3GPP RAN1 meeting #44-bis
`Athens, Greece, 27th–31th March, 2006
`
`10.2.3
`Agenda Item:
`Nortel Networks
`Source:
`On the performances of LTE RACH
`Title:
`Document for: Discussion
`
`R1-060908
`
`1 Introduction
`While several issues on RACH have been raised in the previous meetings, one of them is how to send RACH
`message. The RACH message can be scheduled on the shared channel or transmitted on RACH along with the
`preamble. The main purpose of this contribution is to discuss the performances on LTE RACH.
`
`In the first scheme, we considered the preamble performances since scheduled message is out of scope in RACH.
`Additionally, the supported simultaneous preambles are simulated if an interference cancellation scheme is applied.
`In the second scheme, we investigated the RACH message performance sent along with the preamble. Since we
`didn’t consider pilot symbols, the RACH preamble is used for the estimation.
`
`In section 2, we configured RACH structure for each case and the performance results can be shown in section 3.
`
`2 Considered RACH configurations
`2.1
`Case 1 : Sending only preambles
`Figure1 shows the transmission structure for LTE RACH preamble. Zadoff-Chu CAZAC is used for the code
`signatures of RACH preamble. They are mapped to the sub-carriers in localized manner before N point FFT is
`accomplished. We should note that Zadoff-Chu CAZAC has the following features.
`
`- Low PAPR than random-code signatures
`- Zero autocorrelation for the delay over one-symbol duration
`- Applicable to SC-FDMA transmission block
`
`The transmitted RACH symbol can be identically repeated in order to make the receiver simple and accumulate the
`symbol energy over the time domain. Through identical symbols, the detector can be designed simply. As shown in
`figure 2 and figure 3, the UE transmits preamble symbols over RACH and then the Node B detects a preamble
`sequence from RACH preamble detection window. The detected symbols are compensated by all the possible
`codes. The receiver can recognize the preamble sequence with threshold detector i.e. if the received energy of a
`code is more than the threshold, the receiver presumes that the code is detected.
`
`In this section, we consider only preambles since the message can be scheduled on UL SCH if the preamble is
`detected. However, this scheme can be extended to support SC-FDMA as well as OFDMA.
`
`Time-domain
`samples
`
`
`
`Symbol repetition
`
`P/S
`
`N
`
`(N point)
`
`FFT
`
`N
`
`Subcarrier Mapping
`
`(localized)
`
`Zadoff-Chu CAZAC
`Signature
`{1,2,…,K}
`
`M
`
`Figure 1. Tx structure for RACH preamble.
`
`
`
`
`
`SAMSUNG 1011-0001
`
`

`
`
`
`RACH symbol RACH symbol RACH symbol RACH symbolRACH symbol RACH symbol RACH symbol RACH symbol
`
`
`
`UE side : UE side :
`
`
`
`PreamblePreamble
`
`
`
`PreamblePreamble
`
`
`
`Node-B side : Node-B side :
`
`
`
`RTDRTD
`
`
`
`PreamblePreamble
`
`
`
`PreamblePreamble
`
`
`RACH RACH
`
`PreamblePreamble
`
`DetectionDetection
`
`WindowWindow
`
`Figure 2. The RACH preamble signal format
`
`
`
`
`
`ChannelsChannels
`
`
`Fading andFading and
`
`Round Trip DelayRound Trip Delay
`
`
`
`Code #KCode #K
`
`
`
`Code #1Code #1
`
`
`2048-chip 2048-chip
`
`FFTFFT
`
`
`Code ResponseCode Response
`
`CompensationCompensation
`
`
`
`IFFTIFFT
`
`
`ThresholdThreshold
`
`DetectorDetector
`
`
`M-chip RACH signalM-chip RACH signal
`
`with Zadoff-Chu CAZAC with Zadoff-Chu CAZAC
`
`
`LocalizedLocalized
`
`MappingMapping
`
`Figure 3. An example of RACH detector
`
`
`
`
`
`
`
`Case 2 : Sending messages along with preambles
`2.2
`We had investigated RACH message transmitted along with the preamble. In that scheme, the RACH message
`signals are transmitted after RACH preambles and identically repeated similar to the preamble symbols. It is shown
`in the figure 4. Zadoff-Chu CAZAC is used for preamble signatures and for channel estimator. In order to send
`RACH message FEC, symbol repetition, interleaver and BPSK modulation are assumed. Note that for convenience,
`tail biting convolution coding with K=7 and R = 1/2 is used. In order to coherent RACH message demodulation,
`the channel estimation is extracted from the preamble since the pilot symbols are not devised in the structure.
`
`
`
`
`
`
`
`RACH symbol RACH symbol RACH symbol RACH symbolRACH symbol RACH symbol RACH symbol RACH symbol
`
`
`
`UE side : UE side :
`
`
`
`PreamblePreamble
`
`
`
`PreamblePreamble
`
`
`
`MessageMessage
`
`
`
`MessageMessage
`
`
`
`Node-B side : Node-B side :
`
`
`
`RTDRTD
`
`
`
`PreamblePreamble
`
`
`
`PreamblePreamble
`
`
`
`MessageMessage
`
`
`
`MessageMessage
`
`
`RACH RACH
`
`PreamblePreamble
`
`DetectionDetection
`
`WindowWindow
`
`
`RACH RACH
`
`MessageMessage
`
`DemodDemod
`
`WindowWindow
`
`
`
`Figure 4. The preamble and message signal format
`
`SAMSUNG 1011-0002
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`

`
`CAZAC codes for
`Preamble identification
`
`BPSK
`modulation
`
`RACH
`message
`(30 bits)
`
`Rate 1/2 K=9
`Conv encoder
`(tail biting)
`
`Channel
`Interleaver
`
`20 times repetition
`over sub carriers
`(distributed)
`
`QPSK
`modulation
`
`Channels
`
`2048-chip
`IFFT
`
`Localized
`Mapping
`
`600-chip OFDMA
`RACH signal
`
`Fading and
`Round Trip Delay
`
`2048-chip
`FFT
`
`Code #K
`
`Code #1
`
`OK
`
`Code Response
`Compensation
`
`2048-chip
`IFFT
`
`Threshold
`Detector
`
`Channel & timing
`Estimation
`
`CAZAC codes for
`Preamble identification
`
`Localized
`Mapping
`
`2048-chip
`FFT
`
`Localized
`De-mapping
`
`Channel
`Compensation
`
`QPSK
`demodulation
`
`Channel
`De-interleaver
`
`Channel
`Decoder
`
`
`
`Figure 5. The modem structures for RACH preamble and message (An example)
`
`
`
`3 Simulation results
`Based on [1], we designed a random access channel. The symbol parameters are same with that of UL SCH LB.
`Table 1 shows the assumptions and the parameters for simulation. The used subcarrier, Nu is 600 since RACH BW
`is 10MHz, while the used subcarrier for RACH preamble, M is 599 which is the maximum prime number less than
`600. The channel estimator for RACH message can be either perfect or real. Two symbols for RACH message are
`transmitted after RACH preamble so that the message demodulation is simpler. To get sufficient performance for
`the message, 10 times repetition is used and distributed over frequency domain.
`
`Table 1. The parameters and assumptions for RACH simulation
`
`RACH symbol duration
`Number of Rx antennas
`N (FFT Size / Samples in a symbol)
`Nu (used subcarriers in a system BW)
`Number of subcarriers for RACH BW
`M (used subcarriers for RACH preamble)
`K (number of signature in a cell)
`Fading Channel
`Txed RACH symbols
`Round trip delay
`RACH preamble Detector
`RACH preamble codes
`False-alarm probability
`Channel estimation
`RACH message information
`
`RACH message channel codes
`RACH message channel interleaver
`RACH message repetition
`
`
`
`66.67 sec
`2
`2048 samples (20MHz)
`1200 subcarriers (10MHz)
`600 subcarriers (10MHz)
`599 subcarriers
`16
`TU1, 3Km/h
`2 (but effectively, 1 symbol for detector)
`Less than 1 symbol duration
`Threshold detection
`Zadoff-Chu CAZAC
`0.001
`Perfect or Real
`30 bits (RRC request)
`5 bits (CQI)
`1/2 K=7 tail-biting convolutional codes (30 bits)
`Modified Reed Muller (5 bits : HSDPA)
`Block interleaver
`10 (30 bits, distribution scheme over subcarriers)
`30 ( 5 bits, distribution scheme over subcarriers)
`
`While keeping 0.1% false-alarm probability, the missing probabilities were simulated as shown in figure 6. The
`0.01 missing probability at 10MHz BW can be obtained at -9.5dB CINR, which means about 18dB Ep/No is
`required for RACH preamble. If 2.5MHz BW for RACH preamble is used, 7.5 dB is needed additionally. In figure
`
`SAMSUNG 1011-0003
`
`

`
`7, the detector performance with IC technique is compared with that of a conventional detector. With the
`conventional detector, 3 signatures are decoded while with IC detector, 7 signatures are simultaneously detected at
`-2.5 dB CINR.
`
`One RACH Preamble signature, System BW=20MHz
`
`100
`
`10-1
`
`Missing Probability
`
`RACH=2.5MHz
`RACH=5MHz
`RACH=10MHz
`
`-18
`
`-16
`
`-14
`
`-12
`
`10-2
`-20
`
`
`Figure 6. The missing probability of RACH preamble over TU1 channels (2 Rx antennas)
`
`-10
`CINR [dB]
`
`-8
`
`-6
`
`-4
`
`-2
`
`0
`
`RACH BW=10MHz, System BW=20MHz
`
`100
`
`10-1
`
`Missing Probability
`
`10-2
`-20
`
`U1
`U3, NoIC
`U3, IC
`U7, NoIC
`U7, IC
`U9, NoIC
`U9, IC
`
`-15
`
`-10
`
`-5
`CINR [dB]
`
`0
`
`5
`
`
`Figure 7. The performance comparison between a conventional detector and a detector with IC technique
`
`
`
`Figure 8 shows the BER/BLER performances of RACH message with perfect estimator and real estimator. To get
`the results, we assumed Node-B know the transmitted preamble sequences, and channel estimator operates with the
`received preamble signals. For perfect estimation, the required CINR is achieved at -10dB to get 1% BLER.
`However the performance with real estimation needs -6dB that is about 4dB greater than the performance of RACH
`preamble at 10MHz BW. The performance gap is caused from channel estimation error. As shown in figure 9, the
`MSE of channel estimation is about 0.63 at -10dB CINR. This is so poor that the channel estimator can not be
`operated properly.
`
`In Figure10, RACH information bits are replaced by 5-bit CQI information as HSDPA cases. CQI is encoded to 20
`bits as the same way with HSDPA. The number of symbol repetition for a coded bit is 30. The required CINR to
`get 5% BLER is about -13dB CINR. The performance difference between ideal estimator and real estimator is
`about 7dB. Compared with the operating point of RACH preamble, CQI operating point is lower than about 4 dB.
`However the information is not CQI, the performance requirement becomes tighter and the performance gap may
`decrease.
`
`SAMSUNG 1011-0004
`
`

`
`2 Antennas, TU1, RACH BW=10MHz, System BW=20MHz
`
`BER w/t Perfect Est.
`BLER w/t Perfect Est.
`BER w/t Real Est.
`BER w/t Real Est.
`
`100
`
`10-1
`
`10-2
`
`10-3
`
`BER/BLER
`
`10-4
`-12
`
`-11
`
`-10
`
`-9
`
`
`Figure 8. The BLER of RACH message (TU1, 2 Rx antennas)
`
`-8
`CINR [dB]
`
`-7
`
`-6
`
`-5
`
`-4
`
`N=2048, N1=1024, Ant=2, Nsym=2, Used Code=1/16
`
`1
`
`0.9
`
`0.8
`
`0.7
`
`0.6
`
`0.5
`
`0.4
`
`0.3
`
`0.2
`
`Channel Error
`
`0.1
`-12
`
`-11
`
`-10
`
`-9
`
`
`Figure 9. The MSE of channel estimation (TU1, per Rx antenna)
`
`-8
`CINR[dB]
`
`-7
`
`-6
`
`-5
`
`-4
`
`2 Antennas, TU1, RACH BW=10MHz, System BW=20MHz
`
`CQI BLER w/t Perfect Est.
`CQI BLER w/t Real Est.
`
`-20
`
`-18
`
`-16
`CINR [dB]
`
`-14
`
`-12
`
`-10
`
`
`
`100
`
`10-1
`
`10-2
`
`CQI BLER
`
`10-3
`
`10-4
`-22
`
`
`
`Figure 10. The CQI BLER (TU1, 2 Rx antennas)
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`SAMSUNG 1011-0005
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`

`
`4 Conclusion
`We investigated RACH preamble and RACH message and simulated them. From the result of RACH preamble and
`message, the preamble is operated at -10dB CINR (18dB Ep/No). In addition, if a detector with IC is used with
`10MHz RACH BW, seven simultaneous preamble signatures can be detected at the same time.
`
`For the performance of message, the 30-bit message with real channel estimation is operated at -6dB CINR. This
`means the RACH duration for message is 2.5 times longer than that for preamble.
`
`On the other hand, if CQI is transmitted, the operating point of message is -14dB, which means the duration for
`CQI is 40% of the duration for preamble. Since we assume the operating point is 5% BLER, the duration of
`message increases if the message information changes to UE ID or something else.
`
`To help understanding, the required RACH durations for the considered cases are shown in the following table.
`When multiple RACH message signals are received at the same time, it required much duration to keep
`orthogonality among the received message signals
`
`RACH BW = 10MHz
`Preamble
`30-bit message (1% BLER)
`5-bit message (5% BLER)
`5-bit message (1% BLER)
`
`Required CINR
`-10dB
`-6
`-13dB
`-11.5dB
`
`Relative duration
`1
`2.5
`0.5
`0.7
`
`
`
`5 References
`[1] 3GPP TR 25.814, version 0.4.1, “Physical layer aspects of Evolved UTRA”
`
`SAMSUNG 1011-0006