Uplink Component Carrier Selection for LTE-Advanced Systems with Carrier Aggregation
Summary (2 min read)
Introduction
- The simulation results show that CA can enhance the throughput, fairness and latency compared with independent carrier scenario for different traffic models.
- Section II gives a general overview of the most important RRM functionalities in uplink CA for LTE-Advanced systems, with special attention on CC selection together with uplink power control.
- Section III outlines the simulation methodology and main assumptions.
- Finally, some conclusions are drawn in Section V.
II. RADIO RESOURCE MANAGEMENT
- The RRM framework for multi-component carrier LTEAdvanced system is illustrated in Fig. 1. Separate RRM blocks operate independently on each component carrier.
- It has been agreed within 3GPP working group to adopt independent Link Adaption (LA) and Hybrid ARQ (HARQ) per CC in coherence with LTE Rel’8 assumptions [3].
- The admission control module in base station decides whether an incoming connection should be accepted or not.
- Then the CC selection module allocates one or multiple CCs to the incoming user based on the Quality-of-Service (QoS) requirements, terminal capability, etc.
- Since the UEs are limited by the maximum transmission power, power control is also an important issue in uplink.
A. CC Selection and Load Balancing
- The main difference of LTE-Advanced RRM framework compared to Rel’8 is the CC-selection functionality which is responsible for configuring a CC set for each user.
- These secondary CCs can be activated/deactivated by signalling [2].
- The legacy Rel’8 users can only be assigned on one CC, while LTE-Advanced users can be assigned on multiple CCs.
- The main difference between uplink and downlink is the transmission power constraint of a UE.
- For power limited users transmitting at (or close to) maximum transmission power, such cost might counterbalance the gain brought by multiple-CC transmission, and even results in a performance loss compared to the case where the SC-FDMA properties of the transmitted signals are maintained (single CC assignment).
B. Proposed pathloss-threshold based CC Selection
- The basic idea behind the proposed pathloss-threshold based CC-selection algorithm is to distinguish between powerlimited and non-power-limited LTE-Advanced UEs, and assign only one CC to power-limited users, but may assign multiple CCs to non-power-limited users.
- The authors assume that eNodeB is deploying Load Adaptive Power Control (LAPC) independently on each CC.
- Previous studies [8] show that system performance in terms of coverage and cell throughput is highly dependent on the setting of P0.
- The pre-allocation of transmission power on each assigned CC depends on the UEs’ metrics and the scheduling algorithm.
III. SIMULATION ASSUMPTIONS
- The performance evaluation is based on a detailed multicell system level simulator which follows the guidelines in [12].
- Two contiguous component carriers, each with 20 MHz bandwidth, are configured to form a wide band of 40 MHz.
- Rel’8 UEs are only assigned on one CC with RR load balancing, while LTE-Advanced UEs can be assigned on one CC or both CCs depending on the CC-selection algorithm.
- A dynamic traffic model with Poisson arrival per cell is assumed, each call having a finite buffer of 2 Mbits payload.
- Table I summarizes the main parameter settings used in the system-level simulations.
IV. SIMULATION RESULTS
- The authors start their analysis by evaluating the effectiveness of the derived path-loss threshold.
- Therefore, if the threshold value is set to be high, the average user throughput decreases, while the cell edge user throughput remains steady since the cell edge UEs usually experience high path-loss and they are assigned only on one CC.
- As a result, those power-limited cell edge LTE-A UEs will experience performance loss from being scheduled over multiple CCs due to the further reduction of maximum UE transmission power when transmitting over multiple CCs.
- Since CC selection is critical on the overall system performance, next the authors evaluate and compare the performance of proposed pathloss-based CC-selection algorithm with other CC-selection algorithm, e.g., ’blind’ CC-selection (all LTEA UEs are assigned on all CCs).
- Fig. 4 shows the cell edge user throughput versus the offered load with 4 dB power backoff in different scenarios.
V. CONCLUSIONS
- The authors have investigated the uplink performance of carrier aggregation in LTE-Advanced systems with different CC selection algorithms.
- In order to optimize the system performance, the authors have derived a pathloss threshold that separates the UEs into two categories: power-limited and non-powerlimited UEs, and assign only one CC to power-limited LTE-A UEs, but assign multiple CCs to non-power-limited LTE-A UEs.
- The simulation results verified that their derived pathloss threshold can achieve the optimal performance in terms of cell edge and average user throughput under various traffic loads.
- By comparing performance between different CC selection algorithms, it is shown that there is a performance loss at cell edge if all LTE-A UEs are assigned on multiple CCs (’blind’ CC-selection), due to the limitation of maximum UE transmission power and the additional power back-off needed when transmitting over multiple CCs.
- On the other hand, assigning LTE-A UEs to one or several CCs depending on their pathloss can achieve the same cell edge performance as Rel’8 case, but shows quite high gains in average and cell center user throughput compared to Rel’8 case.
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Citations
175 citations
Cites background from "Uplink Component Carrier Selection ..."
...[40] Enhanced Channel quality, round robin....
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...An exception, however, can be found in [40], where the authors propose a scheduler that can support both LTE and LTE-Advanced UE....
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170 citations
Cites background or methods from "Uplink Component Carrier Selection ..."
...management (RRM) framework for LTE-Advanced systems [26]....
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...Especially for uplink transmission, the maximum transmission power becomes a constraint [26]....
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...Objective Allocation algorithm Characteristics/Challenges To improve fairness between UEs of different channel conditions Path-loss based CC Selection [26] - allows separation of users into power-limited and non-power-limited - can allocate one CC to the power-limited users and all CCs to non-power-limited users - also proposes the method of path-loss threshold calculation based on the 95th-percentile user path loss - provides better performance than allocation of all CCs to all UEs - But only considers the case of intra-band CA RB scheduling with max RB number limit [55] - converts limit of the maximum transmission power into the maximum number of RBs - proposes a priority metric matrix and allocates RBs to users to maximize the priority sum - But only considers the case of equal power allocation among subcarriers Feedback reduction scheme [31] - proposes HARQ bundling, CQI compression, and hybrid of the two methods - proposes a weighted PF scheme for the users at the cell-edge - How to set the path-loss threshold to decide a proper reduction technique and how to set the weights for different feedback reduction techniques remain challenging User grouping resource allocation [56] - UEs are spatially grouping [57], [58] - Only one user in each group sends feedback CQI on behalf of the entire group - supports prioritization of the edge UE groups for resource allocation - uses PF scheme for inter-group/intra-group scheduling in the time and frequency domains - But is only suitable for the low-speed mobile UEs...
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...Since efficient methods to set path-loss threshold are required, in [26], a threshold calculation method based on the 95th-percentile user path loss in the corresponding cell, is proposed as shown in Eq....
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...Since the unbalanced traffic loading across multiple CCs leads to performance degradation, advanced rules for user allocation to CCs are proposed in [26] to help balance the loading across CCs....
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66 citations
Cites background from "Uplink Component Carrier Selection ..."
...mance analysis of CA in uplink (UL) and downlink (DL) are investigated in [2]-[4], respectively....
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56 citations
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References
446 citations
"Uplink Component Carrier Selection ..." refers methods in this paper
...In order to meet the technical requirement defined by IMTAdvanced, which targets to achieve peak data rates up to 1 Gbps in downlink and 500 Mbps in uplink respectively [1], the 3 Generation Partnership Project (3GPP) started a new study item in March 2008 on evolving from Long Term Evolution (LTE) towards LTE-Advanced with the target of finalizing the Release-10 (Rel10) specifications between the year 2010 and 2011....
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...Parameters Settings Propagation scenario Macro case #1 Layout 7 sites - 3 sectors/site - wrap around Component carriers 2 × 20 MHz contiguous @ 2GHz band 92 available PRBs per CC eNode-B receiver 2-Rx MRC UE bandwidth ATB: [1, 92] PRBs per CC Packet scheduling Proportional Fair with ATB Traffic model Finite buffer with Poisson arrival Fixed file size of 2 Mbits per UE Offered load: [5 : 5 : 35] Mbps Available MCSs BPSK (R=1/5,1/3) QPSK (R=1/4,1/3,1/2,2/3,3/4) 16QAM (R=2/3,3/4,5/6)...
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382 citations
215 citations
89 citations
"Uplink Component Carrier Selection ..." refers background in this paper
...bandwidth allocation and packet scheduling together to exploit the envelope of UEs’ metrics [9]....
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87 citations
"Uplink Component Carrier Selection ..." refers background in this paper
...The downlink performance of CA over deploying independent carriers is investigated in [4]....
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