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27th IET and AIAA International Communications Satellite Systems Conference (ICSSC 2009), January 2009 page 613
RADIO RESOURCE MANAGEMENT POLICY FOR MULTICAST TRANSMISSIONS IN HIGH ALTITUDE PLATFORMS A. Raschellà, G. Araniti, A. Iera, A. Molinaro A.R.T.S. Laboratory - Dept. DIMET - University “Mediterranea” of Reggio Calabria, ITALY
Keywords: Multicast, MBMS, HAP, RRM, HSPA.
Abstract In this research work we aim at investigating how efficient High Altitude Platforms (HAPs) can be in supporting Multimedia Broadcast/Multicast Service (MBMS) in scenarios in which the terrestrial coverage is not available. In particular, we propose to implement an efficient policy of Radio Resources Management (RRM) into the HAP Radio Network Controller (H-RNC). The proposed technique allows to increase the overall system capacity by selecting the most efficient multicast transport channel in terms of power consumption, amongst Dedicated Channel (DCH), Forward Access Channel (FACH), and High Speed Downlink Shared Channel (HS-DSCH).
1 Introduction In the last years, manifest improvements in the field of both mobile terminals and network infrastructures provided users with innovative and enhanced communication capabilities. Thanks to this technological deployment, nowadays Third Generation (3G) cellular wireless networks, such as Universal Mobile Telecommunication System (UMTS), are able to bring in both video conferencing and streaming video [1]. At the same time, the introduction of many novel services has led to the need of point-to-multipoint (PtM) transmission capabilities enabling communications among one sender and several receivers. Multicasting and Broadcasting are efficient modalities to provide PtM services. Hence, third generation partnership project (3GPP) introduced a new system, called Multimedia Broadcast/Multicast Service (MBMS), specifically intended to provide groups oriented services [2]. A terrestrial-only MBMS segment is not adequate for disadvantaged regions such as rural areas and disaster areas. Therefore, the integration of space segments into the MBMS architecture deserves a special attention from the scientific communities. High Altitude Platforms (HAPs) are space segments, usually located at an altitude of 17-22 km, which could be used for MBMS services, in particular when disadvantages areas are taken into account. Some of the advantages can be summarized in the following: relatively low platform upgrading cost, broadband capability, larger coverage area then terrestrial systems, flexibility to respond
to traffic demands, low propagation delay and less groundbased infrastructure [3]. HAPs could be either utilized as base stations in the sky (standalone case) or as an overlapping coverage (integrated system case). We have investigated how the HAP can efficiently support MBMS services in a scenario in which the terrestrial network is not available (standalone case). In particular, we propose an efficient Radio Resources Management (RRM) policy in the HAP that allows reducing as much as possible the consumed power required by MBMS service support. Thanks to the saved power, the overall system capacity increases; this reduces the impact of multicast services on the pre-existing unicast traffic which share the same radio resources. Transmit power limitations necessitate the use of the most efficient transport channel. MBMS services can utilize: Common (Forward Access Channel, FACH), Dedicated (Dedicated Channel, DCH), defined by UMTS Release 99, and Shared transport channel (High Speed Downlink Shared Channel, HS-DSCH), subsequently introduced by High Speed Downlink Packet Access (HSDPA) Release 5 [4]. The selection of the most efficient transport channel is strictly related to the number of users receiving MBMS data; in fact if such a number increases, then the system would need a higher amount of DCHs to keep the offered level of service at acceptable values. Hence, a key decision concerns the maximum number of users that can use the DCHs, while allowing a power saving. Such a number represents a Switching Threshold from dedicated channel to either common or shared ones. The state of the art in term of MBMS radio resource management can be depicted as such, that several research works have been conducted for the determination of Switching Thresholds taking into account the UMTS terrestrial network only [5], [6], [7]. While other studies aimed at defining Switching Thresholds by considering the integration of HAPs into the MBMS architecture, but only utilizing Dedicated and Common transport channels [8]. By starting from the latter study, our work contributes to the state of the art as the RRM policy that we propose aims at defining the Switching Thresholds including also the use of the Shared transport channel, when considering MBMS transmissions from HAPs systems in a standalone case. We also highlight how the introduction of the HS-DSCH can lead to an improvement of the MBMS performance in term of consumed power; moreover we explain how the limitation in terms of
users number served by the Shared channel affects the selection of the thresholds. The paper is structured as follows. Section 2 provides a brief overview of the integration of HAPs into the MBMS architecture. Section 3 illustrates the transport channels features used for MBMS services, from HAPs systems. While the results of a exhaustive simulation campaign, aiming at defining the RRM policy, are the focus of Section 4. Finally, conclusive remarks are given in Section 5.
function of the transmitter-receiver distance raised to a power of 2. While, in wireless terrestrial systems, where the Okumura Hata model is taken into account, the received power decays as a function of the transmitter-receiver distance raised to a power of 4.
Futures
Terrestrial Wireless
HAP
2 MBMS/HAP Applications
Breadth of geographical coverage
A few kilometres per base station
The inclusion of HAPs into the MBMS architecture to delivery multicast/broadcast services is illustrated in Figure 1. This inclusion implies the addition of an UMTS-HAP Control System (U-HCS) to the MBMS architecture, with functionality of HAP Radio Network Controller (H-RNC).
Cell diameter Shadowing from terrain
0.1 – 1 km Causes gaps in coverage; requires additional equipment Okumura Hata Constant
Hundreds of kilometres per platform (up to 200 km) 1 – 10 km Problem only at low elevation angles Free Space Variable
Path Loss Antenna Gain
Table 1. Differences between terrestrial and space segments.
3 MBMS/HAP Transport Channels
Figure 1: UMTS/HAPs integrated systems for multicast services delivery. HAPs include an UMTS payload, at an altitude of 22 km above the service area. Hence, the new node of the UMTS terrestrial network used to support HAP systems (U-HCS) is depicted in Figure 1. In particular, one can note the presence of the following elements: (i) SGSN (Serving GPRS Support Node): it carries out user individual service control functions gathering together all individual users of the same MBMS service into a single MBMS group; (ii) GGSN (Gateway GPRS Support Node): it terminates the MBMS GTP tunnels from the SGSN and links these tunnels via IP multicast with the MBMS data source; (iii) Home Location Register (HLR): it is a database that stores the MBMS users’ services profile; (iv) BM-SC (Broadcast-Multicast Service Centre): it is the new element introduced into such an architecture to provide MBMS data; (v) Border Gateway (BG): it manages the data delivery from the Multicast Broadcast Source. The proposed MBMS/HAP architecture is very similar to the MBMS/UMTS terrestrial network; nevertheless, several differences (reported in Table 1) between terrestrial and space segment have to be taken into account in the determination of the Switching Thresholds [9]. The most relevant difference is represented by the propagation environment. In fact, HAPs enjoy more favorable Path Loss characteristics compared to wireless terrestrial links, since in the Free Space the received power decays as a
The HAP system can use either the Dedicated, or the Common, or the Shared transport channel for MBMS applications. Next we provide a brief overview of the modality to assign the transmission power to such channels. The overall downlink transmission power allocated to the DCHs varies depending on the following issues: (i) the number of multicast users; (ii) the position of users (for instance, users close to the centre of the area covered by HAP need a lower amount of power than users at area border); (iii) the application bit rate. Equation (1) defines the total transmission power assigned to a number of DCHs equal to “i” scattered across the cell [10]. Nc
∑G(θ , d )P + (1− α )P PT ,i = (C / I )i
j =1
j
j
T
Tot (i −1)
G(θi , di ) + PpG(θi , di ) + Nd
G(θi , di )
(1)
Where Nc is the number of interfering neighboring cells, PT is the interference from such cells, α is the orthogonality factor that can be zero in the case of perfect orthogonality, PTot(i-1) is the power allocated to a number of users equal to i-1, Pp is the power devoted to common control channels, Nd is the Additive Gaussian White Noise (AGWN). While G( θ j, dj) is the link gain and C/I is the Carrier-to-Interference ratio respectively computed by applying Equation (2) and (3).
G(θi , di ) dB = g (θi ) dB − Lp (di )dB (2) (C / I ) dB = ( Eb / N 0 ) dB − (G p ) dB (3) Where θ i is the angle representing the boresight direction [11], g( θ i) is the Antenna Gain calculated in the boresight direction, Lp(di) is the attenuation value caused by the Path
Loss for the user at a distance equal to di from the centre of the area covered by the HAP. Eb/N0 is the Energy per Bit-toSpectral Noise Density and Gp is the Processing Gain. A FACH channel transmits by using a fixed power level since fast power control is not supported in this kind of channel. It is a broadcast channel which shall be received by every UE scattered across the cell served by the HAP, regardless of the considered Path Loss and the Antenna Gain. Hence, the fixed power has to be high enough to guarantee the services in the whole coverage area [12]. The bit rate of the MBMS services and the needed coverage area of the cell influence the power allocated to the FACH channel. The delivery of high data rate MBMS services over FACH is not always feasible, due to the excessive downlink transmission power that would be required. High bit rates can only be offered to users located close to the centre of the area covered by the HAP. HS-DSCH is the shared transport channel that HSDPA exploits to carry out users data in the downlink direction. HSDPA aims at increasing packet data throughput and rate, thanks to link adaptation and fast physical layer retransmission. Moreover, in HSDPA the Adaptive Modulation and Coding (AMC) replaces the variable Spreading Factor (SF) and the fast power control procedures. AMC enables to change the kind of modulation and coding depending on the radio channel conditions; such information are represented by the Channel Quality Information (CQI). The transport block size, the number of used physical channels, and the modulation technique can be evaluated from the CQI value [4]. In a wireless network scenario, the chosen Eb/N0 corresponds to a particular Block Error Rate (BLER) for a given data rate. Nevertheless, the Eb/N0 metric is not an appropriate measure for HSDPA. In fact, the bit rate on the HS-DSCH can vary every Transmission Time Interval (TTI), by utilizing different modulation schemes, effective code rates, and number of physical channel codes. Hence, in HS-DSCH environment the Eb/N0 is replaced by the Signal to Interference Noise Ratio (SINR) representing a more appropriate measurement metric. Equation (4) defines such a parameter.
SINR = SF16
PHS − DSCH pPown + Pother + N d
(4)
Where PHS-DSCH is the HS-DSCH transmission power, Pown is the own cell interference, Pother is the interference from neighboring cells, Nd is the AGWN, p is the orthogonality factor (that can be zero in the case of perfect orthogonality) and SF16 is the SF equal to 16. A further useful parameter that has to be considered is the geometry factor, evaluated according to the following Equation:
G=
Pown Pother + N d
(5)
This parameter indicates the distance from the centre of the area covered by the HAP. A lower G-factor is expected when
a user is close to the area edges, where the interference from the neighboring cells is higher than the own cell interference. Finally, by rearranging Equation (4) and considering Equation (5), we can define the average HS-DSCH SINR as:
SINR = SF16
PHS − DSCH 1 Pown p + G −1
(6)
SINR is an important parameter for our research work; in fact, as explained later , it affects the CQI that can be selected which in turn influences the maximum number of users served through HS-DSCH. The PHS-DSCH value can be fixed by two different modalities: (i) the H-RNC explicitly allocates an amount of transmission power to the HS-DSCH and it could update such a value any time later; (ii) the H-RNC allows utilizing any unused power in the cell (i.e. the residual power after serving other channels) for the HS-DSCH. In our research work we assume the (i) case and we consider the update of the PHS-DSCH value to vary between 35% and 60% of the overall HAP transmission power [6, 13].
4 Obtained Results Table 2 summarizes the main assumptions [14, 15] of our evaluation study, while parameters not listed are varying during the different campaigns.
Parameter
Value
HAP High Cell Radius Cell Layout Number of Neighboring Cells Maximum HAP Tx Power Other BS Tx Power Common Channel Power Path Loss Multipath Channel Orthogonality Factor BLER Target Gmax Thermal Noise
22 Km 2.6 Km Hexagonal grid 7 40 W 10 W 2W Free Space Vehicular A (3Km/h) 0,5 10 % 32,3 dBi -100 dBm
Table 2: Simulation Campaign Assumptions. Table 3 reports the Switching Thresholds from DCHs to FACH expressed in terms of number of users in the coverage area , according to [8]. Such thresholds are evaluated by considering: 64, 128, and 256 kbps services and several cell coverage extensions. In particular, when the number of multicast users is lower than the Switching Threshold from DCHs to FACH value, the power used by the DCHs is smaller than the one used by the FACH, so the RRM foresees to use Dedicated channels. While, when the number of multicast users exceeds the Switching Threshold from DCHs to FACH, then the Broadcast channel is utilized. Moreover, from Table
1 it clearly emerges that the Switching Threshold values are undefined in the following cases: (i) when 128 kbps is the application bit rate for 95% cell coverage; in fact, an amount of power equal to 31,6 W would be needed; such a value is too high considering that the overall HAP transmission power is 40 W (see Table 2); (ii) when 256 kbps is the application bit rate for 75% cell coverage; in this case an amount of power equal to 28,8 W would be needed; this power value is still too high; (iii) in case of 256 kbps services and 95% cell coverage, the FACH transmission power is not defined because it would be greater than the overall HAP transmission power.
Bit Rate (Kbps)
Power Allowed to FACH (W)
Switching Threshold from DCHs to FACH
64
5,6
33
128
12,6
22
256
16,8
11
64
8,8
43
128
17,8
24
256
28,8
---
64
15,2
49
128
31,6
---
256
None
---
DSCH assigned power increases and/or the cell coverage percentage value decreases. As a further step, we want to highlight in what extent obtained SINRs limit the maximum number of users served by the HS-DSCH. In fact, in HSDPA, such a number of users depends on the CQI parameters that, in turn, are closely connected to the SINR values.
Maximum Cell Coverage (%)
50%
75%
95%
Table 3: Switching Thresholds when varying Cell Coverage and Bit Rate. Our RRM policy aims at redefining the Switching Thresholds showed in Table 3 as a consequence of HSPA introduction. In particular, it allows to define a first Switching Threshold between DCHs and HS-DSCH. How already mentioned, HSDSCH is limited in terms of served users number. As a consequence such a restriction allows to define the second Switching Threshold between HS-DSCH and FACH channel, being the latter one a broadcast channel and thus not limited in terms of served users number. How the overall system can benefit from the introduction of the HS-DSCH will be explained in the next subsections. From Equation 6 it emerges that the SINR depends on the assigned HS-DSCH transmission power and on the area covered by the channel (depending on the furthest user position from the centre of the area covered by the HAP). Hence, Figure 2 illustrates the SINRs that have to be guaranteed, when varying the cited parameters. HS-DSCH could be active to cover respectively 50%, 75%, or 95% (in according to the study made in [8]). As expected from Equation 6, the guaranteed SINR improves when the HS-
Figure 2: SINR vs. Tx power for different cell coverage when FACH is not active. Therefore, in Figure 3 the relationship between SINR and CQI is illustrated for four different target Block Error Rates (BLER). We obtained such a figure through a simulator giving in output a SINR value, when a CQI parameter and a BLER target are selected. We carried out the simulations by using Simulink and implementing the following steps of HSDSCH coding and modulation chains [16], [17]: (i) CRC attachment, (ii) Scrambling, (iii) Segmentation, (iv) Turbo Coding, (v) Hybrid ARQ, (vi) Interleaving, (vii) 16QAM constellation rearrangement, (viii) Modulation Mapper, (ix) Scrambling, (x) Modulation. Table 4 illustrates the mapping between the CQI values and the maximum number of users served by the HS-DSCH. Such a number is obtained by dividing the maximum Physical Bit Rate by the service bit rate [4]. For instance, by considering the target BLER equal to 10% (according to the simulation campaign assumptions), for a given guaranteed SINR (i.e. SINR equal to 25 dB) we find the greatest CQI supported by the HS-DSCH (i.e. CQI equal to 22, see Figure 3). From the CQI value we can determine the maximum number of users that could be served by HS-DSCH (i.e. 75, 37 and 18 users considering respectively 64, 128 and 256 kbps services, see Table 4). These numbers represent the Switching Thresholds from HS-DSCH to FACH. According to our RRM policy, H-RNC continuously keeps track of the position (that can be included within either 50%, or 75%, or 95% cell coverage), the number of multicast users utilizing DCH channels, and the application bit rate (that can be either 64, or 128, or 256 kbps). In so doing, the H-RNC
can dynamically change the Switching Thresholds among the channels. We focused our attentions on two different sample situations illustrated in the next subsections. In particular, in the first the 35% of the overall HAP transmission power (14 W) is assigned to the HS-DSCH while the cell coverage varies; differently, in the second one the cell coverage is fixed to 95% and the HS-DSCH transmission power is varied up to the 60% of the overall HAP transmission power (24 W).
The philosophy of our RRM policy that allows to re-define the Switching Thresholds among the channels is based on the following principles: (i) the HS-DSCH transmission power value is first chosen; (ii) such a value of power allows defining the first Switching Threshold between DCHs and HS-DSCH; (iii) the assigned HS-DSCH transmission power gives a SINR value depending on the area covered by the HAP (see Figure 2); (iv) such a SINR gives a maximum number of users served by the HS-DSCH depending on the application bit rate (see Figure 3 and Table 4) that represents the second Switching Threshold between HS-DSCH and FACH. 4.1 Varying the cell coverage radius Table 4 summarizes the new Switching Thresholds for 64, 128, and 256 kbps MBMS applications when 14 W is the assigned HS-DSCH transmission power and the cell coverage varies.
Cell Coverage (%)
Figure 3: SINR to guarantee per CQI Physical Users Users # Users CQI Modulation Physical Bit Rate Number Number Number Channel (kpbs) 64Kbps 128Kbps 256Kbps 5
QPSK
1
480
7
3
1
8
QPSK
2
960
15
7
3
10
QPSK
3
1440
22
11
5
11
QPSK
3
1440
22
11
5
12
QPSK
3
1440
22
11
5
13
QPSK
4
1920
30
15
7
14
QPSK
4
1920
30
15
7
15
QPSK
5
2400
37
18
9
16
16-QAM
5
4800
75
37
18
17
16-QAM
5
4800
75
37
18
18
16-QAM
5
4800
75
37
18
19
16-QAM
5
4800
75
37
18
20
16-QAM
5
4800
75
37
18
21
16-QAM
5
4800
75
37
18
22
16-QAM
5
4800
75
37
18
23
16-QAM
7
6720
105
52
26
24
16-QAM
8
7680
120
60
30
25
16-QAM
10
9600
150
75
37
26
16-QAM
12
11520
180
90
45
27
16-QAM
15
14400
225
112
56
28
16-QAM
15
14400
225
112
56
29
16-QAM
15
14400
225
112
56
30
16-QAM
15
14400
225
112
56
Table 4: HS-DSCH parameters.
Bit Rate (kbps)
64 50
128 256 64
75
128 256 64
95
128 256
Number of Users (Nu)
Utilized Channel
Nu ≤ 33 Nu > 33 Nu ≤ 22 Nu > 22 Nu ≤ 11 11 < Nu ≤ 26 Nu > 26 Nu ≤ 43 Nu > 43 Nu ≤ 24 24 < Nu ≤ 37 Nu > 37 Nu ≤ 5 5 < Nu ≤ 18 Nu ≤ 49 49 < Nu ≤ 75 Nu > 75 Nu ≤ 25 25 < Nu ≤ 37 Nu ≤ 5 5 < Nu ≤ 18
DCHs FACH DCHs FACH DCHs HS-DSCH FACH DCHs FACH DCHs HS-DSCH FACH DCHs HS-DSCH DCHs HS-DSCH FACH DCHs HS-DSCH DCHs HS-DSCH
Table 4: Switching Thresholds when varying the Cell Coverage Radius and assigning 7 w to the HS-DSCH. From the Table we can draw the following conclusions: (i) when the FACH channel needs an amount of power lower than 14 W, our RRM does not foresee the use of the HSDSCH; therefore, in these cases we consider the Switching Thresholds defined in [8]; (ii) when the FACH channel needs an amount of power greater than 14 W then the use of the HSDSCH allows an improvement of the MBMS performance in
term of consumed power; for instance, when 256 kbps is the application bit rate and 50% is the considered cell coverage, then the FACH needs 16,8 W; hence when the number of multicast users is included between 11 and 26, then the use of the HS-DSCH allows saving 2,6 W that could be used for unicast applications; (iii) the use of the HS-DSCH is introduced when FACH channel is not utilized because of its too high transmission power; in this case the maximum number of users served by the HS-DSCH corresponds to the general maximum number of users receiving the multicast application because of the shared nature of the channel. 4.2 Varying the HS-DSCH transmission power Table 5 summarizes the Switching Thresholds for 64, 128, and 256 kbps MBMS applications when the cell coverage is fixed to 95% of the area covered by the HAP and the HSDSCH transmission power varies. Under these assumptions FACH always allows saving power in comparison with HSDSCH, when 64 kbps is the service bit rate.
HS-DSCH Power (W)
Bit Rate (kbps) 64
16
128 256 64
18
128 256 64
20
128 256 64
22
128 256 64
24
128 256
Number of Users (Nu)
Utilized Channel
Nu ≤ 49 Nu > 49 Nu ≤ 27 27 < Nu ≤ 37 Nu ≤ 5 5 < Nu ≤ 18 Nu ≤ 49 Nu > 49 Nu ≤ 29 29 < Nu ≤ 37 Nu ≤ 5 5 < Nu ≤ 18 Nu ≤ 49 Nu > 49 Nu ≤ 31 31 < Nu ≤ 52 Nu ≤ 6 6 < Nu ≤ 26 Nu ≤ 49 Nu > 49 Nu ≤ 32 32 < Nu ≤ 52 Nu ≤ 6 6 < Nu ≤ 26 Nu ≤ 49 Nu > 49 Nu ≤ 34 34 < Nu ≤ 60 Nu ≤ 5 5 < Nu ≤ 30
DCHs FACH DCHs HS-DSCH DCHs HS-DSCH DCHs FACH DCHs HS-DSCH DCHs HS-DSCH DCHs FACH DCHs HS-DSCH DCHs HS-DSCH DCHs FACH DCHs HS-DSCH DCHs HS-DSCH DCHs FACH DCHs HS-DSCH DCHs HS-DSCH
Table 5: Switching Thresholds when varying the HS-DSCH and fixing the Cell Coverage Radius to 95%.
Moreover, in the cases when the FACH is not used, then the maximum number of users served by the HS-DSCH (then the general maximum number of users) can reach up to 60 and 30 for 128 and 256 kbps applications respectively, if 24 W are assigned to the shared channel.
5 Conclusions In MBMS environment the choice of the most efficient transport channel is a key aspect, since a wrong transport channel selection could adversely affect the overall capacity of the system. In this paper we defined a RRM policy aiming at identifying the Switching Thresholds among DCH, HSDSCH, and FACH channels, by considering the radio channel conditions, the cell coverage radius, and several MBMS application bit rates. Obtained results demonstrate that a wise selection of transport channels performed thanks to the proposed RRM policy, leads to the effective management of MBMS services in a HAP standalone scenario. In particular we highlighted how the introduction of the HS-DSCH can lead to an improvement in the MBMS performance in term of consumed power, compared to the case when DCHs and FACH only are used.
Acknowledgments This work has been supported by Italian Research Program (PRIN 2007) Satellite-Assisted Localization and Communication system for Emergency services (SALICE). Web site: http://lenst.det.unifi.it/salice. Special thanks go to Anna Umbert Juliana of Signal Theory and Communication Dept. - Universitat Politecnica de Catalunya (UPC) for her valuable suggestions on the conducted study.
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