Xem mẫu
Multiple Access Protocols for Mobile Communications: GPRS, UMTS and Beyond Alex Brand, Hamid Aghvami
Copyright 2002 John Wiley & Sons Ltd ISBNs: 0-471-49877-7 (Hardback); 0-470-84622-4 (Electronic)
6 MULTIDIMENSIONAL PRMA
The PRMA protocol extended for operation on a hybrid CDMA/TDMA air interface is defined in the following. This extended version of PRMA is referred to as multidimen-sional PRMA or MD PRMA. First, the basic protocol suitable for frequency-division duplexing is described. Then, the implications of different approaches to time-division duplexing on the protocol operation will be discussed. Finally, the two investigated approaches to access control, namely load-based access control and backlog-based access control, will be introduced. Before tackling the main issues of interest, a little digres-sion is required to discuss the terminology used in conjunction with the research efforts presented here, or more precisely, the names used in previous publications when referring to this PRMA-based protocol.
6.1 A Word on Terminology
The following comments are provided to avoid potential confusion when looking at some of our earlier publications, since although the investigations documented in the next few chapters on MAC strategies are centred fundamentally on one protocol, this protocol has evolved over time, and so did the names we used when referring to it.
Initially, the protocol was referred to as the Joint CDMA/PRMA protocol in Refer-ences [28–31], where random coding was considered, single time-slots could carry several packets, but individual code-slots were not discerned. In References [48] and [49] we suggested a protocol for operation on a rectangular grid of resource units, where the basic unit would normally be a code-time-slot, but it could also be a frequency-time-slot if the protocol were to be used with a hybrid FDMA/TDMA multiple access scheme. Apart from the different channel models considered, as discussed in detail in the previous chapter, and a different approach to channel access control, the protocol is essentially the same as Joint CDMA/PRMA, but since the focus was extended to hybrid FDMA/TDMA, a new ‘umbrella name’ was required.
Multidimensional PRMA (MD PRMA) was chosen as a name, with reference to the fact that resource units are defined in two dimensions rather than only one, and could in theory even be defined in three dimensions, when using FDMA, CDMA and TDMA all together. With a few exceptions, this is the only term which will be used in the following. While ‘MD PRMA’ does not specify which multiple access scheme is being used, the focus will be on CDMA/TDMA in the next few chapters. In the context of enhancements to EGPRS, the FDMA/TDMA version of the protocol could also be interesting, as discussed in Chapter 11.
258 6 MULTIDIMENSIONAL PRMA
6.2 Description of MD PRMA
6.2.1 Some Fundamental Considerations and Assumptions
In a cellular communications system, a certain amount of the downlink resources available in a cell will have to be reserved for signalling channels, which require resource units at regular intervals. These may be synchronisation or pilot channels, broadcast channels carrying system information, and common control channels, as known from 2G systems. Since traffic is normally symmetric or downlink biased, but rarely uplink biased, it is possible in FDD systems to reserve the corresponding resource units on the uplink as well, without wasting capacity. This resource could, for instance, be used to provide some guaranteed random access capacity for high-priority users or initial access purposes1.
If both ‘circuit-switched’ and ‘packet-switched’ transmission modes are to be supported over the air interface, a common pool of physical resources should be shared, to enable efficient system operation. ‘Circuit-switched traffic’ (or rather: traffic carried on dedicated channels) can coexist without problems with ‘packet-switched traffic’ (traffic carried on shared or common channels) supported by MD PRMA. If a circuit is set up, one of the resource units will simply have to be reserved on a per-call basis rather than a per-packet-spurt basis. During the lifetime of the call, this resource unit will not be available for packet-switched traffic.
For the MD PRMA results reported in the next few chapters, the interest is exclusively in services supported on ‘packet-switched’ bearers. All considered terminals are already admitted to the system, such that initial access procedures need not be studied. Guaranteed random access capacity is not provided, and it is assumed that all the resources in a cell are available for MD PRMA operation.
6.2.2 The Channel Structure Considered
As in conventional PRMA [8], N time-slots of fixed length are grouped into frames (or TDMA frames, to distinguish them from voice frames). Depending on the context, a particular time-slot may either be specified using discrete time t (starting from t = 0, with unit increments for each time-slot), or by the time-slot number ns (from 1 to N) together with the frame number nf , where ns = (t modulo N) + 1. In the case of the physical layer model with code-time-slots described in Sections 5.3 and 5.4, each time-slot is subdivided into E code-slots, such that the basic resource unit is one of U = N · E code-time-slots or simply slots (see Figure 6.1)2. Since MD PRMA is an in-slot protocol, each such unit can either be a C-slot available for contention, or an I-slot used for information transfer. This implies that a particular time-slot can feature both C-slots and I-slots. If the ‘pure’ random coding model described in Section 5.2 is used and code-slots are not distinguished (but time-slots still are), then every time-slot may carry a number of packets irrespective of the codes selected, but subject to a packet error rate determined
1 In GSM, for instance, the time-slot onto which synchronisation, broadcast, paging and access grant channels
are mapped on the downlink, carries the random access channel on the uplink (see Sections 3.3 and 4.3).
2 With E = 1, MD PRMA degenerates to conventional PRMA, and with N = 1, the protocol is essentially
the same as a protocol proposed in Reference [35], which will be discussed further in Chapter 8.
6.2
Sub-slots (code-slots)
Frame nf
DESCRIPTION OF MD PRMA 259
Implicit resource assignment through ACK on downlink
Frame nf + 1
XXX
XXX XXX
XXX XXX
XXX
XXX XXX
1 2 3 4 5 6 1 2 n
= I-Slot, idle
3 4 5 Time-slots
= C-Slot, idle
= I-Slot, reserved = C-Slot, success XXX = C-Slot, collision
Figure 6.1 Code-time-slots and implicit resource assignment in MD PRMA
by the MAI experienced. Since there are no code-time-slots with this model, the notion of C-slots and I-slots does obviously not apply in this case.
In the case of time-division duplexing, the time-slots are shared between the two link directions, as discussed in more detail in Section 6.3. With frequency-division duplexing, the above description refers to the uplink channel only, while the exact structure of the downlink channel does not matter for MD PRMA operation, except for possible constraints regarding downlink signalling. However, as argued in Chapter 3, for complexity reasons it is considered desirable to use the same basic multiple access scheme and thus the same fundamental channel structure in both link directions.
The channel parameters are adapted to the bit-rate of the standard service (e.g. the rate of the full-rate voice codec) such that during a packet spurt with this service, one packet per frame is generated, which needs to be transmitted on one single slot. Due to this periodic resource requirement, such a source is termed a periodic information source3.
6.2.3 Contention and Packet Dropping
On the uplink, resources are allocated on the basis of packet spurts. With the traffic models considered, packets to be transferred during a packet spurt will either carry data from a talk spurt, an IP datagram, or an email message. To obtain a resource reservation, terminals must go through a contention procedure. This procedure is first described for the code-time-slot case. Subtle differences in the random coding case are outlined subsequently.
6.2.3.1 The Code-Time-Slot Case
Terminals that are admitted to the system, but do not hold a reservation of resources, may only access C-Slots in contention mode with some time-slot and service or access-class specific access permission probability px[t] signalled by the base station (for voice, x = v).
3 Traffic generated by so-called random data sources defined in Reference [8] is not considered here for reasons outlined in Section 5.6.
260 6 MULTIDIMENSIONAL PRMA
A terminal with a new packet spurt will switch from idle mode to contention mode and wait for the next time-slot which carries at least one C-slot. It then determines whether it obtains permission to access this time-slot t by performing a Bernoulli experiment with parameter px[t]. In the case of a positive outcome, it will transmit the first packet of the spurt on a C-slot, which may have to be selected at random, if more than one such slot is available in the respective time-slot. In the CDMA context, selecting a C-slot means spreading the packet with the code-sequence which is assigned to the respective code-slot. If this packet is received correctly by the BS, it will send an acknowledgement, which implies a reservation of the same code-time-slot (now an I-slot) in subsequent frames for the remainder of the spurt. This way of assigning resources was already earlier referred to as implicit resource assignment, and is illustrated in Figure 6.1. The MS in turn switches to reservation mode and enjoys uncontested access to the channel to complete transmission of its packet spurt. In the case of a negative outcome of the random experiment, a collision on the channel with another contending terminal, or erasure of the packet due to excessive MAI, the contention procedure is repeated.
With delay-sensitive, but loss-insensitive services, packets are dropped when exceeding a delay threshold value Dmax, in which case contention will have to be repeated with the next packet in the spurt. As packet dropping will cause deterioration of the perceived quality of, for instance, voice or video, some maximum admissible packet dropping ratio Pdrop will normally have to be specified.
The state diagram for the MAC entity of the mobile terminals is depicted in Figure 6.2. Note that the transition from CON to IDLE is only possible for a terminal that drops packets and may have to drop an entire packet spurt in exceptional cases. For loss-sensitive and delay-insensitive services (that is, NRT services such as email and Web browsing), packets are, at least in theory, never dropped at the MAC and therefore this transition is not possible.
6.2.3.2 Differences in the Random-Coding Case
There are subtle differences in the contention procedure for the ‘pure’ random-coding case. Since no code-slots are discriminated, the notion of C-slots and I-slots does not apply. The access permission probability to time-slots for contending users is controlled based on the number of users having a reservation on that time-slot, as outlined in Section 6.4. The equivalent of a time-slot without C-slot is a time-slot with access permission probability zero. If the probability is greater than zero, and the outcome of the Bernoulli experiment performed as a result is positive, contention may only fail due to the packet being erased by MAI, code-collisions are not possible.
RES
IDLE CON
Figure 6.2 State diagram of mobile terminals (MAC entity)
6.2 DESCRIPTION OF MD PRMA 261
6.2.4 Accounting for Coding and Interleaving
In conventional PRMA and basic MD PRMA introduced above, each packet, whether sent in contention or in reservation mode, carries an addressing header, some further signalling overhead and user data.
Once a logical context is established between a mobile terminal and the network and the latter knows for instance the destination of a mobile originated call, there is no need to transmit the full addressing information in every packet over the air interface4. The full header is therefore only required in the contention packet, if at all. In some cases, even only a temporary ID which identifies both the contending mobile and the relevant context unambiguously, will do. On the other hand, given the adverse propagation conditions in a mobile environment, data need to be error coded and interleaved over several time-slots to provide some protection against deep fades (see also Section 4.2).
These considerations lead to the following evolution of the basic protocol: when a packet spurt arrives, the MS generates a dedicated request burst for contention fitting into one slot and containing a temporary mobile ID, which is unambiguous in the considered context, and most of the signalling overhead required for the packet spurt, but no user data. Upon successful contention, the MS sends its user data in groups of bursts using rectangular interleaving, each burst again fitting into one slot (but for the standard data-rate, it sends again only one burst per TDMA frame, exactly as in the basic scheme). The group size is determined by the interleaving depth dil. For the basic voice service, dil is chosen here such that the transmission time of these bursts corresponds to the voice frame duration Dvf . The choice of air-interface parameters must then ensure that the data in one voice frame fits onto the payload of the bursts in one group. In Chapter 5, the term RLC frame was introduced for such a group of bursts. For data services, the data transmitted in dil bursts is also referred to as an RLC protocol data unit or RLC-PDU.
In the case of the voice service, once a reservation is obtained, the voice frame most recently delivered by the RLC to the MAC is transmitted (no queuing is applied at the RLC or higher layers), while any older voice frame is dropped. This is equivalent to saying that Dmax corresponds to Dvf . Dropping occurs frame-wise rather than packet-wise, such that Pdrop denotes the frame dropping ratio.
In the case of NRT data services, the RLC delivers its PDUs either when the MAC is in IDLE state (in which case the delivery of a PDU triggers transition to the CON state) or, while in RES state, immediately after successful transmission of the previous PDU by the MAC. Dropping at the MAC does not occur.
At least as far as dedicated request bursts are concerned, this scheme bears some resemblance with burst reservation multiple access (BRMA) proposed in Reference [264].
6.2.5 Duration of a Reservation Phase
In PRMA as defined in Reference [8], an MS with periodic traffic may hold a reser-vation as long as needed to transmit successfully all packets in its spurt. If the MS leaves the allocated resource idle, the BS interprets this as the end of the spurt and
4 In the case of IP traffic, address information may have to be transmitted with every single datagram. In this case, it is included in the datagram header (or IP header), which is considered to be part of the payload transmitted over the air interface. IP headers may be compressed, as discussed in Chapter 11.
...
- tailieumienphi.vn
nguon tai.lieu . vn