Control and Data Signaling in SC-FDMA Communication Systems
`Aris Papasakellariou and Joonyoung Cho
`June 4, 2007
`
`678-3235 (P15725-US/ST)
`
`1. Introduction
`The disclosed invention considers the transmission of positive or negative acknowledgement signals
`(ACK or NAK, respectively) and channel quality indicator (CQI) signals in a single-carrier frequency
`division multiple access (SC-FDMA) communications system as it is known in the art and is further
`considered in the development of the 3GPP E-UTRA long term evolution (LTE). The invention
`assumes the uplink (UL) communication corresponding to the signal transmission from mobile user
`equipments (UEs) to a serving base station (Node B). The ACK/NAK and CQI signals may also be
`referred to as the physical uplink control channel (PUCCH) or simply the control channel. The
`invention further assumes that ACK/NAK or CQI signals are transmitted together with data signals
`carrying the service information.
`
`The ACK or NAK signal is in response to the correct or incorrect, respectively, data packet reception in
`the downlink (DL) of the communication system which corresponds to signal transmission from the
`serving Node B to a UE. The CQI signal transmitted from a reference UE is intended to inform the
`serving Node B of the channel conditions for channel-dependent scheduling of DL data. Either or both
`of the ACK/NAK and CQI signals may be transmitted simultaneously with or separately from the data
`signal. As previously mentioned, the disclosed invention considers the former case. This case may also
`be referred to as data-associated transmission of the ACK/NAK and/or CQI signals.
`
`The UEs are assumed to transmit control and/or data signals over a transmission time interval (TTI)
`corresponding to a sub-frame. Figure 1 shows the sub-frame structure assumed in the disclosed
`invention. The sub-frame has duration of one millisecond and comprises of two slots. Each slot further
`comprises of seven symbols and each symbol further comprises of a cyclic prefix in order to mitigate
`interference due to channel propagation effects as it is known in the art. Furthermore, the middle
`symbol in each slot carries the transmission of reference signals (RS), also known as pilots, which are
`used to provide channel estimation and allow coherent demodulation of the received signal (DM RS).
`
`The transmission bandwidth (BW) is assumed to comprise of frequency resource units which will be
`referred to as resource blocks (RBs). Each RB in a symbol is assumed to comprise of 12 sub-carriers
`and UEs are allocated a multiple N of consecutive RBs for the data transmission. Nevertheless, the
`above values are only illustrative and not restrictive to the embodiment of the disclosed invention.
`
`Nx12 sub-carriers
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`Figure 1: Structure of the UL Sub-Frame.
`
`1
`
`

`

`678-3235 (P15725-US/ST)
`
`An exemplary transmitter structure for SC-FDMA signaling is shown in Figure 2. In order to transmit
`the control (ACK/NAK and/or CQI) bits, certain data bits (such as, for example, the parity bits in case
`of turbo coding) may be punctured and replaced by the control bits in order to maintain the single­
`carrier property. The discrete Fourier transform (DFT) of the combined bits is then obtained, the sub­
`carriers corresponding to the assigned transmission bandwidth are selected, the inverse fast Fourier
`transform (IFFT) is performed and finally the cyclic prefix (CP) and filtering are applied to the
`transmitted signal. This time division multiplexing (TDM) between the control (ACK/NAK and or
`CQI) and data signals prior to the DFT is necessary to preserve the single carrier property of the
`transmission.
`
`Coded data Puncture Data
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`
`Figure 2: Multiplexing of Control and Data Signals
`
`2. Placement of Data-Associated ACK/NAK and CQI Signals
`The main object of the invention is the placement in the UL sub-frame of the ACK/NAK and CQI
`signals by considering aspects related to their reception performance and other general SC-FDMA
`communication system requirements. Another object of the invention is the enhancement of the overall
`reception reliability for the data signal.
`
`A first observation for the UL sub-frame structure in Figure 1 is that the demodulation RS (DM RS)
`exists only in the middle symbol of each slot. In case of high speeds, this results to substantially
`degraded channel estimate for symbols located further away from the DM RS (that is, for symbols near
`the beginning and end of each slot). This may be acceptable for data transmission which is coded, has
`typically a relatively large target block error rate (BLER) of 10% or above, and can benefit from
`retransmissions though a conventional HARQ process. Conversely, the CQI and particularly the
`ACK/NAK have much stricter performance requirements .
`
`The second observation relates to the transmission of a sounding RS (SRS) in synchronous systems.
`The SRS has a wideband nature and is transmitted by UEs in order to serve the purposes of UL
`frequency domain channel dependent scheduling, timing estimation for synchronous operation, and
`power control as it is known in the art. Figure 3 illustrates the concept of SRS transmission. The SRS
`typically has larger transmission bandwidth than the data and the DM RS. It is transmitted periodically
`in one of the symbols of the UL sub-frame (in the example of Figure 3, the SRS is transmitted in the
`first symbol and once every two sub-frames).
`
`.
`
`2
`
`
`

`

`Sub-Frame
`
`Sub-Frame
`
`678-3235 (P15725-US/ST)
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`Time
`
`•
`
`•
`
`UE1 DMRS
`
`UE2DMRS
`
`• UE3DMRS
`
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`Figure 3: Multiplexing of Control and Data Signals
`
`Frequency
`
`Because the DM RS is asswned to have a contiguous spectrum, the SRS cannot be transmitted at the
`middle symbol of each slot because it will create mutual interference with the DM RS (clearly, the DM
`RS transmission is necessary for the data demodulation and cannot be omitted). Distributing the CQI
`and ACK/NAK signals substantially over the entire UL sub-frame will either severely restrict the
`placement of the SRS or introduce additional complexity and performance loss in the reception of
`ACK/NAK and/or CQI signals as puncturing will be dynamically needed in a symbol depending on
`whether or not the SRS is transmitted in that symbol. Having as many as possible locations for the SRS
`transmission is desirable in synchronous systems because, for proper UL CQI and power control
`measurements, the SRS should capture interference from data transmission and not from other SRS
`transmission, that is, SRS transmission from neighboring cells and Node Bs should not coincide.
`
`Subsequently, a brief set of simulation results for the raw bit error rate (BER) is provided to illustrate
`the impact of inaccurate channel estimation on the data reception quality as a function of the symbol
`position in the slot and the UE speed. Table 1 provides the simulation asswnptions which provide the
`most optimistic setup for the performance loss of symbols further away from the DM RS because:
`a)
`Transmission bandwidth is 1 RB. This maximizes power per sub-carrier.
`b)
`Channel frequency selectivity is large and there are 2 uncorrelated receiver antennas. This
`maximizes the slope of the raw BER curve.
`Operating signal-to-interference and noise ratio (SINR) is large. This minimizes the impact
`of inaccurate channel estimation.
`
`c)
`
`Table 1: Simulation Assumptions
`
`Parameters
`Operating Bandwidth @ Carrier Frequency
`Modulation Scheme
`Data Transmission Bandwidth (BW)
`UE Speed
`
`Transmission Type
`
`Channel Model
`Number of Receive Antennas
`Number of Transmit Antennas
`
`...
`
`Assup:iptions
`5MHz@ 2.6 GHz
`QPSK
`1 RB
`3, 30, 120 and 350 kmph
`Localized (at same RB) over the sub-frame at 3, 30 Kmph
`Frequency Hooping Between Slots at 120 and 350 Kmph
`GSM-TU6
`2
`1
`
`3
`
`
`

`

`Figure 4 presents the raw BER. At symbol locations symmetric to the DM RS, the BER is typically the
`same. At 120 Kmph and 350 Kmph, the transmission in the first slot is assumed to occur at a different
`BW than the one in the second slot (frequency hopped transmission per slot). As only 1 DM RS per slot
`is available for channel estimation, the BER is the same at symbols symmetric (equidistant) to the DM
`RS. At low speeds, such as 3 Kmph, this is also the case because the channel does not change over the
`sub-frame. Some variability does exist for medium UE speeds, such as 30 Kmph, but, for simplicity,
`the average BER of symbols equidistant to the DM RS is shown in Figure 4.
`
`678-3235 (P15725-US/ST)
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`10·1
`
`a:
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`
`Figure 4: Raw BER as a Function of the Slot Symbol and the UE Speed.
`
`Even under the previous, most optimistic, assumptions for the raw BER degradation at symbols further
`away from the DM RS, at 350 Kmph, the BER saturates at the 1 st17th and 2"d/6th symbols. However, the
`impact on the BER of the Yd/5th symbols is rather contained and saturation is avoided (the difference
`relative to the BER at 3 Kmph is also partly due to the fact that the latter uses both RS in the sub-frame
`for channel estimation, that is, channel estimation is operating with 3 dB more SINR). The BER at 120
`Kmph, relative to the one of the 3rd/5th symbols at about 1 % BER, is also degraded by about 3 dB for
`the 1 st;ih symbols and bl about 1.5 dB for the 2"d/6th symbols. Obviously, due to the flattening of the
`BER curves for the 1 st17 and 2"d/6th symbols, the degradation will be much larger for operating points
`below 1% as needed for the NAK reception.
`
`Based on the results in Figure 4 it becomes apparent that the data-associated PUCCH transmission
`should be placed immediately next to the DM RS. Figure 5 shows an example for such placement
`when a UE transmits both ACK/NAK and CQI during a sub-frame by applying TDM with data.
`I ACK/NAK
`
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`Figure 5: Placing of ACK/NAK and CQI Transmission
`
`4
`
`
`

`

`Figure 6 shows an example for such placement when the UE transmits only ACK/NAK bits together
`with the data signal during a sub-frame while Figure 7 considers the case of only CQI and data
`transmission.
`
`678-3235 (P15725-US/ST)
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`-RS
`
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`
`To minimize channel estimation losses, the ACK/NAK should be placed with priority in the symbol
`after the first DM RS. Notice that this does not impact demodulation latency as a channel estimate is
`available only after the first DM RS ( clearly, there is no use from earlier transmission with respect to
`latency).
`
`Subsequently, to address low SINR or coverage issues, the ACK/NAK can be placed in the symbol
`before the second DM RS. The reason is that for medium UE speeds, this second ACK/NAK placement
`benefits from improved channel estimation and time diversity while for high UE speeds, it benefits
`from frequency and time diversity. The tradeoff is the increased latency which however is not critical
`for the ACKINAK transmission. Typically, data non-associated PUCCH transmission is assumed to be
`over the entire sub-frame and therefore, the resulting latency for the proposed data-associated PUCCH
`transmission is not larger. Moreover, CQI and ACKINAK performance targets are similar for all UE
`speeds while they are most challenging to achieve for high UE speeds.
`
`Provisioning for ACK/NAK transmission in the number of sub-carriers over 2 symbols is typically
`comfortably adequate to achieve the desired BER for the ACK reception even for the lowest SINRs and
`transmission only over 1 RB ( corresponding to the fewest number of available sub-carriers).
`Nevertheless, since the NAK reception has lower BER requirements, it is appropriate for robustness
`and to achieve time and frequency diversity, to have the ACK/NAK transmission over the number of
`sub-carriers in 1 symbol in each slot.
`
`If further ACK/NAK transmissions are needed, because of low SINR or coverage issues, or for some
`interference randomization in asynchronous systems, the other symbols next to the RS in the 2 slots
`
`5
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`

`

`678-3235 (P15725-US/ST)
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`may also be used. However, the number of symbols where ACK/NAK and CQI transmission may
`occur should be minimized as it directly affects the number of available symbols for SRS transmission
`for synchronous systems.
`
`Moreover, depending on the number of information bits carried in the CQI reporting, which are several
`times more than the ACK/NAK information bits, the symbols immediately adjacent to the DM RS may
`not suffice for the CQI transmission especially for coverage or SINR limited UEs that are also typically
`assigned small bandwidth allocations (a small number of RBs). In such cases, the CQI transmission
`may also extent to one or more symbols that are adjacent to the symbols also carrying CQI information
`that are adjacent to the symbols carrying the DM RS. In the exemplary setup of Figures 5-7 these
`additional symbols are either of the second or sixth symbols in a slot.
`
`To minimize CQI reception latency, the corresponding transmission may be confined in the first slot of
`the sub-frame. This may he feasible for UEs in good SINR conditions but, in general, it also leads to
`worse performance due to losses from less accurate channel estimation and less diversity. Nevertheless,
`Figure 8 illustrates an exemplary setup (in a direct modification of Figure 5) where the CQI
`transmission is only in the first slot (the corresponding modification for Figure 7 is straightforward).
`
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`
`I ACK/NAK
`
`li]CQI
`
`1 slot
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`Figure 8: Placing of CQI Transmission only in the First Slot of a Sub-Frame.
`
`Another object of the invention is the improved data packet detection performance for high speed UEs.
`It is assumed that the serving Node B can obtain an estimate of the Doppler shift for the received signal
`from a UE, and in this way obtain an estimate of the UE speed. Since the reception reliability of the
`data bits depends on the symbol in which they are placed, the receiver can account for this inequality in
`the reliability of the received data bits by appropriately weighting the log likelihood ratio (LLR) for
`each bit prior to providing to a turbo decoder. Since channel interleaving, as it is well known in the art,
`typically applies to the coded data prior to transmission, the weighting of data bits at the receiver is
`either on a modulation symbol basis prior to the de-interleaving operation or on a bit basis after the de­
`interleaving operation.
`
`Therefore, another object of the invention is the application of weight coefficients to the data bits
`depending on their location in the symbols of the sub-frame. Those coefficients may have the same
`value for low UE speeds, while for high UE speeds, the larger the distance of a symbol where the data
`bit is located from the DM RS, the smaller the coefficient value. For medium speeds, and without
`frequency hopped transmission across the two slots of a sub-frame, the value of the coefficient may be
`selected specifically for each symbol, as the ones between the 2 DM RS have better reception reliability.
`
`6
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`

`678-3235 (P15725-US/ST)
`
`3. Conclusions and Preliminary Invention Claims
`Due to the non-uniformity of the DM RS across the UL sub-frame, the reception of symbols carrying
`data or control information that are closer to the symbol carrying the DM RS is considerably more
`reliable at medium and high UE speeds than the corresponding reception of symbols located at a larger
`distance in time from the DM RS. Therefore, the placement in the sub-frame of ACK/NAK and CQI
`signals, which have high reception accuracy requirements and are not able to benefit from HARQ,
`should be immediately next to the DM RS.
`
`The unequal reception reliability of data bits located at the various symbols of the sub-frame can also
`be exploited at the receiver by appropriately weighting the log-likelihood ratio (LLR) of the data bits
`provided to the turbo decoder.
`
`The first claim of the disclosed invention considers a sub-frame structure comprising of symbols
`carrying data or control signals and demodulation reference signals wherein the bits corresponding to
`an acknowledgement signal, that is transmitted together with data bits from a reference user equipment
`to the serving base station, are placed only at one or more of the symbols adjacent to one or more
`symbols carrying demodulation reference signals.
`
`The second claim of the disclosed invention considers a sub-frame structure comprising of symbols
`carrying data or control signals and demodulation reference signals wherein the bits corresponding to
`an acknowledgement signal, that is transmitted together with data bits from a reference user equipment
`to the serving base station, are placed only at two symbols that are adjacent to two symbols carrying
`demodulation reference signals. The second claim may be restricted by having the two symbols
`carrying the acknowledgement signal further placed between the two symbols carrying the
`demodulation reference signals.
`
`The third claim of the disclosed invention considers a sub-frame structure comprising of symbols
`carrying data or control signals and demodulation reference signals wherein the bits corresponding to
`an acknowledgement signal, that is transmitted together with data bits from a reference user equipment
`to the serving base station, are placed only at four symbols that are adjacent to two symbols carrying
`demodulation reference signals.
`
`The fourth, fifth, and sixth claims are the same with the first, second, and third claims, respectively,
`with the acknowledgement signal replaced by a channel quality indicator signal.
`
`The seventh claim of the disclosed invention is the weighting of the data bits, transmitted by a user
`equipment at various symbols of a sub-frame, at the base station receiver prior to the decoding
`operation with a value that depends on the velocity of said user equipment and the time distance of said
`various symbols from one or more symbols carrying a demodulation reference signal.
`
`7
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`