Algorithms and Performance Results for Dynamic HSDPA Resource Allocation Klaus Ingemann Pedersen and Per Henrik Michaelsen Nokia Networks, Niels Jernes Vej 10, DK-9220 Aalborg East, Denmark Email:
[email protected] Abstract— Cell specific algorithms for dynamic sharing of transmit power and channelization codes between HSDPA and Release’99 dedicated channels on the same carrier are proposed. A fast Node-B based HSDPA power allocation algorithm is derived, which is able to track the fast power control fluctuations of dedicated channels. Fast channelization code allocation is handled by the centralized RNC. The presented performance results show an improved utilization of the transmission resources. The cell throughput is improved significantly by using these algorithms as compared to using semi-static allocation schemes previously published in the open literature.
I. I NTRODUCTION High-speed downlink packet access (HSDPA) includes a number of performance enhancing features over Release’99 wideband code division multiple access (WCDMA), including a distributed architecture with location of the medium access control layer (called MAC-hs) in the base station (Node-B) [1]-[2]. In this paper we focus on configurations with mixed HSDPA and Release’99 on the same carrier with emphasis on adaptive transmission resource sharing between these two domains. In this respect the shared transmission resources per cell are power and channelization codes. Ideally, these transmission resources should be dynamically shared between HSDPA and Release’99 depending on the offered traffic and the QoS attributes for the services carried on these two domains. The resource allocation algorithms are an integral part of the radio resource management (RRM) algorithms, which includes both algorithms at the Node-B and at the centralized radio network controller (RNC). Given the specified RNC/Node-B functionalsplit, we present new performance results for scenarios with mixed traffic on HSDPA and Release’99 on the same carrier. Previous studies of mixed HSDPA and Release’99 on the same carrier have assumed fixed power and channelization code resources for HSDPA [8]. QoS studies for HSDPA have mainly focused on admission control aspects [3]-[4] and QoS aware MAC-hs packet scheduling algorithms [5]-[6]. However, QoS differentiation between HSDPA and Release’99 on the same carrier has not been studied extensively in the open literature, so we address this topic here. The rest of the paper is organized as follows: The RRM functional split between the RNC and the Node-B is introduced in Section II. The algorithms for dynamic transmission resource allocation are presented in Section III. Performance results are presented in Section IV. Finally, concluding remarks are given in Section V.
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II. S ETTING THE SCENE Fig. 1 shows a simplified sketch of the considered architecture. Admission control (AC) at the RNC is responsible for admission decisions, including mapping of new calls to either HSDPA or Release’99 dedicated channels (DCH). The packet scheduler (PS) for non real-time (NRT) DCHs is also located at the RNC, while the PS for HSDPA is at the Node-B as part of the MAC-hs entity [1]-[2]. The resource manager (RM) at the RNC is responsible for book-keeping of the used power and channelization code resources for each cell. The Node-B also has algorithms for adaptive HSDPA resource allocation. Although not explicitly indicated in Fig. 1 the Node-B and UE also contain algorithms for downlink inner and outer loop power control for Release’99 DCHs [1]. RNC
Node-B
AC
RM
Iub
HSDPA resource allocation
PS
MAC-hs
UE
NBAP protocol RRC protocol
Fig. 1. Simplified block diagram of RRM functional split between the RNC and the Node-B.
The control plane (c-plane) signaling between the RNC and the Node-B is carried on the Node-B application protocol (NBAP) [9]. As an example, the “NBAP Physical Shared Channel reconfiguration Request” message is used by the RNC to allocate HSDPA transmission resources to the MAC-hs. The radio resource control (RRC) protocol carries c-plane signaling between the RNC and the UE [10]. Among other things, the RRC protocol is used for configuration of DCH connections such as initial channelization code assignment, NRT DCH bit rate adjustments via channelization code re-assignment. The RRC protocol requires air interface capacity, so frequent messaging to many UEs on this protocol should be avoided when designing dynamic channelization code allocation algorithms [12]-[13]. The cost of sending NBAP messages is considered to be marginal as it only involves RNC/Node-B communication. Given the allocated power and channelization codes for HSDPA transmission, the MAC-hs in the Node-B is responsible for fast link adaptation, hybrid ARQ, and packet scheduling of the HSDPA users on the high speed downlink shared channel
(HS-DSCH) in the cell [2]. The experienced performance for the HSPDA users depends on how often the users are scheduled on the HS-DSCH, the experienced HS-DSCH SINR, and on the number of available high speed physical downlink shared channel (HS-PDSCH) codes. At HS-DSCH SINRs above 5 dB the HSDPA throughput increases with the number of allocated HSPDSCH codes, as it is more spectrally efficient to transmit on multiple HS-PDSCH codes as opposed to increasing the modulation order and effective code rate [1], [2], [4]. III. R ESOURCE ALLOCATION ALGORITHMS A. Power allocation algorithms The Node-B downlink power budget for one cell is summarized in Fig. 2. The total transmit power is divided between common channels, real-time (RT) DCH, NRT DCH, and HSDPA. The power for common channels includes the transmit power for the primary common pilot channel (P-CPICH). The average RT DCH power is controlled by the AC algorithm while the NRT DCH power is controlled by the PS algorithm in the RNC. The Node-B can be configured to report average measurements of the total transmit power and the non-HSDPA power, denoted by P total and P non−hsdpa , respectively [14]. These measurements are averaged over at least a 100 ms period. The maximum transmit power of the power amplifier (PA) for the cell is denoted by Pmax .
the Node-B is allowed to allocate all unused power for HSDPA. The difference between the RNC and Node-B based HSDPA power allocation schemes are pictured in Fig. 3 and 4. The non-HSDPA power for Release’99 channels is time-variant due to e.g. fast closed loop power control. The RNC based HSDPA power allocation is not capable of tracking the fast Release’99 power variations, since the RNC only receives average power measurements from the Node-B. The RNC based HSDPA power allocation algorithm will therefore have to reserve a power control headroom before allocating HSDPA power. On the other hand, the fast Node-B based HSDPA power allocation algorithm can more accurately track the fast power control fluctuations, so the PA can be better utilized as illustrated in Fig. 4. The fast Node-B power based HSDPA power allocation algorithm is therefore the preferred solution as it results in more HSDPA transmission energy (i.e. improved HSDPA coverage). Furthermore, this solution does not require frequent NBAP messages over the Iub to dynamically adjust the HSDPA power. Carrier power Maximum power
HSDPA power
Carrier power Maximum power Total Tx power measurement
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Non-HSDPA power Time
Ptarget Fig. 3. Slow RNC based HSDPA power allocation principle.
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Fig. 2. Downlink Node-B power budget for one cell.
HSDPA power
The RNC AC and PS algorithms aim at controlling the DCH traffic so [1], [7] Ptarget > P non−hsdpa ,
(1)
where Ptarget is a radio network parameter (dynamic adjustment of Ptarget is discussed later in this section). Thus, a capacity request (CR) for admitting new RT DCH connections or increasing the bit rate of NRT DCH connections is only granted if Ptarget > P non−hsdpa + ∆P, (2) where ∆P is the estimated power increase from increasing the DCH traffic [1]. The RNC can dynamically allocate HSDPA power to the MAC-hs by sending NBAP messages to the NodeB. If no NBAP message with HSPDA power allocation is sent,
Non-HSDPA power Time
Fig. 4. Fast Node-B based HSDPA power allocation principle.
The fast Node-B based HSDPA power allocation algorithm adjusts the HSDPA power every T seconds according to1 Phsdpa = Pmax − Pnon−hsdpa − Pmargin ,
(3)
1 The allocated HSPDA power is dynamically split between HS-SCCH and HS-PDSCH transmission, assuming HS-SCCH power control every TTI.
where Pnon−hsdpa is the measured non-HSDPA power during the last T seconds and Pmargin is a power safety marging, which is needed to account for unpredictable variations of the non-HSDPA power (e.g. due to fast power control). It is assumed that T = 10 ms. The RNC is in control of the overall power sharing between HSDPA and Release’99 via setting of Ptarget . Note that Ptarget limits the maximum Release’99 power and therefore also explicitly determines the available power for HSDPA. The RNC should therefore dynamically adjust Ptarget ∈ [P99min ; P99max ] depending on the offered traffic and specified QoS constraints, where P99min and P99max are the minimum and the maximum power for Release’99 channels. For the sake of simplicity, we assume that all traffic on HSDPA is NRT, although RT applications such as constant bit rate streaming [4],[6] and VoIP [16] are also attractive for HSDPA. Release’99 power congestion is detected if a DCH CR is rejected according to the criteria in (2). If the latter happens, Ptarget should be increased by ∆Ptarget if Release’99 has priority over HSDPA and Ptarget + ∆Ptarget < P99max . We assume that RT DCH always has priority over NRT HSDPA. If the CR is for NRT DCH, a parameter (W ) defines the priority of NRT DCH versus NRT HSDPA. This parameter can be statically defined by the operator, or it can be computed online depending on e.g. the UMTS bearer QoS attributes for the NRT DCH and NRT HSDPA users [15]. If HSDPA power congestion (i.e. all the allocated HSDPA power is constantly used) is detected, Ptarget should be reduced by ∆Ptarget if: Ptarget − ∆Ptarget > P99min and W gives HSDPA priority. B. Channelization code allocation algorithms It is assumed that one scrambling code is assigned per cell with an associated set of orthogonal channelization codes. The set of channelization codes can be represented by a binary code tree as shown in Fig. 5. Strategies for assignment of codes to Release’99 users with a DCH have been studied extensively in the open literature [12]-[13], where various algorithms have been proposed. In these studies, the primary objective has been to minimize the blocking probability of Release’99 CRs and the number of required code re-assignments. In this context, code re-assignment refers to allocation of new codes to Release’99 users to reduce fragmentation of the code-tree. Code re-assigments are in general undesired as they require RRC signaling and NBAP signaling to the UE(s) and Node-B(s), respectively (see Fig. 1). The Release’99 code assignment algorithms in [12]-[13] are applicable together with HSDPA by primarily allocating Release’99 codes from the left in the code tree as shown in Fig. 5. The RNC is responsible for allocation of HS-PDSCH codes with SF16 and HS-SCCH codes with SF128 to the MAChs. Assignment of more than one HS-SCCH code allows the MAC-hs to use code-multiplexing, where more than one HSDPA UE is scheduled on the HS-DSCH per TTI. The allocated HSDPA codes can be changed frequently with minimal cost, as it only requires NBAP signaling (no RRC signaling).
SF1 SF2 SF4 SF8 SF16 Rel'99 codes
Buffer zone
HSDPA codes
Fig. 5. Channelization code-tree allocation strategy. Only the first five levels in the code tree are pictured although spreading factors up to 512 are allowed.
Given the amount of assigned Release’99 codes and HS-SCCH code(s), the unused SF16 codes are assigned from the right to HS-PDSCH transmission, leaving a buffer zone of Cmargin codes between the Release’99 and HSDPA code domain. The buffer zone of Cmargin is included to leave room for minor Release’99 code assignments without modifying the allocated HSDPA codes. Note furthermore that HS-PDSCH codes should be on contiguous nodes in the channelization code tree [1]. If a Release’99 CR is initially code-blocked (due to lack of available channelization codes), the required code is immediately made available by down-grading the number of HSDPA codes, if Release’99 has priority over HSDPA. The same rules as described in Section III-A are applied to determine the relative priority between NRT DCH and NRT HSDPA, while RT DCH by default is given priority. On the contrary, if the Release’99 domain is reduced due to for instance end of Release’99 calls, the HSDPA code domain is increased if possible. HSDPA codes are only allocated if there are allocated calls on HSDPA in the cell. Finally, we assume that the codes in Table I are constantly allocated for Release’99 common channels from the left in the channelization code tree [1]. TABLE I S UMMARY OF CHANNELIZATION CODES FOR COMMON CHANNELS . Channel P-CPICH PCCPCH SCCPCH AICH PICH
Spreading factor (SF) 256 256 64 256 256
IV. S IMULATION CAMPAIGN A. Simulation methodology Dynamic Monte-Carlo system simulations are conducted for a 3-sector macro cellular network. Layer-1 to Layer-3 are explicitly simulated, using the simulation methodology outlined in [8] with the parameter settings summarized in Table II. Each HSDPA user in the network receives the HS-DSCH, the HSSCCH, and an associated DCH which carries Layer-3 signaling information. The associated DCH is configured to 3.4 kbps so it occupies one channelization code with SF 256. Dynamic link adaptation is used for transmission on the HS-DSCH as described in [8], including explicit simulation of Hybrid ARQ
TABLE II S UMMARY OF THE MAIN DEFAULT SIMULATION PARAMETERS . Parameter Site-to-site distance Path loss model Shadow fading std. Power delay profile Max cell Tx power (Pmax ) P-CPICH Tx power Max Rel’99 power (P99max ) Min Rel’99 power (P99min ) Max HS-SCCH power Number of HS-SCCH codes Common Rel’99 codes Code margin (Cmarging ) Node-B power allocation period (T ) ∆Ptarget MAC-hs scheduler Hybrid ARQ combining UE receiver type HSDPA UE category Soft handover window Geographical UE distribution
Setting 2.8 km COST231 Hata 8 dB ITU Veh-A @ 3 kmph 20 W 2W 12 W 4W 2.5 W 1 (SF128) See Table I One SF128 code 10 ms 0.5 W Proportional Fair Incremental redundancy 1-Rx Rake Category 10 [1] 4 dB Uniform
New calls are generated according to a homogeneous Poisson process. A simple closed loop TCP web-browsing model is used to simulate NRT traffic [8]. The packet call size is sampled from a truncated log-normal distribution with a mean of 100 kbytes and a minimum and maximum of 20 kbytes and 1 Mbyte, respectively. A constant bit rate streaming model at 64 kbps is used for RT traffic with a mean call length of 40 seconds [4]. Unless otherwise mentioned, RT is simulated on DCH and NRT on HSDPA. B. Simulation results At first, a series of simulations are executed to determine the optimal value of Pmargin . These simulations are configured so that there are 4-6 active NRT HSDPA users per cell with data to transmit. The offered RT DCH traffic is adjusted in the range 200-600 kbps/cell. The optimization criteria is to minimize the value of Pmargin while still guaranteeing that the PA is only saturated with a maximum probability of 2%, i.e. P r{Ptotal ≥ Pmax } ≤ 2%. The result of this exercise is Pmargin = 3 W, so this parameter setting is by default used in the following. As an example, Fig. 6 shows the cumulative distribution function (cdf) of the transmit power from one cell for a scenario where the average offered RT DCH traffic equals
270 kbps/cell. The total power, HS-SCCH power, HS-DSCH power, and non-HSDPA power cdf are reported. In this particular case, Ptarget converges to 6 W. The HS-SCCH power is observed to reach the maximum value of 2.5 W (see Table II) with a probability of 2%, while it is less than 1 W with 86% probability. The total transmit power is fairly constant around 17 W due to the use of fast Node-B based HSDPA power allocation every T = 10 ms. 1.0
non-HSDPA power
0.9 0.8
Cumulative distribution
with soft combining [11]. One user is scheduled on the HSDSCH every TTI according to the proportional fair scheduling (PF) algorithm [2], [8], [17]. HS-SCCH power control is used every TTI. HSDPA mobility is simulated by assuming synchronized serving HS-DSCH cell changes when users move from one cell to another. Release’99 users use the conventional soft handover algorithm [1]. Note that UEs with an active set size larger than one occupy DPCH channelization codes in multiple cells. PC, AC, PS, and RM as described in Section II for Release’99 are simulated [1], including the algorithms for fast Node-B based HSDPA power allocation and RNC based HSDPA channelization code allocation in Section III.
0.7
HS-SCCH power
0.6
HS-DSCH power
0.5
Total power
0.4 0.3 0.2 0.1 0.0 0
2
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6 8 10 12 14 Node-B transmit power [W]
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Fig. 6. Cumulative distribution of the Node-B transmit power for one cell.
Fig. 7 shows the average carried throughput per cell on DCH and HSDPA when the proposed dynamic power/code resource sharing algorithms are used (solid line). Results are also presented for a fixed allocation of 5 HS-PDSCH codes and slow RNC based HSDPA power allocation (dashed line). The latter results are from [8]. Without any HSDPA load in the cell, the carried RT DCH traffic can reach 780 kbps/cell with a 5% RT DCH call blocking probability. In cases with a lower offered RT DCH traffic, the carried NRT HSDPA throughput increases, as more and more power/codes are made available for the MAChs. Notice here that the offered HSDPA traffic is adjusted so we always have 4-6 active HSDPA users per cell. The numbers indicated in the solid line are the total cell throughputs, i.e. the sum of DCH and HSDPA throughputs. For the dashed line, the maximal total cell throughput equals 1.3 Mbps for a DCH throughput of 400 kbps. With dynamic power/code allocation, the total cell throughput equals 1.7 Mbps for the same offered DCH throughput, indicating a clear gain from using fast dynamic power/code resource sharing. When comparing the results for no DCH traffic in the cell, it is observed that the HSDPA throughput increases from 1.2 Mbps to 2.2 Mbps by increasing the number of HS-PDSCH codes from 5 to 15. The latter observation is in coherence with the results in [2]. In order to gain further insight, Fig. 8 shows the cdf of the allocated and used HS-PDSCH codes for two different load conditions. Here the average non-HSDPA power equals approximately 6 W and 12 W for the DCH load of 270 kbps/cell and 535 kbps/cell, respectively. It is observed in both cases that the MAC-hs seldomly transmits on all the allocated HS-
HSDPA cell throughput [Mbps]
2.5
resources can be optimally adjusted. The fast resource allocation algorithms adapt to the offered traffic for Release’99 and HSDPA, respectively. Results from dynamic macro cell network simulations show cell throughputs of e.g. 400 kbps on Release’99 plus 1.25 Mbps on HSDPA on the same carrier, i.e. a total cell throughput of 1.65 Mbps. For an HSDPA-only carrier, the cell throughput equals 2.2 Mbps. For comparison, the maximal available cell throughput equals 1.3 Mbps (0.4 Mbps for DCH and 0.9 Mbps for HSDPA) if the number of HS-PDSCH codes is fixed at 5, while using a semi-static RNC based HSDPA power allocation algorithm.
Dynamic power/code allocation
2.0
1.81 Mbps 1.5 1.62 Mbps 1.0
RNC based HSDPA power allocation and 5-codes
1.45 Mbps
0.5
VI. ACKNOWLEDGEMENTS 0.0 0
100
300 200 400 500 600 Rel'99 DCH cell throughput [kbps]
700
800
Fig. 7. Average cell throughput for HSDPA and Release’99 DCH on the same carrier frequency.
Thanks to David Steele and Søren Corneliussen for implementing and testing the dynamic resource allocation algorithms in the simulator, and for contributing with various ideas. R EFERENCES [1]
PDSCH codes. The reason is that UEs seldomly experience sufficiently high HS-DSCH SINR to use all the allocated HSPDSCH codes, i.e. the simulated scenario is power limited rather than code limited. However, in environments with better HS-DSCH SINR conditions (e.g. in micro cells or cases with dual antenna terminals), the used HS-PDSCH codes will converge towards the allocated number of HS-PDSCH codes. The number of used HS-PDSCH codes exceeds 5 with a probability of 73% and 65% in the two considered cases. This indicates a need for code-multiplexing and two HS-SCCH codes if some of the HSDPA users only support 5 HS-PDSCH codes (e.g. UE category 6). 1.0
[3]
[4] [5] [6] [7]
Allocated HS-PDSCH codes Used HS-PDSCH codes
0.9
[8]
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Cumulative distribution
[2]
RT DCH load 535 kbps/cell
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[9]
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[10]
0.5 RT DCH load 270 kbps/cell
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[11]
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[12]
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[13] 0.0 1
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5 6 7 8 9 10 11 12 13 14 15 Number of HS-PDSCH codes
Fig. 8. Cumulative distribution function for the number of allocated and used HS-PDSCH codes for two different load situations.
[14] [15] [16]
V. C ONCLUDING REMARKS We have studied a scenario with Release’99 and HSDPA on the same carrier frequency. With a fast Node-B based HSDPA power allocation algorithm and a fast RNC based channelization code allocation algorithm, the shared transmission
[17]
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