Resource Management in Satellite Networks part 15 pot

Physical Channel Direction DescriptionDPDCH Both Carries the DCH transport channel DPCCH Both Layer 1 control information for DPDCH PRACH Uplink Carries the RACH transport channel P-CPIC

Physical Channel Direction Description

DPDCH Both Carries the DCH transport channel DPCCH Both Layer 1 control information for DPDCH PRACH Uplink Carries the RACH transport channel P-CPICH Downlink Phase reference for downlink channels S-CPICH Downlink Phase reference for dedicated downlink channels P-CCPCH Downlink Carries the BCH transport channel S-CCPCH Downlink Carries the FACH and PCH transport channels SCH Downlink Synchronization (spot search)

AICH Downlink Acquisition indicators (random access results)

Physical Channels

DPDCH Dedicated Physical Data Channel

DPCCH Dedicated Physical Control Channel

PRACH Physical Random Access Channel

P-CPICH Primary Common Pilot Channel

S-CPICH Secondary Common Pilot Channel

P-CCPCH Primary Common Control Physical Channel

S-CCPCH Secondary Common Control Physical Channel

SCH Synchronization Channel

AICH Acquisition Indicator Channel

PICH Paging Indicator Channel

MICH MBMS Indicator Channel

Transport Channels

DCH Dedicated Channel RACH Random Access Channel BCH Broadcast Channel FACH Forward Access Channel PCH Paging Channel

Table 5.1: Transport and physical channels.

supported in the satellite air interface RACH is characterized by open-loop power control and a collision risk in every transmission RACH is crucial for the operation of the UMTS air interface, since it is used not only for initial channel access to the network (e.g., call origination, paging response

and registration messages), but also for sending short data bursts (e.g., Short Message Service, SMS), as investigated in the following simulative study.

In 3GPP specifications [6], the PRACH transmission is based on a Slotted-ALOHA (S-Slotted-ALOHA) approach with fast acquisition indication The User Equipment (UE) can start the random access procedure at the beginning

of a number of a well-defined time intervals, called access slots, by sending

Fig 5.1: Structure of the message transmission on RACH.

To construct the preamble, the UE uses two components: the preamble

scrambling code (there are 8192 such codes available) and the preamble signature code (16 signatures to choose from, obtained as a repetition of a

Hadamard codeword) These codes, sequences of chips with values +1 or−1,

are combined to determine the complex preamble transmission code More details can be found in [4]

The 10 ms message is split into 15 slots, each of 2560 chips (each slot of these has half duration with respect to access slots) The message consists

of two parts: the data part and the control part, which are transmitted simultaneously (see Figure 5.2) using different channelization (spreading) codes that both depend on the signature used to construct the preamble part

The control part has a Spreading Factor (SF) of 256 and the data part can

have different spreading factors in the set{32, 64, 128, 256} The content of

the data bits depends on the higher layers The 8 pilot bits of the control part are used to support channel estimation for coherent detection and the

Transport Format Combination Indication (TFCI) bits are used to indicate

the spreading factor and the format of the data part

Access Service Class (ASC) represents a certain PRACH partition (i.e.,

sub-channels and signature codes, as explained below) and an associated access persistency value (i.e., a probabilistic check to determine whether a preamble transmission can be attempted in the current access frame) There

Fig 5.2: Structure of the RACH message part (slots are here shorter than the

access slots; in this case, a slot contains 2560 chips so that 15 slots correspond to 10 ms)

are 8 ASCs, numbered from 0 (highest access priority) to 7 (the lowest access priority) [8] ASC 0 shall be used for emergency calls A PRACH sub-channel defines a sub-set of the access slots There are a total of 12 sub-channels Typically, every 8 frames the allocation pattern of the different access slots

to the different sub-channels repeats The higher layers communicate to the physical layer the available signatures and sub-channel groups for each ASC There are at most 16 PRACH channels per cell; each of them corresponds

to a different preamble scrambling code On a given access slot of a PRACH,

up to 16 simultaneous transmissions are possible by using distinct (orthogonal) signatures codes A PRACH channel is defined by the following parameters: preamble scrambling code, spreading factor for data part, available signatures for each ASC, available sub-channels (i.e., slots) for each ASC and power control information Available sub-channels and signature codes are broadcast through the BCH channel When there is data to be transmitted, the UE performs PRACH selection randomly Then, MAC selects the appropriate ASC for the traffic type to be managed Consequently, an access slot and a signature are randomly selected among those available for the selected ASC

In the PRACH access mechanism, the main difference with respect to the classical S-ALOHA system is that, besides the time of the transmission, the

UE also randomly chooses the signature and the scrambling code that will be used to transmit the preamble

Once the preamble is sent, the UE waits for an acquisition indication

(a sort of acknowledgment message) sent by the Node-B on the Acquisition Indicator Channel (AICH), a downlink physical channel that is received in

the entire cell or part of the cell in case of sectorization This transmission

one-to-one mapped to the scrambling code used for the preamble.

The remainder of this sub-Section is devoted to the performance evalu-ation of RACH in a GEO bent-pipe scenario A C++ simulator has been implemented with a slightly simplified access procedure with respect to that

in Figure 5.3 (i.e., no power ramping has been considered; only one PRACH has been simulated) We refer to a GEO bent-pipe satellite scenario, where the Node-B that manages the RACH protocol is on the Earth: the UE exchanges messages with the Node-B via the GEO satellite In this study the Earth station provides a feedback to the UE about its transmission attempts Hence, there is a round-trip propagation delay of about 560 ms to know the outcome

of this transmission (τ pa timer has been set accounting for such propagation delay)

In order to evaluate whether the access attempt has been successful or not,

we have to consider collision events and the uplink interference conditions typical of CDMA transmissions An access (i.e., preamble transmission) is considered successful if the following conditions are fulfilled [9]:

1 No other UE selects the same access slot and the same signature code on the same PRACH (otherwise there is a collision event; the capture effect

is not considered in this case)

2 The received Signal-to-Interference Ratio (SIR) at the satellite exceeds a given threshold, SIR t

The above SIR issues (point 2) can be taken into account in the access

phase by assuming a maximum number of transmissions (MaxUE) that can

be tolerated in the same access slot for interference reasons Hence, when

there are n concurrent access attempts with n > MaxUE, there is a too high interference level (i.e., SIR < SIR t ) so that all n transmission attempts (using

different signature codes) are unsuccessful We can consider that MaxUE is proportional to 1

SIR t The simulator numerical settings are detailed below:

Fig 5.3: Random access process on the PRACH channel (PRC, Power Ramping

Control, denotes a mechanism to increase the transmission power of the access burst

in subsequent attempts)

• We have considered a GEO bent-pipe satellite scenario with round-trip

propagation delay of 560 ms

• Only one PRACH has been simulated (i.e., one scrambling code is used).

• We consider two different cases for the interference conditions concerning

the preamble transmission: MaxUE = 6 (mild interference conditions) and MaxUE = 3 (severe interference conditions) More appropriate MaxUE values could be determined with a complex analysis of the interference con-ditions deriving from the simultaneous transmissions of different preambles

on the same access slot with different signature codes and the same scrambling code Such a study is beyond the scope of this work

• There are 10 sources per ASC The OFF state sojourn time is exponentially distributed with mean message arrival rate denoted with λ As soon as

the source leaves the OFF state, a procedure is started to transmit a 10

ms message

• After the successful transmission of the preamble, message transmission

requests are served according to the priority order of the related ASC A

‘virtual’ message transmission queue corresponds to a PRACH (messages from ASC0 are prioritized with respect to ASC1, etc.) These message transmissions use a suitably shifted scrambling code with respect to the scrambling code of the preamble transmission that also combines this code with a signature code We neglect interference between simultaneous message and preamble transmissions related to the same PRACH Hence, preamble transmissions and message transmissions use separated resource spaces Of course the message part can be received at the Node-B with

errors according to a certain Frame Error Rate (FER) value.

• Simulation runs have a duration of 500 s.

We evaluate through simulations both the mean preamble delay (from

the arrival of the message for the S-RACH transmission to the instant when the terminal receives the acknowledgment -AICH message- that the random

access is successful) and the mean message delay (from the instant when

the AICH message is received to the instant when the message transmission

completes) The total mean message delay (from message arrival to message

transmission) is the sum of the two above mean delay components Results are shown in Figure 5.4 considering both the cases MaxUE = 6 and MaxUE =

3 The ideal preamble delay (lower bound) only contains a frame duration and a round trip delay As expected, the mean preamble delay increases

with the mean arrival rate λ and reduces with the MaxUE value Moreover,

the mean preamble delay increases from ASC0 to ASC1 and to ASC2 (i.e., the higher priority ASC0 permits to achieve lower mean preamble delay values) As expected, the mean message delay increases with the mean arrival

rate λ and is practically insensitive to the MaxUE value variation (the

message transmission on PRACH can be described as a simple queuing system -M/D/1-like queue with state-dependent arrival rate- with no interference with preamble transmissions, as previously assumed) Moreover, the mean message delay for ASC0 is lower than that for ASC1 that, in turn, is lower than that for ASC2

Note that for all the ASCs, the mean preamble values are not so different, thus proving the robustness of the preamble access protocol: the time-code space is a sufficiently wide resource space also for the ASCs with lower number

of assigned sub-channels The random access scheme for preamble, based on different sub-channels and signature codes, has an intrinsic stability since

it uses a form of special capture effect due to the codes In addition to this, the mechanism that a source in the ON state cannot generate a new message, allows reducing the load of random preamble attempts and the load

of messages to be transmitted on the PRACH ‘virtual’ queue This mechanism further provides stability to both the random access phase and the subsequent message transmission queue

As a final consideration, we may note that these results prove that the total message delay is high in a GEO bent-pipe scenario A possible improvement has been proposed for the GEO satellite case in [9] where the message transmission immediately follows the preamble transmission

Fig 5.4: PRACH performance in the presence of traffic on three ASCs with

differently allocated resources and two cases for MaxUE values

slots of duration T s that are grouped into frames of duration T f = T s N Each slot has two states: available and reserved.

The figure below shows the state diagram for a UT in the simplest case where a UT is allowed to reserve only one slot at a time [more complex state diagrams result when more than one reserved slot per UT is allowed in a

frame: N −1 or 2(N −1) states are added, depending on the mechanism used

to reserve additional slots] The UT starts in the silent state When a talkspurt begins, the UT moves to the contending state where it attempts to reserve

one slot in order to transmit the voice data Random access transmissions are only allowed in available slots and occur according to a permission probability scheme We assume that a UT monitors the state of the slots (using a downlink control channel) and therefore knows which of them are available

If a transmission is correctly received by the base station (no collisions), the transmitting UT is notified via a downlink control channel (this channel is often broadcast and can be used by UTs for slot state monitoring) In this case,

the UT moves to the active state and the slot becomes reserved This means

that only the reserving UT is allowed to transmit in that slot in subsequent frames When the talkspurt ends, the UT releases the slot by sending a special signal and moves again to the silent state while the slot becomes available If the random access burst is not correctly received, usually due to a collision with other UTs that transmit their random access burst in the same slot, the base station informs the UTs that a collision has occurred and the UT remains in the contending state and schedules a retransmission attempt The

UT behavior in the access phase is depicted in the diagram in Figure 5.5 During the access phase, a packet can be dropped (front-end clipping

phenomenon): if the voice packet transmission delay D (i.e., the time between

the packet generation and the packet successful transmission) exceeds a

certain limit (30-40 ms), D max, the packet is dropped and the UT will attempt to transmit the next one following the same procedure Of course, the probability that a packet gets dropped is an important performance parameter and must be kept very low (lower than 1%) for guaranteeing a good voice

Fig 5.5: State diagram of the PRMA protocol.

quality

As shown in [10], PRMA outperforms the classical S-ALOHA protocol in terms of packet dropping probability and is therefore more preferable It is also flexible enough to accommodate data and voice traffic Moreover, there have been proposals where a UT can reserve more than one slot per frame

to accommodate more demanding real-time traffic There are certain issues however that are critical for PRMA performance, some of them are even more important in the case where it is used for satellite systems These issues are:

• Frame and slot duration, channel bandwidth and voice codecs In our

discussion above, we mentioned that in order to transmit a talkspurt the

UT reserves one slot per frame This assumes that the channel bandwidth, the slot and frame durations and the codec used must be coordinated in order to receive the required voice quality at the receiver This means that

if the channel bit-rate is R c and the codec voice bit-rate is R s, then the maximum number of slots per frame is

Nmax= T f R c

R s T f + L h

(5.1)

where L h is the length of each packet header

• Scheduling retransmissions and resolving collisions We can assume that

as soon as a talkspurt begins, the UT selects the next available slot to transmit the random access burst in order to make a reservation as soon as possible If there is a collision and all UTs that participated in the collision select another available slot in deterministic manner (e.g., they all select the next available slot), then they will enter a collision deadlock since all of them will select exactly the same slot to transmit To avoid such deadlocks,

a probabilistic collision resolution mechanism must be employed In the

simplest case, each UT may decide to transmit with a probability p, known

to RTD In other implementations, the base station does not reply after

a failed transmission and the UTs assume that they failed after not receiving a response within a given timeout This means that in every transmission (or retransmission) the round trip delay is directly added

to the contention phase While this is not an issue in terrestrial systems with very small RTD values, it is quite critical for satellite systems To

cope with this problem, a modified PRMA protocol, called PRMA with Hindering States (PRMA-HS) has been proposed in [11] In this PRMA

version, the UT employs a more aggressive behavior in the contending state

by continuously reattempting random access transmissions during RTD, without stopping for waiting the base station reply It has been proved that while this approach increases the contention load with possibly useless re-transmissions, it still outperforms the classical PRMA scheme in mobile satellite systems

• Available slots versus collision probability In the classical PRMA protocol,

we described above, the number of available slots (i.e., the number of unreserved slots that are available for contention) is variable This means that as more slots become reserved the probability that two or more UTs transmit their random access bursts in the same available slot (collision probability) increases There are cases where this phenomenon is not desirable Therefore, there have been proposals in which a separate channel

is used for contention (for example, this channel may simply consist of

a certain amount of minislots in a reserved portion of the frame, thus significantly reducing the variations on the collision probability There is obviously a trade-off here, as these contention-dedicated resources may cause a waste of bandwidth

5.2.3 Adopting PRMA-like schemes in S-UMTS

GEO systems cannot adopt PRMA since their long RTD (max 280 ms in the case of a regenerating satellite; max 560 ms for a bent-pipe satellite)