Impact of packet-level FEC and outer block interleaving on MBMS delivery over WCDMA mobile satellite systems U. Mudugamuwa, M. Karaliopoulos, R. Tafazolli, B. G. Evans Mobile Communications Research Group, Centre for Communications Systems Research (CCSR) University of Surrey, Guildford, GU2 7XH, UK
[email protected] Abstract--The inherent broadcast capabilities of satellites make them an attractive solution for the delivery of multimedia services to mobile users in third generation (3G) networks. Reliable delivery of content over a hostile landmobile satellite channel is one of the key technical system requirements that can be addressed at different protocol layers. In this paper, we focus our attention on two of the mechanisms which provide partial reliability: the PacketLevel Forward Error Correction (PLFEC) technique in Radio Access Network (RAN) and different interleaving depths at physical layer (inter frame) in the satellite radio interface. We rely on an end-to-end simulation platform in order to assess the performance gain achieved by the two mechanisms separately, and providing clear indications for the achievable benefits at system level when the two mechanisms act in co-ordination rather than in isolation. Index terms-- Packet level FEC, Higher interleaving depth, MBMS, Wideband CDMA, Satellite UMTS.
I. INTRODUCTION Standardisation work is ongoing within the 3GPP Multimedia Broadcast and Multicast Services (MBMS) framework in order to facilitate emerging high data rate p-t-mp (point-to-multipoint) application (video/ audio streaming, file downloads) in third Generation (3G) mobile networks [1]. Although the standardisation efforts have made significant progress with respect to service and architectural aspects, there are several aspects related to MBMS support in the Radio Access Network (RAN) and Core Network (CN), which merit further research. Such areas are user satisfaction (reliability of the content), handling the user mobility with respect MBMS session, selection of the transmission method (point-to-point or p-t-mp), and group management. On the other hand, the inherent broadcast capabilities and the larger geographical coverage of the satellite network make them more spectrally efficient than the terrestrial network and also it can help to overcome some of the challenges faced by the terrestrial network, the in particular those relevant to user mobility, hence promoting to a promising medium for the delivery of MBMS content. In light of those advantages of satellites, hybrid satellite-terrestrial systems are envisaged[2], where satellite uni-directional downlink carriers deliver MBMS data in close synergy with the terrestrial network. These hybrid system configurations appear advantageous for services featuring large audience with a high degree of geographic dispersion, since users can be reached via a single data flow over one satellite spot-beam, whereas in a purely terrestrial system, the same MBMS content should be transmitted over many cells to reach the same number of users. However, the requirement for reliable content delivery to larger number of users, over the hostile land-mobile satellite channel, still remains at large. Within the context of WCDMA mobile satellite systems, reliability can be addressed at different layers within different network entities: at application or/and transport layer at the source, packet level on the RAN interface, and also at radio interface link layer. In this paper we investigate the impact of the, packet level Forward Error Correction (FEC) mechanism at packet level and the outer block interleaving mechanism at link layer, with respect to the reliability that can be achieved when delivering MBMS content over a unidirectional satellite system. We assess the achievable gain in terms of number of satisfied users, when these two mechanisms act in co-ordination rather than in isolation, by using an end-to-end simulation platform developed in Network Simulator (NS2). The rest of paper is organised as follows, the next section introduces MBMS features and requirements when delivering over the satellite. Then section III will introduce the partial reliability mechanisms considered in this paper followed by the simulation methodology in section IV. Section V provides the simulations results and section VI concludes the paper.
II. MBMS OVER SATELLITE A.
MBMS Features and Requirement
The main types of user services envisaged within the 3GPP MBMS framework [1] are, streaming, file download and carousel services. Those services distinguish each other based on their Quality of Service (QoS) requirements such as reliability of the content (in terms of Bit Error Rate (BER) or Block Error Rate (BLER)), delay, and jitter. Furthermore, those services have different processing requirements at the receiving side, for a example a receiver will play the streaming content “on the fly”, whereas for downloads and carousel during the transmission, receiver will buffer the entire content and thereafter it will access the content from the terminal cache and finally it will play the content. Therefore streaming services will introduces delay variation (jitter) requirements that are related to the availability of play-out buffers at the terminal side and on the other hand carousel and download services will not introduce packet-level requirements, although there may be some requirement for the total download time, namely the time elapsing between the moment the content is requested till the moment the content becomes available at the terminal cache. These types of services have been considered over hybrid network and this paper
mainly emphasises on the reliability requirement, in terms of Packet Loss Rate (PLR) and the percentage of users been satisfied (SU)1, when delivering the MBMS content via satellite radio interface. B. The Satellite Radio Interface The satellite radio interface considered for this research work is the interface engineered within the European Union IST SATIN project [3]. The access scheme features maximum commonalties with the WCDMA access scheme of terrestrial UMTS [4]. The logical, transport and physical channels that are retained in the proposed interface at the data and control plane are a subset of the full WCDMA channel set [5], including only common channels. MBMS data are mapped one-to-one on MBMS Traffic Channels (MTCH) at Radio Link Control sub-layer and Forward Access Channels (FACHs) at the MAC sub-layer, which are then multiplexed at physical layer on to Secondary Common Control Physical Channels (S-CCPCHs). The latter features fixed spreading factor and no power control. Given that in T-UMTS the FACH/S-CCPCH carries important signalling information, the standard practice is to allocate a small rate/large Spreading Factor (SF) to it, so that it can be accessible by all users in the cell. However, in the proposed integrated system S-CCPCHs are used mainly for data transfer purposes, hence their SF can vary in the whole range defined in 3GPP standards, i.e. from 4 to 256. A separate S-CCPCH of low rate, called "master SCCPCH" is reserved for signalling related to service notification. Likewise, the access scheme sub-layers support only a subset of the full functionality described in the 3GPP standards, related to the retained common channels. Multimedia data make use of the UMTS RLC unacknowledged mode [4] over the satellite radio interface, namely the RLC sub-layer provides basic sequencing and protocol maintenance functions without catering for error recovery functions such as ARQ. The MAC sub-layer is primarily responsible for the scheduling of the different services over the air. The latter is performed via selection of the Transport Format Combination at Transmission Time Interval (TTI) level and it involves the prioritisation of certain flows over the others. In the following section, we investigate more closely two partial reliability mechanisms associated in two different layers within the context of MBMS delivery over geo-stationary mobile satellite systems with a WCDMA-based air interface.
III. RELIABILITY MECHANISMS A.
Higher Interleaving depths
Interleaving is a mechanism to convert an error bursts (i.e. a group of consecutive erroneous symbols owing to the burst nature of errors) into random-like errors. Because of this capability, two types of interleaving are performed at WCDMA physical layer [5] to combat the burst bit error losses due signal interruptions at the receiving terminals. This interleaving will improve the Turbo decoder efficiency at the receiver when compared to without any interleaving; this is shown in [6]. However interleaving will add an additional latency to the transmission delay and also will require extra processing.
MAC Layer Physical Layer
Transport Channel (TrCH 1)
Transport Channel (TrCH 2)
Transport Block (L)
Coded TB (=( L+CRC +Tail)*3*RM)
Intra Frame Interleaving (1st Interleaving)
Interleaved coded TB
Radio Frame segmentation
TrCH Multiplexing
Figure 1 shows the interleaving function performed within WCDMA physical layer functions defined in [5]. In both interleaving functions procedures are more similar; input data is stored in row wise, and then inter column permutation is perfumed. Finally data is released column wise. The main differences of those two methods are memory storage parameters and the column permutation pattern. In 2nd interleaving the number of columns is fixed to 30 and the number of rows will depend on the input data size (i.e. data length/30). First one, number of columns of the memory storage will depend on the Transmission Time Interval (TTI), for example 10ms will have only one column whereas a 20ms TTI will have two columns. Furthermore according to the TTI, the inter column permutation will vary as well.
Coded Composite TrCH)
Interleaving Depth (ID) is a function of number of rows and the column permutation, Figure 2 shows the standard interleaving procedure. Physical Channel Mapping Furthermore, the interleaver capability of randomising the error burst Figure 1: Interleaving step for WCDM A fully depends on the interleaving depth, i.e. a higher ID can randomise downlink longer burst. This will improve the turbo decoding efficiency. However just increasing the ID will not necessarily improve the decoding efficiency monotonically. Because if the interleaving depth is not chosen according to the error burst pattern at the terminal, then the interleaver procedure may combine the random errors and will generate error burst, hence reduce the decoding efficiency. Inter Frame Interleaving (2st Interleaving)
TrCH Multiplexing
But in the envisage hybrid system, satellite gateway cannot obtain the real time user feed-back (satellite return link is not available) and also when delivering the content to a larger user group error characteristics between users will vary a lot, therefore interleaving depth selection is very important for the overall network performance (user satisfaction).
1
If the PLR is below or equal to the required PLR (a%) by the application then the user is considered as satisfied.
In addition to above stated two interleaving methods, turbo coding efficiency will be depend on Transport Block (TB) size in WCDMA systems [6], this also due to a interleaving function inside the turbo coder. Hence selecting the TB size also crucial for the user satisfaction. In p u t se q u e n c e
1, 2, 3,
……
, ( L -1 ) , L
Encoded FEC Transport Block (FTB) 8 bits RS code symbol
W ritin g to in te r le a ve r
N u m b e r o f C o lu m n s (N c ) 1
2
Original packet
3 L /N c L -1
Original packet
L
K
C olu m n p e r m u ta tio n 1
3
Original packet
2
Redundant packet L
(N – K)
R e a d f r o m th e in te r le a ve r O u tp u t se q u e n c e
Redundant packet RS code Word
Figure 3: Packet level FEC procedure
F ig u r e 2 : I n t e r le a v in g p r o c e d u r e
B.
Packet Level FEC
Although Forward Error Correction (FEC) has been widely used at physical layer (for example, see [7]), the technique has only recently been introduced at packet level [8]. Packet-level FEC increases the reliability of the transfer in the face of packet loss that may be due to either congestion or link errors or both. In packet level FEC the encoder produces n encoded packets out of k original data packets. These n packets form the FEC Transport Block (FTB) or FEC Transport Group, each one including h=n-k redundant packets in addition to the original packets, see Figure 3. The redundancy overhead Ro may then be defined as, Ro =
h ⋅100% k
(1)
When the FEC decoder receives packets, it will extract the FEC related information from the packet headers. If at least k out of the n encoded packets belonging to the same FTB are correctly received, the decoder can recover the original packets. Otherwise, depending on the FEC code, it may be able to recover only the correctly received original packets. In this case, correctly received redundant packets do not have an impact on transfer reliability; they are rather waste of bandwidth. On the contrary, these packets may be useful, when retransmission of lost packets is enabled, namely when FEC is combined with some form of Automatic Repeat request (ARQ) strategy into a hybrid packet loss recovery scheme [9]. In the FEC encoding process, redundant packets are generated using erasure codes. Various types of erasure codes exist, each one introducing some trade-off among performance, additional capacity consumption, processing time and buffering requirements [9]. In this study we have selected optimal systematic block codes, i.e. Reed-Solomon code defined over GF (28), which correspond to a code word length of 255 (n) symbols (8 bits). During the encoding process different codeword lengths are achieved by performing shortening or puncturing for the RS code, see [9]. In this research paper Packet level FEC analysis is mainly concerned the trade-off between performance and redundancy overhead. Increasing Ro , performance becomes better at the expense of increased session duration and reducing the throughput (i.e. wasting the bandwidth). The redundancy overhead becomes particularly critical in wireless and, in particular, mobile cellular networks, where capacity is costly.
IV. SIMULATION METHODOLOGY For the simulations two main modules were used and those modules have been developed on top of the mainstream 2.26 version of Network Simulator (NS2): the packet-level forward error correction modules and the satellite radio access scheme modules. The latter implement functions of the MAC and RLC sub-layers of the WCDMA air interface such as segmentation and reassembly of packets into/from access layer TBs and concatenation of TBs, and it also includes the packet-scheduling block. The link-layer characterisation was performed using an in-house developed C++ based link level simulator [2]. The results of the independent link-layer simulations are interfaced with the system level simulator via lookup files of {BLER, E b N o } entries.
A.
Simulation scenario and procedure
The simulation hybrid network scenario is depicted in Figure 4, where (a) shows the actual network entity while (b) shows the corresponding NS simulation modules. During the simulations Multicast/Broadcast Service Centre (i.e. data source) will forward the content to the satellite hub, and in here the content will be segmented into Transport Blocks [0,3840] (TBs) and for every TTI [10, 20, 40, 80] Medium Access Controller (MAC) layer will release TBs into the physical layer for the transmission towards the satellite. Then the satellite repeater will broadcast the content towards the coverage area (spot-beam) using WCDMA air interface. Each receiver within the simulation will have a separate air interface and according to the instantaneous channel conditions, system level channel model will set the TB error flag according to the link level look-up tables (Eb/N0 vs BLER curves). Finally the performance monitor is located at the receiver node and it will process the receive content and then will generate the statistical information, such as PLR. In the following, PTXref is the reference transmit power value
GEO satellite S atellite d ow nlin k W C D M A interface
P erform an ce m on ito r
C o ntent p rov id er
S - H ub
S /T ter m inals
(a )
S atellite L in k-lev el S atellite receiver ch ann el-m od el R ep eater (b )
S atellite g atew ay
P ack et le vel D ata F E C agen t so urc e
Figure 4: Hybrid network scenario
considered in the forward link budget [2]. The assumed E b N o requirement ( 10-2 target BLER) is 2.6 dB for 80 ms TTI using Turbo coding scheme of rate 1/3. With this power, a margin of 10.21 dB is achievable at the forward link for a handheld terminal receiving 384 kb/s service flows. For lower TTI, to achieve the same target BLER transmit power is increased with respect to the reference power settings (Table 1). Five hundred user terminals are randomly distributed within the selected spot-beam area, ranging from 44N to 51N and 4W to 8E (effectively covering France). They are subject to correlated Lognormal shadowing with a standard deviation of 3 dB [11].
Application level and packet level simulation parameters are given in Table 2. Furthermore FTB was fixed to 128 symbols in order to be inline with IST MoDiS project simulations [2]. Table 2: Packet level FEC parameters
Table 1: Required Eb/N0 and Transmit power for Different TTI values TTI value (ms)
Required Eb/N0 (dB)
80
B.
Power (dB) PTXref
2.6
ref TX
40
4.0
P
20
4.75
PTXref + 3.5
10
7.25
PTXref + 5.25
+ 1.4
FEC parameter
Value
Source
CBR
Data rate (kb/s)
384
Transfer file size (MBytes)
15
Number of original data packets (k)
128 - 98
RS code length (FTB) (n)
128
Packet length (IP) (Bytes)
960
Simulation metrics
Three different error rates been identified and measured in the simulator. The block error rate (BLER) is measured at access layer as the ratio of corrupted TBs over the total number of transmitted TBs. BLER =
number of corrupted TBs total number of received TBs
(2)
At network layer before the FEC decoding, the packet error rate (PER) is defined as PER
=
number of corrupted IP total number of received
packets packets
(3)
Finally, after the packet level FEC decoding performance is measured in terms of the Packet Loss Rate (PLR) PLR =
number of corrupted packets after decoding total number of transmitte d data packets
(4)
The main performance metric is the percentage of satisfied users S u , defined as the ratio of users, whose PLR is lower than or equal to some threshold a (i.e. 5% PLR). S u (a ) =
number of users with PLR ≤ a ⋅ 100 % total user population
In assessing the two reliable mechanisms, two more parameters become relevant: • •
The transmission overhead related to packet-level FEC. This is represented by the redundancy overhead R o Link level 1st interleaving depth (ID), i.e. Transmission Time Interval TTI.
(5)
V. SIMULATION RESULTS Figure 5-8 demonstrates the performance gain achieved in terms of user satisfaction for varying Ro and TTI values for different transmit power settings. Figure 5 and 6 uses threshold PLR value as 5% to determine the satisfied users where as the other two figures (7 and 8) uses 2% and 0% PLR, the difference of the threshold values are because with the increase of the transmit power overall user satisfaction will increases. For example Figure 5, maximum Su(5%) is 64% whereas in Figure 6 Su(5%) is almost 100%. For higher TTI values (80 and 40) Su variation is small compared to lower TTI values, this is due to physical layer Turbocoding effects (interleaving depth)2. Furthermore for TTI 40ms and 80 ms, user satisfaction will not vary a lot after a certain redundancy overheads; this is due to packet level FEC mechanism, i.e. for 40ms users will have more errors compared 80ms but higher redundancy overheads will compensate that additional packet loss. For example in Figure 5, at Ro zero TTI 40ms has a user satisfaction of 40% whereas for TTI 80 Su is 45% but by adding 13% of redundancy overhead the drift between those two curves been reduced. Figure 9 - 10, plots the trade-off curves between the two reliability mechanisms concerned, showing at the same time the possible combinations of the two mechanisms for achieving a given target objective in terms of percentage of satisfied users. The curves are generated from the previous results via using interpolation. In Figure 10 curves are plotted only for TTI 40ms and 80ms, because due to the higher link margin other TTI values cannot satisfy as many users compared to the plotted TTI values.
2
Figure 5: SU(a
