TSG-RAN WG1 #43
`Seoul, Korea, November 7-11, 2005
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`Source:
`Title:
`Agenda Item:
`Document for:
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`
`Ericsson
`E-UTRA Random Access
`8.6
`Discussion and Decision
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`R1-051445
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`Introduction
`1.
`An orthogonal uplink, where the Node B scheduler is responsible for rapidly allocating resources among UEs
`having data for transmission, has emerged as the main candidate for E-UTRA. To maintain uplink orthogonality
`among UEs, both frequency and time synchronization of the signal transmitted from the UEs are needed.
`Frequency synchronization can be achieved by the UE locking its local oscillator to the downlink signal. The
`remaining frequency misalignment as the Node B is due to Doppler, which is the same situation as for scheduled
`data transmissions and requires no further consideration.
`Time synchronization when the UE is transmitting can be achieved by the Node B measuring on the received
`signal and sending timing advance commands to the UE, which adjusts its uplink transmission timing
`accordingly. Through the use of scheduling request, UEs known to the scheduler can request uplink resources
`and, once the request has been granted, uplink transmissions can take place.
`However, in absence of recent uplink transmissions, the Node B cannot control the transmission timing. Hence,
`there is a need for a (random access) procedure to establish synchronization with the Node B, e.g., at power-on.
`As uplink synchronization not yet has been established, a guard time is required. The random access procedure
`should, as a minimum, support time synchronization of the UE transmission timing.
`To summarize, the following two cases can be distinguished:
` Synchronized uplink transmissions, e.g., scheduling requests or user data.
` Non-synchronized uplink transmissions, i.e., random access. A guard time is required to account for
`timing uncertainties.
`Scheduling requests are transmitted using a time synchronized uplink and does in principle not differ from other
`uplink control signaling used by an active UE. Hence, scheduling requests are not further discussed in this
`contribution.
`2. Random Access
`The main purpose of the E-UTRA random access procedure is to obtain uplink time synchronization within a
`fraction of the uplink cyclic prefix. In WCDMA, the random access is non-orthogonal to the uplink data
`transmission. This provides the benefit of not having to semi-statically allocate any resources for random access,
`but requires a power ramping procedure to control the inter-UE interference. Alternatively, if the random access
`is made orthogonal to the (scheduled) data transmissions, no power ramping procedure is required to control the
`interference between random access and data transmissions, thereby allowing for a faster random access. The
`random access can be separated from data transmissions in the time domain by reserving one sub-frame at
`regular intervals for random access. This is illustrated in Figure 1, where one sub-frame per TRACH-REP period is
`allocated for random access. The value of TRACH-REP could be signaled to the UE using a broadcast channel.
`Within a RACH sub-frame, a random access burst is transmitted. A guard time of 2t = 6.7 s/km is required to
`not interfere with neighboring subframes. For a UE-NodeB distance less than 15 km, 100 s is sufficient, leaving
`approximately 400 s for the random access burst part (around 700 symbols using the numerology in the TR). In
`case of very large cell sizes, additional guard time can be obtained by ensuring the scheduler is not using the
`subsequent subframe for data transmission.
`The bandwidth of the random access should be wide enough to allow for a sufficiently accurate timing estimate.
`An estimation accuracy of less than the cyclic prefix is needed, e.g., the accuracy should be in the order of 1 s.
`This requires a bandwidth of the RACH burst of around 1 MHz, which is in line with the smallest supported E-
`UTRA spectrum allocation of 1.25 MHz. For E-UTRA deployments using larger spectrum allocations, either
`multiple 1.25 MHz random access channels can be defined (providing an increased random access capacity), or
`higher RACH bandwidth can be defined for these cases. The former has the advantage of resulting in a single
`RACH procedure, regardless of the system bandwidth, while the latter may have a diversity benefit. A third
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`ZTE/SAMSUNG 1037-0001
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`APPLE 1037
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`possibility is to use part of the spectrum in the RACH sub-frame for scheduled data transmissions, given that the
`interference from the non-synchronized RACH transmissions is acceptable.
`As a minimum, the RACH burst should contain a signature sequence identifying the random access attempt.
`Additionally, a small payload, e.g., a UE identity (if the UE has been assigned a identity from the system) or user
`data, could be included. However, the payload size may be limited to a relatively small value as the signature
`sequence and guard time occupy part f the subframe.
`
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`
`
`Frequency hopping for
`multiple attempts?
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`Can be used for other RACH channels or data transmission.
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`Data transmission
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`TRACH
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`0.5 ms sub-frame
`TRACH-REP (10 ms radio frame?)
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`(Scheduled) Data transmission(Scheduled) Data transmission
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`RACH burstRACH burst
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`Guard timeGuard time
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`BWRACH
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`Figure 1: Random access subframe.
`The signature sequences used for the random access burst should have good auto-correlation properties to
`provide good timing estimation accuracy in the Node B. The mutual cross-correlation should also be low to
`reduce the interference between users in case of simultaneous random access attempts from multiple users.
`Finally, these properties should also hold at high Doppler as E-UTRA should be functional also at very high UE
`speeds. The number of signature sequences should be kept reasonable small to reduce complexity as the Node B
`has to search for all possible signature sequences in a random access subframe.
`The same structure and principles can be used for both FDD and TDD.
`
`2.1. UE procedure
`A UE performing a random access randomly selects one signature sequence from a small set of sequences to use
`in the RACH subframe. The UE transmission timing (i.e., assumed subframe start) can be obtained from the
`downlink timing. Open-loop power control can be used to obtain a suitable transmission power.
`Once the random access burst has been transmitted, the UE monitors the appropriate downlink control channel
`for response from the Node B. It may be possible to reuse the downlink control channel normally used for
`scheduling assignments (“HS-SCCH-like”) in which case no additional downlink control channels are required.
`As a minimum, the downlink control signaling includes information for timing control of the UE. Additionally, it
`may also contain a resource reservation (“scheduling grant”), providing the UE with resources for transmission
`of the payload. If no resource reservation information is included, a subsequent scheduling request can be used
`instead.
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`Finally, if the UE does not receive a response from the Node B within a certain time interval, a new random
`access attempt can be performed at a subsequent RACH sub-frame, possibly after a random back-off time. In
`case of multiple RACH sub-channels are defined on separate frequencies, a different frequency may be used for
`different attempts to avoid being stuck in a deep fading dip.
`
`2.2. Node B procedure
`The Node B correlates the received signal in the RACH subframe with all possible signature sequences. Once
`the Node B detects a sufficiently strong correlation peak, the timing of the (unidentified) UE performing the
`random access is known. In response, the Node B sends a timing adjustment command on the downlink control
`channel, possibly along with a resource assignment. The identity included on the downlink control channel is
`linked to the identity of the identified signature sequence in the uplink (or the UE ID if included), thus indicating
`in response to which random access attempt the downlink control signaling relates.
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`2.3. Example
`An example of a random access procedure followed by regular scheduled data transmission is found in Figure 2.
`In this example, if the UE has no data to transmit, dummy data (e.g., an empty scheduling request) is transmitted
`at regular intervals to maintain uplink synchronization. This allows for rapid transmission of a scheduling
`request and a corresponding resource assignment. When no user data has been transmitted for a certain time, the
`transmission of dummy data stops and the uplink is allowed to go out-of-sync.
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`
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`Resource Request
`Tim i ng A dj us t m ent
`
`ming Request
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`Ti
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`S c hedul i ng G ra nt
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`Resource Request
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`User data or RRC signaling
`Tim i ng A dj . , Res ou rc e Gra nt
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`Random Access
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`Unsynchonized
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`Uplink synchronized
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`Unsynchonized
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`Random
`Access
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`Scheduled Data
`Transmission
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`Transmission of dummy
`data to maintain
`synchronization (optional,
`in absence of user data)
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`Figure 2: Example of random access and scheduled data transmission.
`3. Conclusion
`A framework for E-UTRA random access has been outlined:
` Orthogonal separation of user data and random access
` Using the random access burst for timing estimation at the Node B
` Responding with a timing adjustment and (optionally) a resource assignment.
` It is proposed to include the text proposal in Section 4 in TR 25.814.
`4. Text Proposal
`--- Begin text Proposal ---
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`ZTE/SAMSUNG 1037-0003
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`Random access procedure
`9.1.2.1
`The random access procedure is used when the UE uplink has not been time synchronized and shall allow the
`Node B to estimate, and if needed adjust, the UE transmission timing within a fraction of the cyclic prefix. The
`random access burst consists of at least a signature sequence. Inclusion of additional data symbols is FFS.
`Random access and data transmission are time multiplexed as illustrated in Figure X, where certain subframes
`are reserved for random access transmissions.
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`
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`Data transmission
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`BWRACH
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`TRACH
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`sub-frame
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`TRACH-REP
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`
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`(Scheduled) Data transmission(Scheduled) Data transmission
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`
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`RACH burstRACH burst
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`
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`Guard timeGuard time
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`Figure X: Reserving a subframe for random access attempts.
`--- End text Proposal ---
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`ZTE/SAMSUNG 1037-0004
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