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LTE – Long Term Evolution

國立中興大學資工系 曾學文

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ITU Activities

International Telecommunication Union (ITU) International Mobile Telecommunications-Advanced (IMT-Advanced)

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Mobile Broadband Technology (1)

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Mobile Broadband Technology (2)

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History

• LTE is the next major step in mobile radio communications, and will be introduced in 3rd Generation Partnership Project (3GPP) Release 8.

• The starting point for LTE standardization was the 3GPP RAN Evolution Workshop, held in November 2004 in Toronto, Canada.

• A study item was started in December 2004. – 3GPP TR 25.913 Feasibility Study of Evolved UTRA and UTRAN

Radio Access Network (RAN)

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History

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Network Evolution : Made Simple

Leveraging Efficient Routing and Low Cost of IP

Gateway GPRS Support Node (GGSN) Serving GPRS Support Node (SGSN)

Radio Network Controller (RNC)

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Decibels (dB)

• Decibels – 10 log10 (x) – Power in decibels

» dB » Y dB=10 log10 (x Watt)

• Power ratio in decibels – dB – Power P1, P2 in Watt – 10 log10 (P1/P2)

• Example: – Input power 100W and output power 1W – What’s the power ratio in decibel? – Ans: 20dB

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dBm

• dBm – Reference power is 1 mW – 10 log10 (Watts/10-3) – Example:

» 0 dB= 30dBm=1 Watt

• Summary – P (dBW) = 10 log (P/1 Watt) – P (dBm) = 10 log (P/1 mWatt)

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High Data Rates in Mobile Communication • Shannon provided the basic theoretical tools needed to

determine the maximum rate (channel capacity). • A radio link, only impaired by additive white Gaussian noise, the

channel capacity C is given by the relatively simple expression

– BW is the bandwidth available for the communication, – S denotes the received signal power, – N denotes the power of the white noise impairing the received signal.

• What factors limit the achievable data rate – The received signal power can then be expressed as S = Eb • R,

» Eb is the received energy per information bit. » R is a certain information rate.

– The noise power can be expressed as N = N0 • BW. » N0 is the constant noise power spectral density measured in W/Hz

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Fundamental Constraints • Together with the above expressions for the received

signal power and noise power, this leads to the inequality:

• By defining the radio-link bandwidth utilization γ = R/BW, • Normalized to the noise power density, for a given

bandwidth utilization γ:

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Fundamental Constraints

γ = R/BW

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Higher-Order Modulation • Higher-order modulation schemes, such as 16QAM

or 64QAM, require a higher Eb /N0 at the receiver for a given bit-error probability, compared to QPSK.

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Propagation Mechanisms

• Reflection (反射) – Propagation wave impinges on an object which is large as

compared to wavelength

- e.g., the surface of the Earth, buildings, walls, etc.

• Diffraction (繞射) – Radio path between transmitter and receiver

obstructed by surface with sharp irregular edges – Waves bend around the obstacle, even when LOS (line of sight)

does not exist

• Scattering (散射) – Objects smaller than the wavelength of the

propagation wave

- e.g. foliage, street signs, lamp posts

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Radio Propagation Effects

Transmitter d

Receiver

hb

hm

Diffracted Signal

Reflected Signal

Direct Signal

Building

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Multi-Carrier Transmission • One way to increase the overall transmission bandwidth is the

use of so-called multi-carrier transmission. • Multi-carrier transmission implies that, instead of transmitting a

single more wideband signal, multiple more narrowband signals, often referred to as subcarriers, are frequency multiplexed and jointly transmitted over the same radio link to the same receiver.

valleys

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Multi-Carrier Transmission

There are two drawbacks for multi-carrier transmission

1.One is that the spectrum of each subcarrier typically does not allow for very tight subcarrier “packing”. (BW consumption)

2.The parallel transmission of multiple carriers will lead to larger variations in the instantaneous transmit power.

– It will have a negative impact on the transmitter power-amplifier efficiency implying increased transmitter power consumption and increased power-amplifier cost.

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Link Adaptation: Power and Rate Control • Dynamic power control dynamically adjusts the radio-link

transmit power to compensate for variations and differences in the instantaneous channel conditions.

– The transmit power is in essence inversely proportional to the channel quality.

• Transmit-power control can be seen as one type of link adaptation

– Adjust for transmission parameters,

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Link Adaptation: Power and Rate Control

• Rate control does not aim at keeping the instantaneous radio-link data rate constant, regardless of the instantaneous channel conditions.

• Thus, rate control maintains the Eb/N0 ~ P/R at the desired level, not by adjusting the transmission power P, but rather by adjusting the data rate R.

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Link Adaptation: Power and Rate Control

• Rate control in principle implies that the power amplifier is always transmitting at full power and therefore efficiently utilized.

• In practice, the radio-link data rate is controlled by adjusting the modulation scheme and/or the channel coding rate.

– The Eb/N0 at the receiver is high and the main limitation of the data rate is the bandwidth of the radio link.

– Higher-order modulation (16QAM or 64QAM)

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Channel-Dependent Scheduling (Downlink)

• Transmissions to different terminals within a cell are typically mutually orthogonal

– there is no interference between the transmissions (no intra-cell interference).

• Downlink intra-cell orthogonality can be achieved in

– Time-Division Multiplexing (TDM) – Frequency-Domain Multiplexing (FDM) – Code-Domain Multiplexing (CDM) – Spatial-Division Multiplexing (SDM)

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Channel-Dependent Scheduling

• Scheduling the user with the instantaneously best radio-link conditions is often referred to as max-C/I (or maximum rate) scheduling

• The max-C/I scheduler can be expressed as scheduling user k given by:

• where Ri is the instantaneous data rate for user i

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Channel-Dependent Scheduling

• A pure max-C/I-scheduling strategy may rise “starve”

– the terminal with bad channel conditions will never be scheduled.

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Channel-Dependent Scheduling

1. max-C/I scheduling: this situation is often not acceptable from a quality-of-service point of view.

2. round-robin scheduling: it is not fair in the sense of providing the same service quality to all communication links. – In that case more radio resources (more time) must be given

to communication links with bad channel conditions.

3. proportional-fair scheduling: the shared resources are assigned to the user with the relatively best radio-link conditions – user k is selected for transmission according to

– where Ri is the instantaneous data rate for user i and is

the average data rate for user i.

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Uplink Scheduling

• The uplink power resource is distributed among the users, while in the downlink the power resource is centralized within the base station.

– The maximum uplink transmission power of a single terminal is lower than the output power of a base station.

• Whether the uplink relies on orthogonal or non-

orthogonal multiple access and the type of link adaptation scheme used, also have a significant impact on the uplink scheduling strategy.

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Non-orthogonal multiple-access scheme

• Power control serves the purpose of controlling the amount of interference affecting other users.

– This can be expressed as the maximum tolerable interference level at the base station in a shared resource.

• Scheduling a terminal when the channel conditions are favorable may not directly translate into a higher data rate as the interference generated to other simultaneously transmitting terminals in the cell must be taken into account.

• Channel dependent scheduling will provide a gain for the system in terms of lower intra-cell interference.

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Orthogonal multiple-access scheme

• Intra-cell power control is fundamentally not necessary and the benefits of channel-dependent scheduling become more similar to the downlink case.

• A terminal can transmit at full power and the scheduler assigns a suitable part of the orthogonal resources (a suitable part of the overall bandwidth) to the terminal for transmission.

– The remaining orthogonal resources can be assigned to other users.

• Leakage between the received signals or limited dynamic range in the receiver circuitry, may pose restrictions on the maximum tolerable power difference between the signals from simultaneously transmitting terminals.

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Uplink Scheduling

• The inter-cell multiple access is non-orthogonal, regardless of the intra-cell multiple access, which sets limits on the allowable transmission power from a terminal.

• A max-C/I scheduler would assign all the uplink resources to the terminal with the best uplink channel conditions.

– Neglecting any power limitations in the terminal, this would result in the highest capacity.

• Greedy filling: the terminal with the best radio conditions is assigned as high a data rate as possible.

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Link Adaptation and Channel-Dependent Scheduling in the Frequency Domain • Channel variations in the time domain could be

utilized to improve system performance by applying channel-dependent scheduling.

• Through the use of OFDM transmission, scheduling and link adaptation can also take place in the frequency domain.

– every OFDM subcarrier – the power and/ or the data rate of each OFDM carrier can be individually adjusted for optimal utilization.

– different subcarriers are used for transmission to or from different terminals.

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Traffic Behavior and Downlink Scheduling

• Consider three different downlink schedulers: – Round-robin (RR) scheduler, where channel conditions are

not taken into account. – Proportional-fair (PF) scheduler, where short-term channel

variations are exploited while maintaining the long-term average user data rate.

» will ensure some degree of fairness by selecting the user supporting the highest data rate relative to its average data rate

– Max-C/I scheduler, where the user with the best instantaneous channel quality in absolute terms is scheduled.

» all resources are allocated for transmission to the terminal whose channel conditions support the highest data rate.

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Traffic Behavior and Downlink Scheduling (a) bursty packet data

– the users’ buffers will be finite and in many cases also empty

(b) a web page – after transmitting the page, there is no more data to be

transmitted to the terminal until the users requests a new page by clicking on a link.