phones, laptop computers, or personal digital assistant (PDAs), active within its cell. When UE is first activated, it performs an initial cell search to connect to the cellular network. The initial cell search includes a downlink (DL) synchronization process between the BS and the UE in which the BS sends a synchronization signal to the UE.

After DL synchronization, the BS sends control information to the UE that the UE then uses to decode subsequent frames and sub-frames of data received from the BS. The UE must reliably receive the control information for the UE to be able to decode subsequently received data accurately. To guarantee fast and reliable receipt of control information, OFDM systems designate a fixed number of tones for transmitting the control information. Consequently, today's OFDM systems perform reliably and are capable of transmitting data at very high rates. SUMMARY

To address the above, one aspect of the disclosure provides a base station transmitter for use with an OFDM communications system. In one embodiment, the base station transmitter includes a control information formatter configured to represent a number of OFDM symbols used for transmission of a physical downlink control channel (PDCCH) in a

downlink sub-frame as two bits of control information and form a codeword with repetitions of bits representing said control information.

Another aspect of the disclosure provides a method of dynamically allocating OFDM symbols needed for PDCCH in a downlink sub-frame from a base station. In one embodiment, the method includes: (1) representing a number of OFDM symbols needed for PDCCH in a downlink sub-frame as two bits of control information, (2) forming a codeword with repetitions of bits representing said control information, (3) generating groups of control information based on the codeword, (4) spacing the groups across a bandwidth (BW) of the base station transmitter at a substantially equal spacing for transmission, (5) transmitting the substantially equally-spaced groups to user equipment, (6) receiving the substantially equally- spaced groups at the user equipment and (7) allocating, by the user equipment, a number of OFDM symbols in the downlink sub-frame for the PDCCH based on the substantially equally-spaced groups.

Yet another aspect of the disclosure provides a UE receiver for use with an OFDM communications system. In one embodiment the UE receiver includes: (1) a receive unit configured to receive a downlink signal, the signal having groups of control information and (2) a processing unit configured to extract from the groups of control information a number of OFDM symbols needed for PDCCH in a sub-frame of the downlink signal. BRIEF DESCRIPTION OF THE DRAWINGS Example embodiments are described below with reference to accompanying drawings, in which:

FIG. 1 is a diagram of an embodiment of a cellular network constructed in accordance with the principles of the disclosure;

FIG. 2 is a flow diagram of an embodiment of a method of dynamically allocating OFDM symbols for PDCCH carried out according to the principles of the disclosure;

FIG. 3A is a graph representing, in a pedestrian channel A (PA) model, a bit error rate (BER) for a CatO transmission with no transmit diversity versus the number of repetitions for the CatO;

FIG. 3B is a graph representing, in a PA model, a BER for a CatO transmission with transmit diversity versus the number of repetitions for the CatO;

FIG. 4A is a graph representing, in a typical urban 6 (TU6) channel model, a BER for a CatO transmission with no transmit diversity versus the number of repetitions for the CatO;

FIG. 4B is a graph representing, in a TU6 channel model, a BER for a CatO transmission with transmit diversity versus the number of repetitions for the CatO; FIG. 5 is a graph representing the geometry distribution for cases 1 and 3 according to

Physical Layer Aspects for E-UTRA (Release 7), version 7.1.0; and

FIG. 6 is a graph representing the PDCCH block error rate (BLER) with code rate 1/12 for the PA and TU channels with and without transmit diversity. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS Unlike a CDMA system, which can assign channels as needed for transmitting control information, an OFDM system has only a limited number (i.e., 600) of frequencies (or "tones"). Because of this, a fixed number of tones are designated for transmitting control information. The amount of control information, however, can vary between sub-frames transmitted from the BS to the UE. It has been discovered that reserving a fixed number of tones for control information for each DL sub-frame may unnecessarily limit the number of tones that could be used for data transmission. An apparatus, method or system to reliably communicate control information in an E-UTRA environment and enhance overall data transmission that takes advantage of this discovery would be useful.

The disclosure provides a category zero transmission (CatO) in a DL sub-frame of an E-UTRA system to achieve a target bit error rate (BER) for the CatO with a limited impact on the overall dedicated physical control channel (DPCCH) block error rate (BLER). The CatO may be considered as a first transmission of a BS to a UE after synchronization. The CatO, also referred to as a physical control format indicator channel (PCFICH), is used to transmit two bits of information that indicate the number of OFDM symbols needed to transmit a physical downlink control channel (PDCCH) in a DL sub-frame. An E-UTRA DL sub-frame may have 14 OFDM symbols. In some embodiments, the DL sub-frame may have 12 OFDM symbols. The two bits of information may be referred to as a control format indicator (CFI). The PDCCH is sent on the first OFDM symbol of the DL sub-frame. The BER of the CatO is considered not as the error probability for one bit but as the probability that any of the two CatO bits is in error (in this sense, it is a stricter measure than the conventional BER and simply corresponds to the probability that CatO is erroneously received).

For transmission of CatO, the disclosure repeats a representation of the two bits of control information to obtain a satisfactory BER of the CatO without needlessly sacrificing transmission overhead. The disclosure provides a simulation that demonstrates the overhead requirements for a CatO transmission to achieve BER of about 1% at the lowest points of the geometry distribution. It is shown that with transmission capturing the frequency diversity of the operating BW of a BS, CatO repetition greater than ten achieves the desired BER targets. The frequency diversity of the BW can be exploited by equally spacing the CatO across the BW for transmission to the UE. In some embodiments the frequency diversity of the BW can be exploited by using a spacing that is approximately equal. For those BWs where the number of tones may not support exact equal spacing, the spacing may be substantially equal between the transmitted control information.

FIG. 1 illustrates a diagram of an embodiment of a cellular network, generally designated 100, constructed in accordance with the principles of the disclosure. The cellular network 100 includes a cellular grid having a single cell employing a base station BS. The cellular network 100 also includes user equipment UE, which is located in the cell. The cellular network 100 may include additional UE and cellular grids with each additional cellular grid having a base station. The cellular network 100 may be an OFDM network (e.g., an OFDM or an OFDMA network) and may operate in compliance with E-UTRA. The base station BS includes a BS transmitter 110 having a control information formatter 112, a code processor 114 and a transmit unit 116. As noted above, the cellular network 100 may be an E-UTRA network. Accordingly, the BS transmitter 110 may be an E-UTRA compliant transmitter. In different embodiments, the BS transmitter 110 may operate at various BWs including 1.25 MHz, 1.4 MHz, 2.5 MHz, 3 MHz, 5 MHz, 10 MHz and 20 MHz. The control information formatter 112 is configured to represent a number of OFDM symbols used for transmission of a PDCCH in a downlink sub-frame from the BS to the UE as two bits of control information. The control information formatter 112 is further configured to form a codeword with repetitions of bits representing said control information. The number of repetitions needed can be based on a desired BER for the two bits of control information with respect to the overall DPCCH BLER. Thus, a target BER can be achieved without adversely impacting the overall DPCCH BLER. As illustrated in FIGS. 3 A, 3B, 4A,

and 4B, about 12 repetitions (e.g., 10 to 12) may be used to achieve an acceptable BER and DPCCH BLER.

The control information formatter 112 may employ block coding to form the codeword from the two bits of control information. The two bits of control information may be coded with a 1/16 block code. In one embodiment, the codeword may be 32 bits and the control information formatter 112 may represent the two bits of control information as three bits and repeat the three bits until obtaining 32 bits to form the codeword. Table 1 represents how the two bits of control information may be represented and repeated to form a 32-bit codeword. The two bits of control information in Table 1 are referred to as CFI (control format indicator) bits.

Table 1 - Example of Forming a Codeword

For example, if one OFDM symbol is needed for transmission of PDCCH in a sub- frame, the number of needed OFDM symbols may be represented by the three digits 0,1,1. The three digits are then repeated until obtaining a total of 32 bits to form a codeword. Repetitions of the three digit representation 0,1,1, provides a sufficient BER for the control information. The process is repeated similarly for two needed OFDM symbols (1,0,1) and three needed OFDM symbols (1,1,0).

The code processor 114 is configured to form groups of control information based on the codeword and substantially equally-space (which may include equally- space) the groups across the BW of the base station transmitter for transmission. For example, using the codeword discussed above with respect to Table 1, the code processor 114 may convert the 32-bit codeword to quadrature phase-shift keying (QPSK) symbols and form the groups from the QPSK symbols. In one embodiment, the code processor 114 may convert the 32-bit codeword to 16 QPSK symbols and form four quadruplets from the 16 QPSK symbols to be substantially equally-spaced across the BW for transmission. In this embodiment, considering the BS transmitter 110 operates at a BW of 10 MHz, the code processor 114 can

space each of the four quadruplets 150 tones apart for transmission. For some BWs in different embodiments, the number of tones may not support exact equal spacing. Accordingly, the code processor may space the groups using substantially equal spacing wherein the number of tones between the groups may not be exactly the same. A scrambling process may be used on the 32-bit codeword before modulation.

Standard QPSK modulation may be used to form the 16 QPSK symbols. With multiple antennas, mapping of the symbols and precoding may be performed. With one transmission antenna, this may be skipped.

The transmit unit 116 is configured to transmit the substantially equally- spaced groups (which may be equally-spaced groups) to the UE. The transmit unit 116 may be an E- UTRA compliant transmit unit. The UE may also be E-UTRA compliant.

The UE includes a user equipment receiver 120 having a receive unit 122 and a processing unit 126. The receive unit 122 is configured to receive a downlink signal from the BS. The downlink signal includes the groups of control information. Each of the groups of control information is substantially equally- spaced (which may be equally- spaced) across a BW of the BS. The processing unit 126 is configured to extract from the groups of control information a number of OFDM symbols needed for PDCCH in a sub-frame of the downlink signal.

After extracting the number of needed symbols, the processing unit 126 is further configured to allocate OFDM symbols for the PDCCH based on the extracted number. In one embodiment, the processing unit 126 may allocate up to 3 OFDM symbols for the PDCCH. By extracting the number of needed OFDM symbols for the PDCCH, the processing unit 126 can dynamically allocate the number of PDCCH for each downlink sub- frame received from the base station. As such, a fixed number of OFDM symbols do not need to be reserved which can result in wasted overhead.

FIG. 2 is a flow diagram of an embodiment of a method 200 of dynamically allocating OFDM symbols for PDCCH carried out according to the principles of the disclosure. At least part of the method may be performed by UE or a BS of an E-UTRA communications network. The method begins with an intent to allocate OFDM symbols for PDCCH in a step 205.

After beginning, a number of OFDM symbols needed for PDCCH in a downlink sub- frame are represented as two bits of control information in a step 210.

A codeword is then formed with repetitions of bits of representing the control information in a step 220. The codeword may be 32 bits and the repetitions may be of a three-bit representation of the two bits of control information. Block coding may be employed to form the codeword. In one embodiment, the codeword may be coded with a 1/16 block code.

After forming the codeword, groups of control information are generated based on the codeword in a step 230. In one embodiment, the groups may be generated by converting a 32-bit codeword to QPSK symbols and forming the groups from the QPSK symbols. The 32-bit codeword may be converted to 16 QPSK symbols and formed into four quadruplets.

The groups are then substantially equally- spaced across the BW of a base station transmitter for transmission in a step 240. After spacing the groups, the substantially equally- spaced groups are transmitted to UE in a step 250. In some embodiments, the groups may be transmitted over a 10 MHz BW and with each of the groups equally separated by 150 tones.

The transmitted groups are received at the UE in a step 260. Subsequently, in a step 270, the UE allocates a number of OFDM symbols in the downlink sub-frame for the PDCCH based on the substantially equally- spaced groups. The UE can allocate the number of OFDM based on extracted control information (e.g., from the two bits of control information) represented in the groups. The method 200 then continues to a step 280 and ends.

To demonstrate the results of repeating the two bits of control information (CatO), a simulation was performed as represented in the following FIGS. The simulations examine the CatO transmission requirements, under various operating conditions, in terms of the required overhead to achieve a target BER which will not result into any appreciable impact of the overall DPCCH BLER. The simulation assumptions are provided below in Table 2.

Table 2 - Simulation Assumptions

CatO is assumed to be transmitted only in the first OFDM symbol of the DL sub- frame because the PDCCH may occupy only one OFDM symbol and its decoding latency should be minimized. For brevity, the CatO BER is presented only for 10 Kmph in order to capture the most important range of UE speeds for E-UTRA (for higher UE speeds, the only difference is an additional fractional decibel penalty due to channel estimation losses).

As noted previously, the BER is considered not as the error probability for one bit but as the probability that any of the 2 CatO bits is in error. As such, it can be considered a stricter measure than the conventional BER and simply corresponds to the probability that CatO is erroneously received.

For the DPCCH transmission, the CatO transmission exploits the frequency diversity of the channel. The channel BW is divided into a number of regions equal to the number of CatO repetitions. For example, for four repetitions, the CatO transmission is at the {0, 1.5, 3, 4.5} MHz regions of the BW. The examined channel models are the typical urban 6 (TU6) and the pedestrian channel A (PA). Since cell edge UE (those having low signal to interference-plus-noise power ratios, or SINRs) typically experiences the full frequency selectivity of the channel, TU6 captures this. However, for smaller BWs or when there is some correlation between the 2 Rx antennas, a flatter channel model is more appropriate and the performance with the PA channel is also evaluated to capture a worst case (and pessimistic) setup and provide a lower performance bound.

FIG. 3A is a graph representing, in a pedestrian channel A (PA) model, a BER for a CatO transmission with no transmit diversity versus the number of repetitions for the CatO. FIG. 3B is a graph representing, in a PA model, a BER for a CatO transmission with transmit diversity versus the number of repetitions for the CatO. In FIG. 4A is a graph representing, in a typical urban 6 (TU6) channel model, a BER for a CatO transmission with no transmit

diversity versus the number of repetitions for the CatO. FIG. 4B is a graph representing, in a TU6 channel model, a BER for a CatO transmission with transmit diversity versus the number of repetitions for the CatO.

In order to avoid CatO impacting PDCCH reception, the target BER for CatO should be below the PDCCH BLER. CatO transmission may be designed considering that it is received with the target BER by scheduled UEs having the worst SINR in any sub-frame. Moreover, adaptation of the transmission power is always possible by reducing it in case the worst SINR of a scheduled UE is above the one required for the CatO provisioned resources and by possibly increasing it above the nominal level if the worst SINR is at the lowest 1%- 2% of the geometry distribution. Nevertheless, robust CatO transmission needs to be ensured without reliance on significantly boosting transmit power.

To avoid over-dimensioning the resources required for CatO transmission, the target BER can be around 1% (or around the PDCCH target BLER) for the worst SINR of a scheduled UE at any sub-frame. This may prove adequate since the PDCCH reception of UEs in better SINR conditions will not be affected by CatO. For a scheduled UE with a worst possible SINR, the overall error probability of incorrect PDCCH decoding only increases from 1% to 2%.

FIG. 5 is a graph representing the geometry distribution for cases 1 and 3 according to the Physical Layer Aspects for E-UTRA (Release 7), version 7.1.0. Case 3 presents the worse setup in terms of SINR and 96% of UEs have SINR above -5 dB. FIG. 6 is a graph representing the PDCCH BLER with code rate 1/12 for the PA and TU channels with and without transmission diversity.

Although the SINR distribution for scheduled UEs with a PF scheduler can be expected to be better than the average long term SINR distribution, because of the larger CQI errors at the lowest SINR regions( and possibly because of delay sensitive traffic), the average SINR distribution can serve as a more conservative indication of the target SINR operating point. From FIGS. 3A, 3B and 6, it can be observed that for flat channels, the PDCCH with the lowest code rate of 1/12 requires larger SINR to achieve 1% BLER than the corresponding one for CatO BER with 12 repetitions to achieve BER of 1%. For TU6 channel, having 12 repetitions for CatO (1 RB total overhead) is also adequate as the required SINR for 1% BER is below the 2% geometry CDF point.

Table 3 below summarizes the overhead CatO transmission over 1 RB represents for the various operating BWs as a percentage of the overall overhead and as a percentage of the total resources. The overall overhead is assumed to be 3 OFDM symbols (including RS and other overhead). Even at 1.25 MHz, this overhead is not significant compared to the ability to dimension the PDCCH with granularity of 0.5 or 1 OFDM symbols.

Table 3 - Relative CatO Overhead

The simulation evaluates the overhead requirements for the CatO transmission to achieve BER of about 1% at the lowest points of the geometry distribution. The simulation demonstrates that with transmission capturing the frequency diversity of the operating BW, CatO repetition of about 12 achieves the desired BER targets. Repetitions of less than 12 may also be sufficient to achieve BER targets while providing less of an adverse affect on the overhead. The corresponding overhead naturally varies with the operating BW but even for the smallest one of 1.25 MHz, this overhead is low compared to the savings in the PDCCH overhead in the range of 0.5 or one OFDM symbols (depending on the mapping between the CatO bits and the PDCCH size) .

At least a portion of the above-described apparatuses and methods may be embodied in or performed by various conventional digital data processors or computers, wherein the computers are programmed or store executable programs of sequences of software instructions to perform one or more of the steps of the methods, e.g., steps of the method of FIG. 2. The software instructions of such programs may be encoded in machine-executable form on conventional digital data storage media, e.g., magnetic or optical disks, random- access memory (RAM), magnetic hard disks, flash memories, and/or read-only memory (ROM), to enable various types of digital data processors or computers to perform one, multiple or all of the steps of one or more of the above-described methods, e.g., one or more of the steps of the method of FIG. 2.

Those skilled in the art to which the disclosure relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of the claimed invention.