RACH Procedures

RACH : Random Access Channel
Done by: Issa Haidar
Supervised by : Dr. Hani el Fawal Date : 13-4-2017
1/30

• RACH: Random access channel
• Why RACH in uplink?
• RACH procedures
• Random Access Slots
• What is a Preamble?
• Preamble Structure
• Preamble format
• Zadoff–Chu (ZC) sequence
• 64 Preamble generation
• Why only 64?
• Preamble timing and Preamble type Determination
• RACH Contention based procedure
• Collisions in contention based procedure
• RACH PROCEDURE OVERLOAD PROBLEM DESCRIPTION
• LTE-A NETWORK ACCESS METHODS AND RESEARCH CHALLENGE
• RACH OVERLOAD CONTROL MECHANISMS.
Contents
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• What is RACH?!
The Random Access Channel (RACH) is an uplink channel in mobile communication
system that is used to transfer control information from a mobile terminal to the
network( Not to carrry data).
• Rach Purpose:
1. To get uplink Synchronization.
2. To Obtain the resource for Message 3
(e.g, RRC Connection Request)
RACH : Random Access Channel
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• Why we don’t use RACH in Downlink!?
• In LTE, the synchronization in downlink achieved by
the special synchronization channel .This downlink
sync signal gets broadcasted to everybody and it is
get transmitted all the time with a certain interval.
• Synchronization in Uplink :
• Broadcasting /always-on synchronization
mechanism is not efficient (actually waste of energy
and causing a lot of interference to other UEs) if UE
is using this kind of broadcasting/always-on
synchronization mechanism.
Why RACH In Uplink? 2/274/30

• Contention free procedure is under the full control of the eNodeB in order to
avoid delayed-constrained access requests therefore M2M devices will have no
effect on such traffic.
RACH Procedures
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• Occurs in Connected Mode:
i. Handover Process
ii. Radio Link Failure
• Occurs in idle mode when UE
doesn’t have uplink resources
yet
- Initial access
5/30

RACH Procedure 4/27
• In Rach , the UE should choose a unique signiture( Preamble )randomly from
the 64 Preambles that are provided by the EnodeB in order to allow this UE to
get access to the network.
• RA preamble is sent in one Random Access (RA) slot.
• Because of the random choosing and the
huge number of UEs that will try to get access,
a possibility may occur and two UEs may take
the same PREAMBLE which will result one of
the two UEs to have failure in connection,
this issue is solved in Contention based procedure.
6/30

• Random Acces channel is divided into Random Access slots ( RA slots).
• These RA slots are periodic time-frequency resources in the uplink channel which are
reserved to allow the UE to send its access requests ( preamble).
• Devices randomly select among these RA slots opportunities to transmit the preamble
• Each RA slot consists of 6 Rbs in frequency domain with 1.08MHz bandwidth (180 KHz x
6) and a duration of 1/2/3 ms in time domain depending on the preamble format.
.
Random Access Slots 5/277/30

• The RA-slots for PRACH are multiplexed with the Physical Uplink Shared
Channel (PUSCH) and the Physical Uplink Control Channel (PUCCH) as
illustrated in the figure.
Random Access Slots 6/278/30

• Preamble: is a signature used by devices to request access to the network.
• The random access preamble is generated using Zadoff-Chu sequences; From each root sequence,
multiple preambles can be obtained by applying different cyclic shifts.
• If all the preambles are generated from the same root sequence they are orthogonal to each other.
In such case, different preambles from multiple M2M Devices can be detected by the eNodeB when
they are transmitted in the same cell at the same time.
Preamble? 7/27
i. Handover Process
• Occurs in idle mode when UE
yet
- Initial access
9/30

• A Zadoff–Chu (ZC) sequence, a spreading code functions and complex valued
mathematical sequence(It is a sequence of special number) which they are generated by
a specific formula in which it generates a family of those sequences.
• A generated Zadoff–Chu sequence that has not been shifted is known as a "root
sequence.
• Each sequence contains 31 terms
• Property 1: A ZC sequence has constant amplitude. The constant amplitude property
limits the Peak-to-Average Power Ratio (PAPR). In addition. Constant amplitude is
desired over wireless channel as it places lesser strain on the power amplifier for
transmission.
• Property 2: Zero autocorrelation for cyclic shifts means the signals are orthogonal in
time. Orthogonality is desired so as to minimize interference between users. Zero auto
correlation means the correlative variance between the users is zero, so no interaction
or interference between the users.
Zadoff–Chu (ZC) sequence, 8/2710/30

• Because of the non-synchronization between the uplink UEs; in the time domain, the
cyclic prefix (CP) and guard time (GT) are used to avoid interference with the previous
and next subframes.
Preamble Structure
9/2711/30

• Preamble Length in Frequency Domain :The
preamble has 893 subcarriers with spacing of
1.25KHz instead of 15KHz) (1.25* 839=1.05MHz)
.
• There will be 15KHz guard band on both the
sides and hence it uses total of 1.08MHz (equal
to 6 RBs).
• Preamble Length in Time Domain including
Guard Time (= CP Length + SEQUENCY Length +
GT Length) can be 1ms or 2ms or 3ms depending
on Preamble Format.
• There is a max. of 1 random access preamble per
a subframe but more than one UEs can use it.
Multiple UEs using same preamble resource
allocations are differentiated by their unique
preamble sequences.
Preamble Structure
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10/2712/30

• Preamble has 4 formats and each of these formats varies in length . (
1ms/2ms/3ms/4ms)
Preamble Formats
11/27
i. Handover Process
13/30

• We have multiple Preamble formats because it’s related to the Cell size:
• The larger the cell-size, the longer the preamble to ensure its proper reception.
• The longer Time sequence (T_SEQ) would help decoding PRACH under noised condition because it
provide longer correlation window to detect PRACH.
• Longer CP would give you better tolerance in fading environment and reducing Inter Symbol
Interference even in highly fading environment.
Preamble Formats
12/27
i. Handover Process
ii. Radio Link Failure• Occurs in idle mode when UE
yet
- Initial access
14/30

• EXAMPLE:
• Let's suppose SIB2 broadcast the parameters as follows:
rootSequenceindex = 22
Highspeedflag = false
zeroCorrelationZoneConfig = 5
• This rootSequenceIndex is a logical value. The real number (physical number) is called 'u' (the
rootSequenceIndex is mapped to u by using a table.
• After finding the rootSequenceIndex and mapping it to “u”: you will get the base Zaddoff-Chu
sequence with u = 1 .
• After some complicated calculations: you will get the Ncs (Cyclicshift interval) = 26
• Now you can get the 64 different PRACH sequence as follows:
•
• PRACH Sequence[0] = base sequence
• PRACH Sequence[1] = do cyclic shift to base sequence by 1 * 26 samples
•
• PRACH Sequence[32] = do cyclic shift to base sequence +1
• PRACH Sequence[33] = do cyclic shift to base sequence +1 by 1 * 26 samples
• PRACH Sequence[34] = do cyclic shift to base sequence +1 by 2 * 26 samples
• PRACH Sequence[63] = do cyclic shift to base sequence+1 by 31 * 26 samples
64 Preambles Generation 13/2715/30

• Answer: With one rootsequenceindex UE
can able to generate MAX of 64 preambles
by doing simple cyclic shift. If it want to
generate more than that preambles it is
complex to generate preamble sequence
,that is the reason it is restricted to 64.
WHY only 64 Preambles?! 14/2716/30

1. How UE know which Preamble format it has to use when it generates PRACH and
transmit?
2. Exactly how many RA Slots (Preambles) should be generated per frame?
3. At which time( When) and at which frame and sub frame(Where) a UE is allowed to
transmit Preambles?
• They are determined by a SIB2 parameter called “prach-ConfigIndex”.
• In total, there are 64 possible configurations we will see in the next slide.
Preamble timing and Preamble type Determination 15/2717/30

• There are, at most, 10 RA slots per LTE frame, one ( RA slot every millisecond). And at least 1 RA
resource every 2 LTE frames ( 1 RA slot every 20 ms).
Preamble timing and Preamble type Determination
Massive Access in the Random Access Channel of LTE for M2M Communications
17/27
Sub frame Sub frame
18/30

Preamble timing and Preamble type Determination 16/27
• SFN and subframe Numbers:
• The transmitter and receiver should operates in
synchronized mode.
• In LTE , the clock synchronization between the transmitter
and the receiver is the same as our analog clock.
• It has two arms:
1. SFN : System Frame Number , is the numbers between 0
and 1023 and its ticks every 10 ms.
2. Sub Frame Number: are the numbers between 0 and 9
which ticks every ticks every 1 ms.
Before the transmitter (eNodeB) and the receiver (UE) in LTE
start communicating each of other they have to set these two
clock arms to be the same number and this synchronization
happens during cell search and timing synch process
19/30

• Many UE may choose the same preamble that will result in a conflict between them. The RACH
procedure has some steps in order to avoid this to happen:
1. Random Access preamble assignment: A UE chooses one RA slot from 64 preambles that are
provided by the EnodeB. ( 2 users may take the same Preamble but we will see how it will be
solved in step4).
2. Random access Response (RAR): A message response from the EnodeB indicating in it the
configurations of the RA slot (time/frequency slot, scheduling). When an UE receives these
configurations, it synchronizes its uplink timing and proceeds to the next step.
3. RRC Message: The UE sends a RRC message to EnodeB in order to establish a connection.
4. Contentions Resolution: If the RCC is successuly decoded by the eNodeB, it replies with a
contention-resolution message on the PDSCH, echoing the mobile terminal identity (mobile ID) in
order to acknowledge the correct reception of its request. Conversely, UEs that do not receive a
contention resolution message from the eNodeB assume that their access requests failed and,
after waiting a random backoff time, perform a new preamble transmission attempt in the next RA
slot.
RACH Contention Based Procedure 18/27
RACH Overload Congestion Mechanism for M2M Communication in LTE-A: Issues and Approaches
20/30

• Collision at Step1: If two or more devices transmit the same preamble, a collision
may occurs and the corresponding requests will not be detected by the eNodeB.
Collisions in RACH 19/27
The challenges of M2M massive access in wireless cellular networks
21/30

• Step1: UEs choose the same preamble and the eNodeB correctly Detects it.
• Step2: EnodeB transmits a Random access response on the PDSCH which includes timing alignment
and the PUSCH resources that have been assigned to the UE for the third step RRC connection
• Step3: The UEs involved in an undetected preamble collision will transmit over the same PUSCH
resource blocks, therefore a new collision will occur and they have to wait for some period and
resend the RACH again.
Collision at step 3 20/27
The challenges of M2M massive access in wireless cellular networks
22/30

• The RA success probability: which is the
probability that a preamble sequence does not
collide with another preamble sequence; it’s
determined by the number of available
preamble sequences and the terminal density in
a cell.
• Problem: Despite of the small packets that are
sent from the M2M devices; the Massive
number of packets sent may result in:
1. low random access success rate and a high
congestion.
2. Unexpected delay, increased packet loss,
and high power consumption.
3. inefficient utilization of radio resources.
4. Huge Load when the increased number of
devices will attempt to re-access the
network after collisions.
• Result: the RA procedure in LTE-A will not be
efficient in the existence of the massive number
of M2M devices and efficient overload control
mechanisms are required for RA-based M2M
communication.
RACH PROCEDURE OVERLOAD PROBLEM DESCRIPTION 21/27
Random Access for Machine-to-Machine Communication in LTE-Advanced Networks: Issues and Approaches
23/30

LTE-A NETWORK ACCESS METHODS AND RESEARCH CHALLENGE 22/27
• LTE-A Network Access Methods:
European Telecommunications Standards Institute (ETSI) addressed three methods of
how to access the LTE-A network:
1. Direct Access: This method will allow both M2M and H2H devices to access the network
via eNodeB without the need of any intermediate device which may result in congestion
problems when there are no enough resources for the massive number of M2M devices,
therefore this access is not suitable.
24/30

2. Gateway Access: Adding an intermediate device to the LTE infrastructure which can help
in managing the high volume data between the eNodeB and M2M devices. This type of
access worth studying especially when we want to apply the QOS for H2H and M2M
traffic.
LTE-A NETWORK ACCESS METHODS AND RESEARCH CHALLENGE23/27
25/30

3. Coordinator Access: In this type of access,
the M2M devices will be collected in one
group and sent to the eNodeB which will
result in reducing the signaling messages.
One M2M device will be selected to play
the role of temporary M2M gateway for
collecting the adjacent devices. Here the
challenging task is to develop an adaptive
algorithm for preamble allocation which
improves the overall network performance
(e.g., "Clustering Techniques“)
LTE-A NETWORK ACCESS METHODS AND RESEARCH CHALLENGE24/27
26/30

Research challenges:
1. PRACH overload control (Massive number of devices influence on RACH
mechanism):massive access requests by M2m devices can overload the PRACH, yielding
an increase of the contention probability and, in turn, of the access delay and failure
rate.
2. Quality of service (QoS) provisioning: Both H2H and MTC devices will contend for their
data transmission channel using the same set of PRACHs. However, the MTC devices
should not collide with H2H devices and at the same time need to satisfy their own QoS
requirements.
3. Tackling energy efficiency : Group management and addressing (i.e., selection of a
group coordinator) of MTC devices could be vital for M2M communication. Therefore we
will have in such mechanisms a low power consumption because it allow multiple
devices data to be aggregated and sent all together.
4. LTE-A inefficient radio resource utilization: M2M devices are made to send smaller
packets than the H2H devices, therefore granting a one PRB for one M2M device will not
be efficient and it will result in overload congestion and wasting of resources.
LTE-A NETWORK ACCESS METHODS AND RESEARCH CHALLENGE25/2727/30

1. Access Class Barring (ACB) scheme: All UEs are members of one out of ten randomly
allocated mobile populations, defined as Access Classes (AC) 0 to 9. The population
number is stored in the SIM/USIM.
a. AC"0" to AC"9”: Represents Normal devices.
b. AC"10”: Represents Emergency calls.
c. AC"11" to AC"15": Represents specific high priority services.
2. RACH resource separation scheme: two approaches for resources allocation:
• Available RA Preambles are divided into two groups:
a. First Group: for M2M devices.
b. Second Group: for H2H devices.
• Available RA Preambles are divided into two groups:
a. First Group: for M2M devices.
b. Second Group: for M2M and H2H devices.
RACH OVERLOAD CONTROL MECHANISMS. 26/2728/30

3. Dynamic resource allocation: The Resources are allocated between both H2H and M2M
devices in a shared dynamic mechanism based on access requests (i.e. No dedication for
any resources).
4. Pull-based scheme: This is a centralize mechanism which let the EnodeB to control the
M2M devices based on the congestion and resources available. The M2M devices
initiates their RA requests when they receive a paging message from the EnodeB.
5. M2M-specific back off scheme: The backoff scheme counteracts the PRACH overload by
distributing different certain delays on RA attempts based on the device type.
6. Grouping or clustering scheme: UEs are grouped based on QoS applications or based on
their geographical locations. One device will be the coordinator of one group which will
play the role of relay agent and communicate with their EnodeB.
RACH OVERLOAD CONTROL MECHANISMS. 27/2729/30

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