System Level Modeling and Optimization of the LTE Downlink

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2. 3GPP Long Term Evolution2.2. LTE Physical LayerThe LTE PHY layer substitutes the Code-Division Multiple Access (CDMA) accesstechnology employed in 3G standards with a new Orthogonal Frequency-DivisionMultiplexing (OFDM) PHY layer, which implies that the PHY resources no longerspan in the code and time domain, but rather the frequency and time domain (inboth cases additionally the spatial domain if MIMO is applied).1 frame: 10 ms1 subframe: 1ms1 2 3 4 5 6 7 8 9 101 subframe: 14 OFDM symbols1 2 3 4 5 6 7 1 2 3 4 5 6 7cyclic prefix: *slot: 0.5 msFigure 2.3: LTE frame structure (FDD mode).*: OFDM symbol nr. 1 employsIn LTE, DL transmissions are organized into radio frames with a duration of 10 ms,both for the Time Division Duplex (TDD) and Frequency Division Duplex (FDD)modes. Focusing just on the FDD case, each radio frame is subdivided into ten subframesof 1 ms each, subsequently divided into two slots and seven OFDM symbolseach. In order to avoid inter-symbol interference, a CP is added to the beginning ofeach symbol, with a length of 5.21 µs for the first symbol or 4.7 µs for the remainingsix symbols (Figure 2.3). A longer CP configuration is also standardized, but is notconsidered in this work [39].Table 2.3.: Available LTE system bandwidths and available resource blocks [40].Channel bandwidth (B channel ) [MHz] 1.4 3 5 10 15 20Number of RBs ( NRBDL )6 15 25 50 75 100Number of data subcarriers 72 180 300 600 900 1200Transmission bandwidth (B TX ) [MHz] 1.08 2.7 4.5 9 13.5 18Bandguard size [% of B channel ] 23% 10% 10% 10% 10% 10%In the frequency domain, the available bandwidth is divided into equally-spacedorthogonal subcarriers. The typical subcarrier spacing, which is employed in thisthesis, is 15 kHz, although a smaller 7.5 kHz spacing is also possible. Subcarriers areorganized in groups of NscRB consecutive subcarriers, which is 12 for the normal-lengthCP and 24 when employing a 7.5 kHz subcarrier spacing. Each of the subcarriergroups, spanning 180 kHz, is referred to as a Resource Block (RB), as depicted inFigure 2.4.The LTE standard defines a set of possible bandwidths [40], which14
2. 3GPP Long Term Evolutionchannel bandwidth:resource blocks:1 RB:180 kHzbandguard: typically10% ofzero DC subcarriersubcarriers: 1 RBFigure 2.4: LTE frequency spectrum distribution for a channel bandwidth (B channel ) of3 MHz and a 15 kHz subcarrier spacing: NRB DLRB= 15 and Nsc = 12.determine the number of RBs, data subcarriers and bandguard size, as listed inTable 2.3.Combining the time and frequency partitioning, a time-frequency grid such as theone shown in Figure 2.5 is obtained. Each element resulting from this time-frequencyseparation is termed a Resource Element (RE), and defines the positions in whichthe transmitted data, be it reference symbols or data/control channels are placed inthe DL frame.Resource Block (180 kHz)subcarrier (15 kHz)subframe: 1ms14 OFDM symbols......1 2 3 4 5 6 7 8 9 10 1112 1 2 3 4 12 1 2 3 4 5 6 7 8 9 10 1112slot (0.5ms)Resource ElementFigure 2.5: LTE time-frequency grid: allocation of the resource elements in an LTE subframeover time (14 OFDM symbols) and frequency (NscRB × NRB DL subcarriers).2.2.1. MIMO TransmissionMIMO techniques are one of the main enablers to achieve the throughput requirementsfor E-UTRAN listed in Table 2.1. The LTE standard defines support for one,two, and four transmit antennas. The supported multi-antenna transmit modesemploy either a Transmit Diversity (TxD) or SM transmission scheme in order toincrease diversity, data rate, or both. These are described in detail for the twotransmit antenna case to illustrate the concepts behind them, considering the fourtransmit antenna case as an extension of this case. SM can be operated in twomodes: Open Loop Spatial Multiplexing (OLSM) and Closed Loop Spatial Multiplexing(CLSM). While both require feedback regarding the number of spatiallayers employed, in OLSM no precoding matrix feedback is employed (hence the15
Page 1: DissertationSystem Level Modeling a
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6. Summary and Outlook6. Summary an
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6. Summary and Outlook6.2. OutlookW
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A. SNR-independence of the CLSM Pre
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B. Correlation Matrices for Shadow
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C. Taylor Expansion of the ZF MSEC.
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C. Taylor Expansion of the ZF MSEAp
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D. Evaluation of Multi-User GainD.
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D. Evaluation of Multi-User GainAvg
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Abbreviations and AcronymsAbbreviat
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Abbreviations and AcronymsKPILLRL2S
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Abbreviations and AcronymsU-PlaneUT
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Abbreviations and AcronymsBibliogra
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Abbreviations and Acronymsdescripti
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Abbreviations and Acronyms[69] Tech
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System Level Modeling and Optimization of the LTE Downlink

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