3GPP TSG RAN1#43
`Helsinki, Finland
`Jan. 23 – Jan 25, 2006
`
`Agenda Item:
`Source:
`Title:
`Document for:
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` R1-060025
`
`5.2.3.1
`Motorola
` RACH Design for EUTRA
`Discussion
`
`
`1. Introduction
`
`The RACH channel is used for initial access to the network as well as to transmit small to medium amount
`of control information and data packets. In [1], preliminary results for RACH preamble design were
`presented. This contribution proposes two options of RACH channel design for E-UTRA.
`
`2. RACH Design Principle
`
`The RACH design requirements include fast acquisition of preamble, ability to transmit short to medium
`packets using the message part, estimation of timing advance, elimination of interference and minimum
`impact on other data/control channels. Usually, RACH signaling includes two parts of transmission
`between UE and Node-B. The first part is the transmission of RACH preamble used for fast acquisition
`and estimation of timing advance.,. The second part is the RACH message transmission, which includes
`transmission of RACH data packets and associated control information. It may be noted that the timing
`advance is not discussed in this contribution.
`On the RACH preamble design, several options, namely TDM, FDM and CDM are available for
`multiplexing between the RACH preamble and scheduled-data channels. Figure 1 illustrates time division
`multiplexing (TDM) and frequency division multiplexing (FDM) of RACH and scheduled data channels.
`Other TDM/FDM combinations are possible such as frequency-hopped preamble design. Both TDM and
`FDM approaches will require slots or sub-carriers to be reserved specifically for RACH access. This
`RACH overhead may affect the system capacity, especially when the channels are not fully utilized.
`
`Figure 1 TDM and FDM of RACH and Scheduled Channels
`Another RACH preamble design is to multiplex the preamble and scheduled data channel in CDMA
`fashion, as illustrated in Figure 2. In this approach, there is no resource (time or frequency) reservation
`necessary for RACH preamble transmission. When an UE needs a RACH access, the preamble is
`transmitted with a long spreading sequence (signature sequence) on top of scheduled data channels. There
`is no RACH preamble overhead in this design. However, interference caused by the preamble exists and
`care should be taken to minimize this effect.
`
`
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`In this paper, both the TDM/FDM and CDM approaches are discussed as candidate RACH methods for
`EUTRA. Preliminary details of the above two methods are presented along with some simulation results.
`
`
`
`Figure 2 CDM of Scheduled Channels and RACH Preamble
`
`
`3. RACH Preamble Design using TDM/FDM
`
`In this scheme a dedicated or special symbol is used for RACH. The RACH symbol can be reserved every
`x frames (e.g. x = 1 … 10.) as shown in Figure 3. The scheme can use either localized or distributed mode.
`In the localized mode the subcarriers are divided into NRB resource blocks with each resource blocks using a
`fixed number of contiguous sub-carriers. Next, for each of the NRB resource blocks, a number of signature
`sequence groups are pre-defined so that every group consists of NS signature sequence and different groups
`can be assigned to different neighboring sectors. Each group also consists of several cyclically shifted
`version of the signature sequences (NSH ). As such, the total number of RACH opportunities per DFT-
`SOFDM symbol is given by NRB*NS.* NSH .
`
`There are different design options for E-UTRA based on this structure. One design example summarized in
`Table 1-(a) provides a large number of RACH opportunities. In this example, for 5MHz bandwidth, all 300
`subcarriers are divided into 20 resource blocks with NRB =20. A RACH signature sequence occupies 15
`subcarriers corresponding to 225kHz bandwidth, thus the length of signature sequence is 15. For the
`scalable bandwidth structure, the length of signature sequence is fixed to 15. The number of RACH
`opportunities will be 160 for 5MHz, and it is variable according to different bandwidth deployment.
`Another design example is summarized in Table 1-(b) with a longer (75) signature sequence and a larger
`minimum resource block (1.125MHz).
`
`Dividing the RACH opportunities into resource blocks provides the opportunity to take advantage of
`channel frequency selective characteristics to further improve the performance. The UE chooses the best
`available resource blocks for RACH preamble transmission based on information of the current frequency
`selective nature of the channel.
`
`The signature sequence are obtained from different “classes” of generalized chirp like (GCL) or Chu-
`sequences which are complex valued and have unit amplitude. The GCL/Chu sequence has low cross
`correlation at all time lags which improves the detection performance.
`
`The number of RACH groups for different bandwidths is summarized in Table-1. The total RACH
`overhead is dependent on the reserved RACH access rate. For example, if the RACH access is reserved
`every 1ms, the RACH overhead is 1/14=7.1%.
`
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`Radio Frame n-1
`
`Radio Frame n
`
`Radio Frame n+1
`
`Radio Frame (10 ms)
`
`Radio Frame Constructed from t ms Sub-frames
`
`Sub-Frames (e.g. 0.5 ms)
`
`RACH Symbol
`
`Data Symbol
`
`Half Pilot Symbol
`
`Figure 3. TDM/FDM RACH Structure
`Table 1. Examples of RACH Parameters for the TDM/FDM Structure
`RACH Parameters in
`Bandwidth (MHz)
`Localized mode
`5.0
`10.0
`min. RB BW (kHz)
`225
`225
`20
`40
`# RB (N RB )
`# of Occupied Subcarriers
`15
`15
`# of Sequences (N S )
`8
`8
`# of Cyclic shifted(N SH )
`1
`1
`# RACH opportunities
`160
`320
`
`1.25
`225
`5
`15
`8
`1
`40
`
`2.5
`225
`10
`15
`8
`1
`80
`
`15.0
`225
`60
`15
`8
`1
`480
`
`20.0
`225
`80
`15
`8
`1
`640
`
`RACH Parameters in
`Localized mode
`min. RB BW (kHz)
`# RB (N RB )
`# of Occupied Subcarriers
`# of Sequences (N S )
`# of Cyclic shifted(N SH )
`# RACH opportunities
`
`Bandwidth (MHz)
`5.0
`10.0
`1125
`1125
`4
`8
`75
`75
`8
`8
`2
`2
`64
`128
`
`2.5
`1125
`2
`75
`8
`2
`32
`
`1.25
`1125
`1
`75
`8
`2
`16
`
`15.0
`1125
`12
`75
`8
`2
`192
`
`20.0
`1125
`16
`75
`8
`2
`256
`
`
`
`(a)
`
`(b)
`
`
`
`2
`
`
`
`e
`
`
`
`when M
`
` is even
`
`
`
`j
`
`pn n
`(
`
`1)
`
`
`
`
`
`
`g
`
`n
`
`
`
`
`3.1. Sequence Design for TDM RACH Preamble
`
`General chirp-like (GCL) [5] or its special case, Chu-sequence [6] can be selected as the signature sequence
`used in each resource block. The Chu-sequence is defined [6] as
`2 1
`
`j
`pn
`M
`2
`
`2 1
`
`M
`e
` is odd
`when
`M
`2
`where p is relatively prime to M. For a fixed p, the Chu-sequence is orthogonal to its time-shift. For
`different p, Chu-sequences are not orthogonal. Note that all GCL sequences (including the Chu-sequence)
`have optimal autocorrelation properties.
`
`
`,
`
`n
`
`
`
`0,1,
`
`,
`
`M
`
`
`
`1
`
`
`
`
`
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`There is an extra benefit of selecting GCL sequence with a prime number length [4]. From [5], GCL
`sequence with a prime number length will yield “optimal” cross-correlation performance. To truncate this
`optimal sequence we can obtain a signature sequence with arbitrary length. However, this approach may
`have other problems for E-UTRA uplink and more studies are needed.
`
`3.2. Performance of the RACH preamble
`
`The RACH preamble detection is similar to the detection algorithm of the CDM-based RACH outlined in
`Appendix A. The basic idea is to detect the received power based on correlation of the received sequence
`to all the possible sequences. The correlation can be carried out either in time or frequency domain. Once
`the detected power is greater than a pre-defined power threshold, a RACH preamble is detected. Naturally,
`the choice of threshold determines detection performance. Figure 4 and Figure 5 illustrate detection
`performance of the TDM/FDM RACH preamble under AWGN and TU propagation channels, respectively.
`The following definitions were used in the performance evaluation:
` False alarm refers to a scenario where a particular code was detected when nothing or a different
`code was transmitted.
` Detection error refers to when a particular code was transmitted but not detected.
`
`
`
`Figure 4 Detection Error and False Alarm Probabilities of TDM-RACH over AWGN Channel.
`
`
`
`
`Figure 5 Detection Error Rate and False Alarm Performance of TDM-RACH over TU Channel at 3
`km/h.
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`3.3. RACH Message part for the TDM/FDM Structure
`
`If the preamble is detected at the Node-B, the Node-B sends an ACK. Upon detection of the ACK at the
`UE, the UE sends the message part in the next slot using the same RB location which was used to send the
`preamble. As an alternative, the message can be scheduled as outlined in a later section if the system is
`lightly loaded. However, more studies need to be performed to optimize the RACH message design.
`
`4. RACH Preamble Design using CDM approach
`
`To minimize uplink interference, the RACH preamble is designed to use time-frequency spreading with
`long spreading factor. With this approach, no reservation of symbols and sub-carriers are required and
`uplink interference generated is minimal when the spreading gain is large enough. In addition, simple
`receiver structure with frequency domain processing can be used to process the preamble.
`In our design, the RACH preamble should fit into the current E-UTRA numerology with 1 ms using two
`0.5 ms sub-frames. The frequency spreading length M usually equals to 300 for 5MHz bandwidth for a
`full bandwidth spreading. Furthermore, a Walsh code with length NW is applied symbol-by-symbol in time
`domain to yield total spreading gain of MNW. Then repetition is applied to cover total of 14 symbols in two
`0.5 ms sub-frames. For each DFT-S-OFDM symbol, NS numbers of signature sequence groups are pre-
`defined so that different groups can be assigned to different neighboring sectors. Each group also consists
`of several cyclically shifted version of the signature sequences (NSH ). The total RACH opportunities are
`NW*NS* NSH . Table 2 presents two RACH design parameters with different combinations of NW and NSH.
`Table 2 RACH Example Parameters of CDM Design
`RACH Parameters of CDM
`Bandwidth (MHz)
`5.0
`10.0
`15.0
`Design
`300
`600
`900
`Chip Length/Sym (M )
`Length of Walsh Code (N W )
`2
`2
`2
`# of Cyclic shifted (N SH )
`10
`10
`10
`# of Sequences (N S )
`8
`16
`24
`160
`320
`480
`# RACH opportunities
`
`20.0
`1200
`2
`10
`32
`640
`
`1.25
`75
`2
`10
`2
`40
`
`2.5
`150
`2
`10
`4
`80
`
`(a)
`
`RACH Parameters of CDM
`Bandwidth (MHz)
`20.0
`5.0
`10.0
`15.0
`2.5
`1.25
`Design
`1200
`300
`600
`900
`150
`75
`Chip Length/Sym (M )
`Length of Walsh Code (N W )
`4
`4
`4
`4
`4
`4
`# of Cyclic shifted (N SH )
`6
`6
`6
`6
`6
`6
`# of Sequences (N S )
`32
`8
`16
`24
`4
`2
`768
`192
`384
`576
`96
`48
`# RACH opportunities
`
`The GCL or Chu-sequence gn discussed previously can be applied as the signature sequence with length M.
`A fixed delay is applied to gn to yield the delayed signature sequence. For the example in Table 2 (a), the
`delayed sequence is
`
`(b)
`
` ,
`
`d
`
`9
`
`
`
`g
`
`
`
`g
`
`,0
`
`
`n
` ,nd
`M
`d
`(
`mod
`)
`30
`
`where the delay is M/NSH = 30. Note that the number of cyclic shifted sequences is based on the maximum
`allowed delay, which should be greater than the length of cyclic prefix of the system, and suitable for
`timing offset estimation. In Table 2 (a), the 30-information chip delay of 5MHz bandwidth equals to
`6.67us. In Table 2 (b), the 50-information chip delay is equivalent to 11.11us.
`To provide temporal spreading, a Walsh sequence of length NW is used. For example, when NW=2, the
`Walsh sequence is
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`0
`
`w
` },1,1{ w
`
`}1,1{
`1
`
`
`This sequence is applied to the signature sequence for symbol-by-symbol spreading. Then, repetition is
`applied to rate-match the preamble sequence length to 1ms.
`The Table 2-(a) example of the RACH preamble sequence is shown in Figure 6. In this case, delayed
`signature sequence g5,n with 150-chip delay is applied together with the Walsh sequence w1. Then the
`resulting sequence, made up of g5,n and –g5,n is then repeated seven times to cover 1 ms.
`
`
`
`Figure 6 RACH Preamble Example
`To mitigate inter-cell interference of RACH channel, different GCL or Chu-sequences can be used for
`different sectors/cells. For the Chu-sequence [6] with even M,
`
`
`
`2
`
`pn
`
`21
`
`2
`
`M
`
`
`
`j
`
`
`
`,1
`
`,
`
`M
`
`
`
`1
`
`
`
`g
`e
` ,
`
`n
`
`n
`different prime number p can yield different group of signature sequences. For example, when M = 300, p
`= {1,7,11,13,17,19,23,29,31,37,…}. Given a fixed p, the corresponding Chu-sequence is orthogonal when
`it is shifted circularly. However, the sequences are not orthogonal for different p and behave as random
`sequences. Thus, by assigning different p to different sector/cell, inter-cell interference can be mitigated.
`For M=300, we can select at least 20 sequences listed in Table 3. With this choice, the total number of
`RACH opportunities in the system is 400, given 20 RACH codes per signature sequences.
`Table 3 p Values of Chu-Signature Sequences for Sectors/Cells
`
`10 11 12 13 14 15 16 17 18 19 20
`9
`8
`7
`6
`5
`4
`Index 1 2 3
`p
`1 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79
`Figure 7-Figure 8 show the RACH preamble detection performance under AWGN and TU channel at 3
`km/h. The RACH preamble detection is outlined in Appendix A. In this case, BW=5 MHz, corresponding
`to a Chu-sequence of length 300. Compared to the power requirement for data transmission, it is seen that
`the transmit power of the CDM RACH preamble is significantly less (20-30 dB lower) per user. As a result,
`interference generated by the RACH preamble is expected to be insignificant for lightly loaded system.
`System studies need to be performed to study the performance of the RACH in a multi-user, multi-cell
`system.
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`Figure 7. RACH Detection Error and False Alarm Performance over AWGN Channel.
`
`
`
`Figure 8. RACH Detection Error and False Alarm Performance over TU-3km/h Channel.
`
`
`4.1. Preamble Interference
`
`There are two interference issues due to the CDM design of RACH preamble. The first issue is the
`interference to intra-cell scheduled transmission. From our simulation in Figure 7 and Figure 8, preamble
`detection can perform at SNR as low as -17dB. For an extreme scenario where all 20 RACH requests are
`sent at the same time, the combined SNR of 20 equal power RACH requests is about -4dB, provided every
`RACH requests operates at -17dB. This -4dB may slightly increase the noise floor required for scheduled
`transmission. However, the interference to scheduled transmission due to RACH preamble is insignificant.
`
`The second interference issue is the inter-cell interference due to large number of RACH requests. When a
`large number of RACH requests (greater than 20 at 1ms period) are present, different sectors/cells use
`different sets of orthogonal signature sequences. For example, there are 100 RACH request present in 1ms
`period. Let us assume that every RACH request operates at -17dB SNR, which is not likely since many of
`these 100 RACH requests are out-of-cell. The combining SNR of 100 RACH requests is 3dB. This
`number may affect performance of scheduled transmission; however, the impact due to preamble
`interference is low.
`
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`4.2. RACH Message Design
`
`To maximize capacity utilization in the uplink, RACH message transmission will be scheduled by the Node
`B on a time-frequency regions reserved specifically for RACH message transmissions. These regions are
`fixed and known beforehand so as to minimize control message overhead. The frequency, size, and
`number of these RACH messages regions will depend on system design and deployment scenarios.
`Naturally, when there is no RACH message transmission, the Node B can schedule other users in these
`regions. At the Node B, once the RACH preamble is successfully received, a 4 bit acknowledgement
`corresponding to the sequence number is transmitted to the UE. This is done even when the UE may not
`be scheduled for some time to prevent the UE from transmitting the RACH preamble again. Subsequent to
`receiving an acknowledgement, the UE monitors the downlink control channel for a period of time for
`scheduling information in order to transmit the RACH message. Due to the use of micro-sleep mode,
`power consumption from monitoring the downlink control channel is not expected to be an issue. In
`addition, the UE may already need to monitor the downlink control channel for possible downlink data
`transmission.
`
`
`
`5. Comparison of TDM/FDM and Hybrid/CDM RACH
`
`The following table compares RACH features between TDM/FDM and Hybrid/CDM RACH:
`
`
`Table 4. Comparison of TDM/FDM and Hybrid/CDM RACH (BW=5MHz).
`
`
`
`TDM/FDM RACH
`
`Hybrid/CDM RACH
`
`Number of RACH opportunities
`(per ms)
`
`Interference generated to scheduled
`users
`
`SNR requirement for 1% false alarm
`and missed detection error
`(TU 3 km/h)
`
`RACH Overhead
`(per ms)
`
`Collision Probability
`
`Inter-cell Interference
`
`System Interference Limited
`
`
`
`N
`N
`N
`N
`
`
`
`SH
`OFDM
`RB
`S
`N
`= number of
`where OFDM
`OFDM symbol reserved for
`RACH per ms
`
`N
`160
`e.g.
`1Table 1.
`
`OFDM
`
` from Table
`
`None
`
`-2 dB
`
`N
`14/
`,
`OFDM
`Minimum 7.14%
`
`Low
`
`Medium
`
`Yes
`
`Formatted: Font: 9 pt
`Formatted: Font: 9 pt
`
`N
`N
`N
`
`
`
`SH
`S
`W
`e.g. 160 opportunities from
`Table 2Table 2
`
`Small
`
`-17 dB
`
`None
`
`Low-Medium
`
`Low
`
`No (Collision limited)
`
`
`6. References
`
`1. R1-051058, “RACH Preamble Design,” Texas Instruments, RAN1#42bis, San Diego, USA, Oct. 2005.
`
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`2. R1-051033, “Further Topics on Uplink DFT-SOFDM for EUTRA,” Motorola, RAN1#42bis, San
`Diego, USA, Oct. 2005.
`3. R1-051173, “Link Comparison of Localized vs. Distributed Pilot and Localized vs. Distributed Data,”
`Texas Instruments, RAN1#42bis, San Diego, USA, Oct. 2005.
`4.
`IEEE C802.16e-04/143r1, “Ranging Improvement for 802.16e OFDMA Phy”, Motorola
`5. B. M. Popovic, “Generalized Chip-Like Polyphase Sequences with Optimum Correlation Properties,”
`IEEE Trans Inform. Theory, vol. 38, no. 4, pp1406-1490, July 1992.
`6. D. C. Chu, “Polyphase codes with good periodic correlation properties,” IEEE Trans. Inform. Theory,
`vol. 18, pp531-532, July 1972.
`
`
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`Appendix A Preamble detection algorithm at Node-B
`
`Assume an UE randomly selects a RACH preamble sequence with sequence identifier number s. The 2M
`length RACH sequence is
`
`k
`
`k
`
`w
`
`(0)
`
`g
`
`d n
`,
`
`or
`
`P
`w
`g
`gw
`[
`
` ],
`
`k
`
`M
`n
`
`,0
`,
`1
`
`
`)0(
`)1(
`
`s
` ,nd
`
`
` , Mnd
`
`where s = 2×d +k. At the receiver side of Node-B, the received signal can be represented as
`y
`x
`h
`z
`,
`  
`n
`n
`n
`n
`where  indicates circular convolution, hn is channel impulse response, and xn is either
`k
`w
`g
`.
`(1)
`
`At the receiver the circular (periodic) correlation of sequence gn and yn.is computed. This yields
`1 M
`1
`
`c
`y g
`
`
`
`m
`n
`n m
`M
`) mod
`
`M
`n
`0
`
`The correlation can be performed either in time or frequency domain. Through some simple manipulations,
`we obtain
`
`*(
`
`d n
`,
`
`c
`m
`
`
`
`30
`
`d
`
`'
`
`k
`z
`M h
`
`
` 
`m
`m
`'
`k
`z
`M h
`
`
`
`
`m
`m
`d
`30
`
`where the term zm’ is the equivalent channel noise. Usually the channel maximum delay is assumed to be
`less than the length of cyclic prefix. Here, it is assumed that the maximum channel delay is less than 30
`signal chips. For 5MHz bandwidth deployment, the length of 30 chips using current E-UTRA numerology
`equals to 6.67us.
`
`Since there are two Walsh sequence for k=0 and k=1, one can combine the nearby two blocks for both
`Walsh sequences. There are a total of 14 blocks of which one 2M RACH sequence uses two blocks. Two
`neighbor cm are added to yield 7 correlation sequences for k=0. For k=1, two neighbor cm are subtracted
`accordingly to yield another 7 correlation sequences for k=1. In the next step, we detect the delay index d,
`so that the RACH sequence identifier number s (s = 2×d +k) can be obtained.
`
`From the correlation sequence cm, when a RACH request with delay index d is present, the channel impulse
`response will appear in the [30d, 30d+30], as illustrated in Figure 9. The figure shows two RACH requests
`with sequence delay 0 and sequence delay 2. The correlation sequence cm indicates corresponding channel
`impulse response at [0, 30) and [60 90) regions. By detecting power in different regions, one can thus
`detect RACH preamble at Node-B.
`
`
`,
`
`0 1
`
`
`Figure 9 Correlation Sequence in the Presence of Two RACH Requests with Delay 0 and Delay 2
`
`
`It is possible to have a ML optimal detection of RACH request if the channel impulse response is known.
`However, usually such channel information is not available to the receiver at the Node-B. A simple
`detection algorithm is the maximum power detection. When the maximum power in a certain region is
`greater than a power threshold, a RACH request corresponding to that region is assumed.
`
`The detection algorithm has three steps. First, calculate average power of correlation sequence. This yields
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`1
`M
`1
`c
`
`m
`M
`m
`0
`The second step is to find the maximum power in all regions to obtain
`1 max |
`d
`30
`29
`
`2
`c
`.
`|
`
`
`m
`d
`P
`m
`d
`30
`
`The final step is to check whether the maximum power is greater than a pre-defined power threshold TH .
`Thus,
`
` 
`
`|
`
`2
`|
`
`.
`
`P
`
`
`
`.
`
`d d
`
`
`
` is present
`RACH request with delay
`
` 
`d
`TH
` is absent
`RACH request with delay
`
` 
`
`d
`TH
`With the detected d, and its corresponding Walsh code index k, the RACH sequence identifier number s,
`can be obtained through s = 2d +k.
`
`
`Appendix B RACH Procedure
`
`The following procedure is used by the UE to reserve and transmit data on the RACH channel:
`
`
`1. Determine available access slot and other transmission parameters.
`2. Randomly select a signature from a total of 16 signatures.
`3. Set minimum transmission power.
`4. Transmit a preamble using the selected slot, signature, and power;
`5.
`If no positive acquisition indicator is detected, increase transmission power with a new
`access slot and a new signature until the maximum number of transmission is reached.
`If positive acquisition indicator is detected, monitor the downlink control channel for a
`fixed amount of time to obtain scheduling information for the RACH message.
`Alternatively, the detection of positive acquisition indicator will tell the UE that a RACH
`message will be sent in the next time slot in the pre-defined time-frequency region.
`
`6.
`
`
`The following procedure is used by the Node B for RACH reception:
`
`
`1. Monitor the channel for the RACH preamble sequence. Note that a simple frequency
`domain correlator can be used to detect all transmitted preamble sequences
`simultaneously.
`2. For any detected RACH preamble sequence, transmit an acknowledgement with the
`preamble sequence number.
`3. When RACH resource is available, schedule the user and transmit scheduling information
`to the UE.
`
`
`
`PETITIONERS 1046-0011
`IPR2016-00758
`
`