cqi-based scheduling algorithms in 3gpp lte
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DESCRIPTIONPrinciple of Novel Scheduling Algorithms in LTESimulation of Best CQI scheduling algorithm
VIETNAM NATIONAL UNIVERSITY HANOIUNIVERSITY OF ENGINEERING AND TECHNOLOGYFACULTY OF ELECTRONICS AND TELECOMMUNICATIONS
STUDENT RESEARCH CONTEST2012 2013 CQI-based scheduling algorithms in 3GPP LTE
Author: D.o.B: ID No.: Advisor:
inh Vit Anh 27 / 09 / 1991 09020006 Class: K54D Dr. Nguyn Quc Tun
Department: Telecommunications System FET, UET, VNU-H
Ha Noi, March 2013
3GPP Long Term Evolution (LTE) was developed based on 3GPP UMTS (Universal Mobile Telecommunications System). LTE allows the subscriber access the Internet from terminals with higher data rate and lower latency. LTE operates in many frequency bands but time and frequency are limited. Therefore, like other network system being implemented, saving radio resources in LTE is a considerable problem. Effective performance of the scheduler in eNodeB of LTE certainly plays an important role in the overall performance of the system. There are many scheduling algorithms was implemented, overall, such scheduling algorithms based on the channel quality indicator CQI is being used widely in large system. In this report, I will take an overview on the system model of 3GPP LTE, including resource allocation and technologies used in data transmission. Then, I will focus on an important factor in shared radio resource allocation in the LTE downlink: the scheduling algorithm. The content of this report will take concentration on the operation of Best CQI scheduling: describes, evaluates and compares with a normal CQIbased scheduling algorithm in order to represent the advantages and also disadvantages of these two scheduling algorithms due to the performance of LTE system.
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In this chapter, we will introduce the concept of LTE and its requirements as well as the highlighted features of LTE network system in part 1.1. Then, we will consider about the matter of radio resource allocation and management in part 1.2. Finally, the work on this report will be mentioned.
1.1. LTE and its requirementsIn the last few years, multimedia applications operate in user terminals using the Internet are being well developed along with the improvement of broadband mobile communications system. These types of applications require higher data rate. The HSPA/UMTS system is being implemented to meet this demand. 3GPP organization also keeps developing the performance of network system. LTE Long Term Evolution, developed by the Third Generation Partnership Project 3GPP, is a standard for wireless communications with high data rate for mobile and other types of terminals. The technologies used in LTE are the improvement of GSM/EDGE and UMTS/HSPA, to increase system throughput by using enhanced radio transmission interface together with a number of improvements in the core network. LTE can support subscribers with a maximum data rate of 100 Mb/s in the downlink and 50 Mb/s in the uplink, corresponding to the spectral efficiency and the bit rate in the downlink 3-4 times, in the uplink 2-3 times greater than HSPA/UMTS system. LTE system has a high flexibility with an operating bandwidth from 1.4 MHz to 20 MHz, supporting the user s movement speed up to 350 km/h, resulting the required user latency is 5ms with 5 MHz or higher spectrum allocation and acceptably 10ms with narrower bandwidth. System capacity is also increased significantly by using MIMO transmission technique along with OFDM technology to save channel bandwidth. LTE can have the best performance in coverage of 5 km radius and guarantee a connection in the radius of 30 km.Page | 2
1.2. What is scheduling?Time and frequency is the two limited resources in any radio communication system. Thus, using these resources effectively is main factor that contributes to the success of any network system. In addition to using multiplexing technology to save bandwidth, an effective scheduling mechanism also maximize the usage of radio resources. The scheduler with an optimized scheduling algorithm is considered as a key element of the base station with functions of resource management and distribution; decide which user will be assigned to the resource block. There are many types of scheduling in wireless network, for example, Best CQI, Round Robin, Proportional Fair, Fast Fair Throughput But Best CQI and Round Robin is two basic scheduling algorithms representing two main factors of scheduling: fairness and throughput. Round Robin has a simple principle of operation and is easy to implement as it only polls over all users, users data will be assigned to the resource block in a fixed interval, then, move to the next user. That, in turn, ensures the fairness between users. Best CQI is a scheduling algorithm based on channel quality. Each time CQI updated, users CQI will be calculated by the base station. User with the best CQI, respectively the best channel, will be chosen to assign its data to resource block. Thus, the channel capacity is always in maximum status because the quality of transmission channel is always the best. However, for users who stay far from base station or travel with high speed, their channel quality is not guaranteed, then, the permission for using system resource is really difficult.
1.3. Reports goalThe main purpose of this report is simulating the operation and evaluating the performance of Best CQI scheduling algorithm. In other hand, another simple CQI-based scheduling algorithm is also proposed in order to compare and analyze the advantages of Best CQI scheduling mechanism. The measurement of performance is the total system throughput.
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2. System ModelIn this chapter, we will provide a general view of LTE network and the technologies used in LTE. Section 2.1 describes the allocation and management of radio resource. Next, the Orthogonal Frequency Division Multiplexing will be briefly introduced in section 2.2. An element which is used to estimate the channel quality will be mentioned in section 2.3. In the final section, we will describe a theoretical system capacity in LTE
2.1. Resource allocation in LTEEach radio frame is Tf 307200 Ts 10 ms long and consists of 20 slots of length Tslot 15360 Ts 0.5 ms , numbered from 0 to 19. A sub-frame is defined as two consecutive slots where sub-frame i consists of slots 2i and 2i 1 . Each slot contains 7 or 6 OFDM symbol (depends on normal or extended cyclic prefix).
Figure 1. Frame structure in LTE 
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In LTE, radio resource in downlink can be imagined as a grid of resource block (RB) in time - frequency domain.
Figure 2. Resource Block in normal cyclic prefix 
Each RB is a part of one slot with a bandwidth of 180 kHz. This bandwidth is divided into 12 sub-carriers and the sub-carrier spacing is 15 kHz.
2.2. Orthogonal Frequency Division MultiplexingOFDM has been adopted as the downlink transmission scheme for the 3GPP LTE. OFDM is a multicarrier transmission scheme because it splits the input bit-stream signal into N parallel signals. These signals, then, are modulated by N sub-carrier mutually orthogonal using different levels of modulation such as QPSK, 16-QAM and 64-QAM. Finally, these subcarriers is multiplexed in OFDM symbol and transmitted on channels. Orthogonal characteristic of sub-carrier allows signals to be modulated overlap but also maintain the separating at the receiver because the peak at central frequency of this sub-carrier locates exactly at the null of other subcarrier. Thus, sub-carriers wouldnt be affected by Inter -Carrier Interference. Furthermore, the overlap of sub-carrier also contributes to bandwidth saving.
Figure 3. Spectrum of Orthogonal Sub-carriers Page | 5
2.3. Reference symbol in LTE downlinkReference symbol (RS) is the symbol that both transmitter and receiver already know. These symbols are put in a RB in order to estimate channel quality.
Figure 4. Location of Reference symbols in sub-frame in normal CP
In time domain, these symbols are added to the OFDM symbol of each slot in the first and the fifth position in the normal CP, or the first and the fourth in the extended CP. In frequency domain, RSs are added every 6 sub-carriers. The unique positioning of the pilots ensures that they do not interfere with one another and can be used to provide reliable channel estimation. All the RS found in a sub-carrier are time averaged across all OFDM symbol, resulting in a column vector containing the average for each reference signal sub-carrier. Thus, for each time slot, there is a fixed number of reference signal sub-carrier transmitted creating a reference signal xRS. The base station will receive an output signal of yRS. Then, with yRS obtained, the BS will find out the channel characteristic by using:
From this characteristic, BS will calculate SNR of the channel and determine the corresponding CQI to feedback to the UE to set the modulation order and coding rate for UE signal transmission.Page | 6
2.4. Channel capacityThe capacity of an AWGN channel can be calculated by the Shannon formula :
where C is channel capacity, B is bandwidth of the channel that occupied by users, and SNR is Signal-to-Noise Ratio. In each time slot of the LTE system, data is transmitted together with Cyclic Prefix (CP) to avoid Inter-Symbol Interference (ISI) and Reference Symbol (RS) to estimate the channel quality. Therefore, a correlative factor F is given to represent the inherent loss of the system for CP and RS.
with Nsc is the number of sub-carrier in each time slot, Ns is the number OFDM symbol in each slot, Tslot is the slot duration, and Tcp represents total time for CP in all OFDM symbols in a frame. Therefore, the channel capacity in LTE is represented by the modified Shannon as followed:
However, this theoretical capacity i