ICLTERPOP Section 3_LTE Link Budgets.pdf

LTE Link Budgets

LTE LINK BUDGETS

© Informa Telecoms & Media

LTE Link Budgets

LTE LINK BUDGETS Intro to Basic Radio System Typical T ypical Link Budget Requirements Requirem ents LTE Link Budget Variables LTE Transmit Power Capability for the UE Additional Factors Affecting UE Power Output eNodeB Power Output Characteristics Typical T ypical Losses in the eNB Antenna Characteristics for the UE Antenna Characteristics for eNB Sensitivity Calculation for the eNB Thermal Noise Nois e in Radio Systems Type T ype of Service Servic e and Impact on Noise Floor Implementation Margin, UE, eNB, (from blue book) Receiver Noise Figure Total T otal Noise Floor Cascaded Noise Typical T ypical SNR for LTE Modulation and Coding Schemes Duplex Gap and Duplex Distance, Effect on Receiver Sensitivity Calculating System Gain Environmental Factors and Noise Rise Shadow Margin (Slow Fading) Building and Foliage Losses Body Loss Uplink and Downlink Noise Rise Propagation Modelling Coverage from Link Budget Comparison of Models COST 231 Propagation Model The WINNER Model Cell Range Calculations from MAPL


4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 48 50 52 54 58 60 64 66 70

LTE Link Budgets

Intro to Basic Radio System Every radio system is a series of components and links, from the transmitter to the receiver. Each element of the system will exhibit some attribute of performance that affects the overall performance of the end to end system. A typical link budget exercise will need to quantify each of these performance attributes and understand the impact it may have on the system performance, i.e. the capacity and coverage. Many factors can be determined determi ned from manufacturers data sheets, thing such as the Tx power, power, feeder losses, losse s, antenna gains etc..however etc..however some parts of the system, the radio i nterface, must be modelled in order to determine a satisfactory plan.

4


Output power Watts, milliwats dBm, dBW Loss @ oper. freq. dB/length

Gain (directivity) dBi dBd

• •

TX

Information source

•

•

Modulator

Amp

Xmission line

•

Antenna

Loss (dB) Operating freqency (MHz, GHz) Distance (km, miles) Environment •

• • • • •

L I N Time dispersion (sec, nsec) Bandwidth (KHz, MHz) K

Operating freqency (MHz, GHz) Bandwidth (KHz, MHz) Signalling rate (Ksps, Msps) Modulation (X-PSK, X-QAM) Error correction overhead

• •

• • •

Fading (selective, flat) Inter-symbol interference

RX

Information source

Demodulator

Sensitivity* Threshold (dBm) C/N (dB) C/I+N (dB) Eb /No (dB) •

Amp

• •

Gain (dB) NFdB

Xmission line Loss @ oper. freq. dB/length NFdB • •

Antenna Gain (directivity) dBi dBd • •

• • •

Fig. 1 – Typical Radio System © Informa Telecoms & Media

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LTE Link Budgets

Typical Link Budget Requirements One of the main aims of calculating a link budget is to determine the maximum path loss allowed across the radio link for a given performance objective. The link loss will be due in part to the performance of the transmitter and receiver components as well as the impact of the environment through which the signal will propagate. The goal of link planning is to determine the parameter MAPL (Maximum Allowable Path Loss) MAPL = System Gain – Margin(fade, body, building, trees) System Gain is a function of the radiated power from the transmitter system and the minimum signal power that can be presented to the face of the receiving antenna. The value of System gain is an indication of the maximum and minimum values in the link budget. Link Margins are subtracted from the System Gain to determine the maximum path loss for a given set of assumptions for the transmitting and receiving system. This MAPL can subsequently converted in to a nominal cell range using an appropriate propagation model. System gain is determined by subtracting the maximum transmit power from the minimum receive power. System Gain = EiRP – IRLmin Where EiRP and IRLmin are given by: EiRP = Tx_PWR – Ltx + Gtx IRLmin = Rx_SENS + Lrx – Grx The values of feeder and connector (and any other) losses can be determined from manufacturer data sheets as can the Tx_PWR of the eNB and UE. It is likely that the Rx-SENS will also be quoted by the vendor for the eNB and UE however the calculation is rather complex and can involve many parameters that will ultimately have a g reat impact on the overall system performance, it is worth therefore, a closer examination.

6


EiRP = Tx_PWR – L tx + Gtx IRLmin = Rx_SENS + L rx – Grx System Gain = EiRP – IRLmin MAPL = System Gain – Margin (fade, body, building, trees)

Fig. 2 – Basic Link Budget Expressions © Informa Telecoms & Media

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LTE Link Budgets

LTE Link Budget Variables The basic expression expressio n from above can be formulated to include aspects of the LTE LTE link budget. The expressions on the opposite opposi te page show how the MAPL for the uplink and downlink may be calculated. It is assumed in these cases that the UE will have no losses due to cables or connectors. Since most cellular systems are limited by the performance of the uplink it is common to being the link budgeting process with the uplink and look for a link balance with the downlink.

8


MAPLUL = (Tx_PWRUE + Gtx_UE ) – (Rx (Rx_SE _SENS NSeNB + Lrx_eNB – Grx_eNB ) – Ma Margi rgins ns(fade, body, building, trees)

MAPLDL = (Tx_PWReNB – Ltx_eNB + Gtx_eNB ) – (Rx (Rx_S _SENS ENSUE – Grx_UE ) – Ma Margi rgins ns(fade, body, building, trees)

Fig. 3 – LTE Link Budget Expressions © Informa Telecoms & Media

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LTE Link Budgets

LTE Transmit Power Capability for the UE The power output of the UE is pretty straight forward since at the prese nt time only a single maximum power output is specified. However it could be possible in future to have different power outputs depending on the power class of the UE in each of the different specified bands. The following maximum max imum output powers can be assumed: assume d: •

10

23 dBm for the UE


E-UTRA band

Class 1 (dBm)

Tolerance (dB)

Class 2 (dBm)

Tolerance (dB)

Class 3 (dBm)

Tolerance (dB)

1

23

±2

2

23

±22

3

23

±22

4

23

±2

5

23

±2

6

23

±2

7

23

±22

8

23

±22

9

23

±2

10

23

±2

11

23

±2

12

23

±22

13

23

±2

14

23

±2

17

23

±2

18

23

±2

19

23

±2

20

23

±2[2]

21

23

±2

33

23

±2

34

23

±2

35

23

±2

36

23

±2

37

23

±2

38

23

±2

39

23

±2

40

23

±2

Class 4 (dBm)

Tolerance (dB)

…

…

Note: 1. The above tolerances are applicable for UE(s) that support up to 4 E-UTRA operating bands. For UE(s) that support 5 or more E-UTRA bands the maximum output power is expected to decrease with each additional band and is FFS 2. For transmission transmissio n bandwidths (Figure (Fig ure 5.6-1) 5.6-1) confined within FUL_low and FUL_low + 4 MHz or FUL_high – 4 MHz and FUL_high, the maximum output power requirement is relaxed by reducing the lower tolerance limit by 1.5 dB 3. For the UE which supports both Band 11 and Band 21 operating frequencies, the tolerance is FFS 4. PPowerClass is the maximum UE power specified without taking into account the tolerance

Fig. 4 – Typical Transmitter Characteristics for UE © Informa Telecoms & Media

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LTE Link Budgets

Additional Factors Affecting UE Power Output The maximum power output of an LTE UE is specified to be 23dBm, however there other factors that might result in a reduced power out put, this first is;

Maximum Power Reduction (MPR) Maximum Power Reduction (MPR) is a reduction in the power output of the UE due to a high order modulation scheme being used, this reduction in power eases some of the problems that occur with high peak values in the power amplifier, it is thought that the disadvantages of reduction in power is out-weighed by reduced complexity in the power amplifier stages of the transmitter. Additional-Maximum Power Reduction (A-MPR) It is possible for the network to signal additional power reductions in specific deployments where there are tighter requirements of Adjacent Channel Leakage Ration (ACLR) and other spectrum emission requirements. The reductions for MPR and A-MPR are shown in the tables opposi te.

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Maximum Power Reduction Modulation

Channel bandwidth/transmission bandwidth configuration (RB) 1.4 MHz 3.0 MHz 5 MHz 10 MHz 15 MHz 20 MHz

MPR (dB)

QPSK

>5

>4

>8

> 12

> 16

> 18

≤1

16 QAM

≤5

≤4

≤8

≤ 12

≤ 16

≤ 18

≤1

16 QAM

>5

>4

>8

> 12

> 16

> 18

≤2

Additional-Maximum Power Reduction Network Signalling Value

E-UTRA Band

Channel Bandwidth (MHz)

Resources Blocks

NS_01

NA

NA

NA

NA

NS_03

2, 4,10, 35, 36

3

>5

≤1

2, 4,10, 35,36

5

>6

≤1

2, 4,10, 35,36

10

>6

≤1

2, 4,10,35,36

15

>8

≤1

2, 4,10,35, 36

20

>10

≤1

NS_04

TBD

TBD

TBD

NS_05

1

10,15,20

≥ 50 for QPSK

≤1

NS_06

12, 13, 14, 17

1.4, 3, 5, 10

n/a

n/a

NS_07

13

10

NS_08

19

10, 15

> 29

≤1

> 39

≤2

> 44

≤3

NS_[09]

A-MPR (dB)

21

TBD

TBD

TBD

–

–

–

–

… NS_32

Fig. 5 – Additional Factors Affecting UE Power Output © Informa Telecoms & Media

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LTE Link Budgets

eNodeB Power Output Characteristics According to the 3GPP specifications there are 3 clas ses of base station. Wide Area Base Stations are characterised by requirements derived from Macro Cell scenarios with a BS to UE minimum coupling loss equal to 70 dB. No upper limit for power output is specified by 3GPP for this class of base station (some regional limits apply, in addition there are CEPT band limits that should also be considered). Local Area Base Stations are characterised by requirements derived from Pico Cell scenarios with a BS to UE minimum coupling loss equal to 45 dB. The limitations on power output depend on the number of antenna ports used and are shown in the table opposite. Home Base Stations are characterised by requirements derived from Femto Cell scenarios. The limitations on power output depend on the number of antenna ports used and are shown in the table opposite. For link budgets the a typical eNB power outputs for macro cell deployments would however be in the range 20 – 60W (43 – 48dBm) depending on channel bandwidth. Typical power outputs may depend on the bandwidth being use d: • •

14

46dBm (10Mhz) 43 dBm (5 Mhz, 1.25 MHz)


Wide Area BS – BS to UE link loss <70dB Local Area BS – BS to UE link loss <45dB BS Class

PRAT (Rated Power Output)

Wide Area BS

(note)

Local Area BS

≤ + 24 dBm (for one transmit antenna port) ≤ + 21 dBm (for two transmit antenna ports) ≤ + 18 dBm (for four transmit antenna ports)

Home BS

≤ + 20 dBm (for one transmit antenna port) ≤ + 17 dBm (for two transmit antenna ports) ≤ + 14dBm (for four transmit antenna ports)

Note: There is no upper limit for the rated output power of the Wide A rea Base Station

Wide Area (macro cell) eNB will typically have a value of 20 – 60W (43 – 48dBm) •

46dBm (10Mhz)

•

43 dBm (5 Mhz, 1.25 MHz)

Fig. 6 – Table of eNB Power Outputs © Informa Telecoms & Media

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LTE Link Budgets

Typical Losses in the eNB Within the eNB system there will be many components that insert loss in to the transmitted and received signals. It is a general rule that losses should be kept to a minimum. The total amount of loss will determine the radiated power (EiRP) and the received signal. Additionally the losses in the receive path will also add noise which change the SINR requirement on the link. Typical components that may be included in the Rx/ Tx system are listed in the table opposite.

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Type

Frequency Range

Typical Losses

Comments

Duplexer Combiner Filters Connectors Cabling Lightning Arrestors

Fig. 7 – Table of Losses for the eNB © Informa Telecoms & Media

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LTE Link Budgets

Antenna Characteristics for the UE There is much which is yet unknown about the LTE UE antenna systems, given that MIMO is likely to be present in the devices, this places a great deal of challenge in the design and implementation of the UE antenna. However for basic link budgeting purposes it is acceptable to assume a low gain figure for the antenna, typically 0dBi. This of course will depend on the type of LTE device, USB dongles, handheld smartphone devices and even cameras and other consumer devices are likely to have differing antenna performances. It will be largely up to the vendors of the devices to provide the relevant figures.

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Samsung LTE USB Dongle

LG Handheld Device

Samsung LTE equipped camera

•

Typical Mobile Antenna Gain – 0dBi

•

Will depend largely on the Device – could be worse than 0dBi

•

There are significant design challenges for LTE mobile antenna systems

Fig. 8 – Antenna Gains for UE © Informa Telecoms & Media

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LTE Link Budgets

Antenna Characteristics for eNB The eNB in most cases can make use of the familiar cellular antennas that have been used for other mobile broadband systems such as WiMAX and UMTS/HSPA. A typical example of LTE antenna specifications is shown on the page opposite. It is possible of course that the operator will implement spatial multiplexing or transmit diversity, this will have an impact on the link budget calculations. It is expected that the vendors of these systems will provide the appropriate figures of gain to be included in any calculations.

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Electrical Poperties Frequency range (MHz)

824-894

880-960

1710-1880

Polarization

± 45°

VSWR

≤ 1.5

Gain (dBi)

(°)

0

4

8

0

4

8

0

4

8

1850-1990

1920-2170

0

0

4

8

4

8

(dB) 16.7 16.8 16.5 17.1 17.2 16.9 16.7 16.9 16.6 17.0 17.2 16.9 17.3 17.5 17.2 Side lode suppression for first side lode above horizon (dB)

(°)

0

4

8

0

4

8

0

4

8

0

4

8

0

4

8

(dB)

18

17

16

18

17

16

18

17

16

18

17

16

18

17

16

3dB beamwidth (horizontal)

67°

65°

67°

65°

63°

3dB beamwidth (vertical)

8.3°

7.2°

8.0°

7.5°

7.0°

Isolation between portsw (dB)

≥ 30

Front to back ratio (dB)

≥ 28

≥ 25

Cross polar ratio (dB) 0°

≥ 18

±60°

≥ 10

Electrical downtilt

0° – 8°

0° – 8°

Intermodulation IM3 (dBc)

≤ -150 (2 x 43 dB carrier)

Max. CW input power (W)

300

200

Max. power per combined input (W)

500

Impedance (Ω)

50

Grounding

DC ground

330°

30°

300°

330°

60°

-20

270° �dB

-10

240°

90° 0dB

120°

210°

150°

30°

300°

60°

-20

270° �dB

-10

240°

90° 0dB

120°

210°

150°

180° 0dB

180° 0dB

824 – 960 MHz

1710 – 2170 MHz

Fig. 9 – Antenna Gains for eNB © Informa Telecoms & Media

21

LTE Link Budgets

Sensitivity Calculation for the eNB In calculating the required or minimum IRL it is necessary to determine the sensitivity of the receiver. It is highly probable that the vendor of the e NB and mobile devices will quote the sensitivity in the spec sheets for their product. However it is important to be able to derive the sensitivity of the receivers for all cases of modulation/coding schemes and resource block usage. The expression on the opposite page show the calculation and all the parameters required to make the calculation. The following pages will explain each parameter.

22


RX sens_eNB = -174dBm/Hz + 10log(Nrb x 180KHz) + NFeNB + SNR + IM

RX sens_UE = -174dBm/Hz + 10log(Nrb x 180KHz) + NF UE + SNR + IM – 3dB – DFB

Where; •

-174dBm/Hz is k x T (Boltzmann Constant x Temperature)

•

Nrb is the Number of Radio Blocks Allocated

•

180KHz is the bandwidth of 1 RB

•

NFeNB is the total noise figure of the eNB system

•

•

SNR is the Signal to Noise Ratio required i.e. for the modulation scheme in use IM is an Implementation Margin depends Modulation and Coding used

•

-3dB is the multiple antenna gain for the UE

•

DFB is a frequency band specific relaxation factor for the UE

Fig. 10 – Calculating Sensitivity for LTE Link Budgets © Informa Telecoms & Media

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LTE Link Budgets

Thermal Noise in Radio Systems Thermal noise is present in all things, it is a measure of the amount of noise power present due to the random motion of the atoms and molecules excited by temperature. In electronic and radio systems the noise is always present and there is little to be done to eliminate the noise completely. In radio systems the noise is present in two forms; • •

Thermal background noise Noise present in the system components

The thermal background noise is present as a result of the “big bang” (cosmic background radiation), the galaxies, the stars, our own sun and natural radiation from the surface of the earth and the object upon it. There is no way that we can prevent this kind of noise entering the radio system but there is a way to quantify the amount of noise present. The expression;

Nt = k TB Where

k is

Boltzmann’s Constant 1.38 x 10-23

T is temperature (normally 290K) B is the Bandwidth of the Channel in Hz shows that noise is propor tional to the bandwidth of the radio systems and temperature. The bandwidth of the radio system under investigation is really the only variable sinc e temperature is taken to be that of the “warm earth” or 290K. The graph opposite show the rise of noise with radio cha nnel bandwidth and the range of LTE radio channel bandwidths plotted for comparision.

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Nt = k TB

Where

k is

Boltzmann’s Constant 1.38 x 10 -23

T is temperature (normally 290K) B is the Bandwidth of the Channel in Hz Basic Expression of Thermal Noise Noise foor (dBm) -100

-105

-110

Nt = k TB -115

-120

-125

1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 . . . . . . . . . . . . . . . . . . . . . . . . . . 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2

Bandwidth (MHz)

Fig. 11 – Basic Expression of Thermal Noise © Informa Telecoms & Media

25

LTE Link Budgets

Type of Service and Impact on Noise Floor LTE is very flexible,not only in terms of the system bandwidth, but als o the amount of bandwidth or Resource Blocks that can be allocated to a singe mobile device. This variable allocation can be demonstrated in the following example. A typical voice call in LTE may require 64Kbps, for example, given that call reliability will be important across the whole radio cell, robust modulation schemes may be allocated for the voice call events, QPSK 1/3 for example, in this case only two RBs will be required, a total allocated bandwidth of 2x180KHz or 360KHz, this figure can be used to work out the thermal noise floor. In contrast an device that has attempting to receive 1Mbps will have to be allocated between 2 and 13 RBs, depending on the selected modulation and coding scheme. Thus the noise floor could rise up to 10dB (or more) for high capacity allocations. The graph opposite shows the potential noise floor rise for RB allo cation between 1 and 25 RB (25 RB corresponds to an channel bandwidth of 5MHz).

26


Voice service at 64Kbps – 2 RB Required (QPSK) Data Service at 1Mbps – depends on modulation scheme But typically

13 RB for QPSK 1/3 2 RB for 64QAM 2/3

Typical Services and Number of RB Required Noise oor (dBm) -105 -107 -109 -111 -113 -115 -117 -119 -121

Nt/RB = -174dBm/Hz + (N RB x 180KHz)

-123 -125

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Number of RB

Fig. 12 – Impact of Number of RB on Noise Floor (5MHz Channel) © Informa Telecoms & Media

27

LTE Link Budgets

Implementation Margin, UE, eNB, (from blue book) Included in the sensitivity calculation is a margin due to the implementation of the modulation scheme. It is not possible for the receiver to be 100% accurate particularly for the higher order schemes therefore an implementation margin is added. Typical values are given below.

Typical Values QPSK

2.5dB

16QAM

3dB

64QAM

4dB

The margin accounts for the difference in the theoretical SINR values and the practical implementation actually possible.

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Implementation Margin A difference in the theoretical SINR values and the practical implementation (accounts for errors during processing in the receiver)

Typical Values QPSK

2.5dB

16QAM

3dB

64QAM

4dB

Fig. 13 – Typical Implementation Margins © Informa Telecoms & Media

29

LTE Link Budgets

Receiver Noise Figure Noise will also be present in the receiver it self. The noise performance of the receiver is normally quoted as the NF (Noise Figure). How much noise is present is largely down to the design of the receiver by the vendor of that component however is expected that the noise will be no more then the example figures given below for a typical eNB and UE receiver. Typical eNB NF

5dB*

Typical UE NF

9dB* (same as WCDMA)

The noise figure (NF) will have an impact on cell range. The LTE documents specify a figure similar to those for WCDMA devices and it is felt that the figure is a compromise between reasonable cell range and practical receiver design performance. It is range of values also allow some scope for the vendors to improve the performance of the device receivers and therefore improve the sensitivity of the devices, this is also a key differentiator in the device market.

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Typical eNB NF

5dB*

Typical UE NF

9dB* (same as WCDMA)

*Figure is likely to be better than this

Fig. 14 – Receiver Noise Figure © Informa Telecoms & Media

31

LTE Link Budgets

Total Noise Floor The overall system noise floor is the sum of the external nois e present and the total component noise. This is illustrated in the figure opposite. Where there are multiple components (active and passive) in the receiver system, the total noise can be calculated using the cascade method.

32


System Noise Floor Total NF Thermal Noise Floor

Fig. 15 – Total Noise in the System © Informa Telecoms & Media

33

LTE Link Budgets

Cascaded Noise When using the Cascade formula, the noise figure reference point can be assigned at any point before the first active (amplifier) component. The first system component will have the greatest influence, meaning that the system NFdB can’t be better than the NFdB of the first component, on the system NFdB. Stages after an amplifier have progressively less impact on total system NFdB. The performance of a cascaded system of components is base d on the configuration and performance parameters of the individual components. The above two systems use the same components in different configurations. The key to performance of these two systems is the placement and performance of the Low Noise Amplifiers (LNA). The first stage in a cascade of stages limits the receiver system NFdB—it can never be better than the NFdb of the first component! The purpose of the LNA is to increase the noise floor high enough to reduce the impact of loss from successive stages while having a minimum effect of the C/N. A high gain LNA with a low NFdB can provide benefit even if it is after a coax loss. Without sufficient gain, benefit is minimum. Too much gain can overdrive the receiver in the presence of a strong receive signal.

System 1: A significant loss in front of the LNA limits the receiver system NFdB. A high gain in the LNA can help minimize the post-LNA losses. This configuration (indoor-mounted LNA) can be beneficial if the coax loss to the LNA is reasonably low and the LNA has sufficient gain relative to the post-LNA losses. A low gain LNA offers little performance benefit in this, or any deployment. An LNA with too much gain reduces the dynamic range of the receiver and could overload the receiver, causing other problems. System 2: Theoretically, this can provide the best performance. If there is a significant amount of gain in the LNA, the post-LNA losses have little impact on the system NFdB. If a small amount of gain is used, the LNA provides little or no benefit. In cellular deployments, this is referred to as a TTA (Tower-Top Amplifier). Since LNAs are typically rated for their operating NFdB at 23° C ambient temperature, there can be a degradation of performance when the ambient temperature increases above this value. Remember, an LNA with too much gain reduces the dynamic range of the receiver and could overload the receiver, causing other problems.

34


Calculating Noise in Cascade Systems FRX = F1 +

F2 – 1 F – 1 + 3 + . G1 G1 G2

F4 – 1 G1 . G2 . G3

+…

Where: Gx = Gain (ratio, not dB) Fx = Noise factor

Example:

3 dB loss

4 dB NF

Coax

7 dB loss

8 dB NF

30 db amp

÷

WiMAX rcvr

2

3

4

1 NFdB reference point F1 = 2 F2 = 2.5 F3 = 5 F4 = 6.3

G1 = 0.5 G2 = 1000 G3 = 0.2

FRX = 2 +

2.5 – 1 0.5

+

5–1 + 0.5 . 1000

6.3 – 1 0.5 . 1000 . 0.2

FRX = 2 + 3 + 0.008 + 0.053 = 5.061 NFdB = 10log10(5.061) = 7dB

Exercise: Calculating Noise in Cascade Systems C/N 3 dB loss

Coax

4 dB NF

7 dB loss

8 dB NF

÷

WiMAX rcvr

3

4

3 dB loss

7 dB loss

8 dB NF

Coax

÷

WiMAX rcvr

2

3

4

30 db amp

1

2

Rx system 1: NFdB = 7.05dB

System NFdB

C/N 4 dB NF

30 db amp 1

Rx system 2: NFdB = 7.05dB

System NFdB

FRX = F1 +

F2 – 1 F – 1 + 3 + . G1 G1 G2

F4 – 1 G1 . G2 . G3

+…

Fig. 16 – Calculating Noise in Cascade Systems © Informa Telecoms & Media

35

LTE Link Budgets

Typical SNR for LTE Modulation and Coding Schemes Given that there are different modulation and coding schemes in use for the LTE radio interface the SINR for each must be determined, this is largely down to the design of the receive and the efficiency of the error coding schemes used, the table opposite shows the expected values of SINR and the respective IM, however the actual number may vary between vendors.

36


System

Modulation

QPSK

LTE UE

16QAM

64QAM UMTS UE

QPSK

Code Rate

SINR (dB)

IM (dB)

1/8

-5.1

-2.6

1/5

-2.9

-0.4

1/4

-1.7

0.8

1/3

-1

1/2

2

2/3

4.3

6.8

3/4

5.5

8.0

4/5

6.2

8.7

1/2

7.9

10.9

2/3

11.3

3/4

12.2

4/5

12.8

15.8

2/3

15.3

19.3

3/4

17.5

4/5

18.6

1/3

1.2

2.5

3

4

SINR + IM (dB)

1.5 4.5

14.3 15.2

21.5 22.6

2

3.2

Fig. 17 – Table of Typical SINR for LTE © Informa Telecoms & Media

37

LTE Link Budgets

Duplex Gap and Duplex Distance, Effect on Receiver Sensitivity The calculation for UE sensitivity includes an extra parameter which is a margin due to the separation between the uplink and downlink radio channels. Where the channel bandwidth is very large and the duplex separation between them is relatively small this causes the UE receiver to fall directly into the shoulders of the transmitter spectral output. This will require better filtering in the UE, filters with the characteristics required to eliminate any significant receiver desensing have a higher insertion loss which therefore contributes to a higher receiver NF. For the bands affected by this problem a relaxation factor is taken into account when calculating the sensitivit y of the receiver, DFB. Typical Figures for the D FB margin appear in the table opposite. It should be noted that this maring only applies to full duplex devices, the margin is note required for TDD or FDD-HD devices.

38


Duplex Gap (DG)

Uplink

DG

Small DD/DG

Downlink

Duplex

Uplink

Downlink

Duplex

frequency

Distance (DD)

UE TX port to antenna duplexer filter

Large DD/DG

frequency

Distance (DD)

UE antenna to RX port duplexer filter

UE TX port to antenna duplexer filter

UE antenna to RX port duplexer filter

Table of DG/DD and Margins Channel bandwidth Band

1.4 MHz (dBm)

3 MHz (dBm)

5 MHz (dBm)

10 MHz (dBm)

15 MHz (dBm)

20 MHz (dBm)

Duplex mode

DD/DG

DFB (dB)

1

–

–

-100

-97

-95.2

-94

FDD

1.46

0

2

-104.2

-100.2

-98

-95

-93.2*

-92*

FDD

4

2

3

-103.2

-99.2

-97

-94

-92.2*

-91*

FDD

4.75

3

4

-106.2

-102.2

-100

-97

-95.2

-94

FDD

1.13

0

5

-104.2

-100.2

-98

-95*

FDD

2.25

2

6

–

–

-100

-97*

FDD

1.29

0

7

–

–

-98

-95

FDD

2.4

2

8

-103.2

-99.2

-97

-94*

FDD

4.5

3

9

–

–

-99

-96

-94*

-93*

FDD

1.58

1

10

–

–

-100

-97

-95.2

-94

FDD

1.18

0

11

–

–

-98

-95*

-93.2*

-92*

FDD

2.09

2

12

-103.2

-99.2

-97

-94*

FDD

2.5

3*

13

-103.2

-99.2

-97

-94*

FDD

1.48

3*

-104.2

-100.2

-98

-95*

FDD

1.67

1*

33

–

–

-100

-97

-95.2

-94

TDD

34

–

–

-100

-97

-95.2

-94

TDD

35

-106.2

-102.2

-100

-97

-95.2

-94

TDD

36

-106.2

-102.2

-100

-97

-95.2

-94

TDD

37

–

–

-100

-97

-95.2

-94

TDD

38

–

–

-100

-97

-95.2

-94

TDD

39

–

–

-100

-97

-95.2

-94

TDD

40

–

–

-100

-97

-95.2

-94

TDD

-93.2*

-92*

14 … 17 …

Fig. 18 – Duplex Gap/Duplex Distance and Margins © Informa Telecoms & Media

39

LTE Link Budgets

Calculating System Gain Once all the equipment operating parameters have been determine the EiRP and Sensitivity can be calculated. From this the System Gain can be determined. System Gain is a measure of the maximum drop of power from the transmit antenna to the receive and antenna, but does not take in to account any additional margin from radio interface effects such as fading and penetration losses.

40


System Gain = EiRP – IRLmin Power Budget Factors dBm

Coax loss

Tx ant gain

+50

Path loss

Rx ant gain

Coax loss

EiRP

D/L

+40 +30 +20 +10 0 -10

System gain

-20 -30 -40 -50 -60 -70 -80

IRLreq

-90 -100

Tx amp out

Tx amp in

Tx ant out

Rx ant in

Rx ant out

Rx in

Fig. 19 – Power Budget Factors © Informa Telecoms & Media

41

LTE Link Budgets

Environmental Factors and Noise Rise Having worked out the System gain it is now possible to determine the MAPL. The Maximum Allowable Path Loss is the system gain less any environmental margi ns. Typical margins include: • • • • • • •

42

Shadow Margin Building loss Foliage loss Clutter margin Fading margin Body loss Noise rise margin


MAPL = System Gain – Margin (fade, body, building, trees)

Typical Margins; •

Shadow (fading) Margin

•

Building loss

•

Foliage loss

•

Clutter margin

•

Body loss

•

Noise rise margin

Fig. 20 – Environmental Factors and Noise Rise © Informa Telecoms & Media

43

LTE Link Budgets

Shadow Margin (Slow Fading) When deploying NLOS implementations, shadow fading (due to path obstructions) must be considered. Measurements have shown that for any distance from a base station, the path loss at different locations is random and has a log-normal distribution. Over a large number of measurement locations having the same distance between subscriber unit and base station, the random shadowing effects are described by a log-normal distribution. This is often referred to as Log-normal Shadowing (or fading). A common approach is to calculate the lognormal probability of adequate signal strength in a coverage area. The probability is a function of the path loss exponent and the standard deviation of signal values for a given environment. The amount of margin determined from the environmental values is based on coverage objectives for a given implementation. Mobile radio (cellular) prioritizes the area service objective, while fixed wireless services may consider margin for area or edge coverage. The propagation constant (n), also called the path loss exponent, accounts for the distance dependent mean of the signal level based on the propagation environment. The standard deviation ( ) statistically describes the path loss vari ability for arbitrar y locations with the same distance between subscriber unit and cell site. The ratio of  /n is used to determine the amount of margin required to satisfy an area reliability objective.

44


Propagation index n

Signal at UE

 �

eNB

Signal threshold

Environment

Path Loss Exponent

Free space

2

Urban

2.7 to 3.5

Shadowed urban

3 to 5

In-building (LOS)

1.6 to 1.8

In-building (NLOS)

4 to 6

ST dev 4 to 6 dB ≡

ST dev 6 to 8 dB ≡

ST dev ST dev 8 to 10 dB 10 to 12 dB ≡

≡

Dense urban Urban r e t t u l C

Suburban

Rural

Customer density/performance requirements

Fig. 21a – Shadowing in Cellular Networks © Informa Telecoms & Media

45

LTE Link Budgets

46


1 1–2ab 1–ab Fu = 1-erf(a) + exp 1-erf 2 b2 b

[

(

Where: (x0–a) a= and b = 10n log10 σ√2

(

e σ√2

)(

)]

)

Fu = fraction of useful service area within a circle X 0 = minimum receivable signal strength at subscriber α = signal strength at mobile unit for radius r σ = standard deviationof possible signal values n = propagation constant (path loss exponent) Example: Given a path loss exponent of 3.5, a minimum signal level of -94 dBm and a standard deviation of 10 dB, what is the margin required for an area coverage probability of 90%? ........................ dB

Fig. 21b – Calculating a Margin for Shadow Fading © Informa Telecoms & Media

47

LTE Link Budgets

Building and Foliage Losses Many studies provide penetration loss data based on frequency, but without other key information, the values provided can only provide a general idea of what can be expected. Key information: • • • •

Angle of incidence Material composition Material thickness Material texture

Foliage loss is a function of absorption and scattering. Building loss is primarily absorption loss. Wet surfaces will generally increase the amount of energy reflected rather than transmitted thus increasing overall penetration loss. In both foliage and building loss, it is important to establish local parameters to be used during planning processes.

48


Frequency

Building Material

Loss

2.30 GHz

Stone faced bldg

12.8 dB

2.40 GHz

University bldg

20 dB

2.57 GHz

Suburban houses

9.1 dB

5.85 GHz

Brick house

12.5 dB

5.85 GHz

Wood siding house

8.8 dB

5.85 GHz

Concrete wall house

22.0 dB

5.85 GHz

Interior plaster walls

4.7 dB

9.60 GHz

2 dry ¾" plywood sheets

4.0 dB

9.60 GHz

2 wet ¾" plywood sheets

39.0 dB

28.8 GHz

2 dry ¾" plywood sheets

6.0 dB

28.8 GHz

2 wet ¾" plywood sheets

46.0 dB

Frequency

Follage Type

Loss

5.85 GHz

Small deciduous tree

3.5 dB

5.85 GHz

Large deciduous tree

10.7 dB

5.85 GHz

Large conifer tree

13.7 dB

9.60 GHz

Single conifer tree

15.0 dB

28.8 GHz

Single conifer tree

15.9 dB

28.8 GHz

Single deciduous tree

7.0 dB

Fig. 22 – Building and Foliage Penetrations Losses © Informa Telecoms & Media

49

LTE Link Budgets

Body Loss In mobile cellular systems, handheld devices will incur an additional loss due to absorption by the human body. The actual figure will depend on the use of the device i.e. held near the head, away from the body holding angle of the device. UE antenna radiation patterns may also affect the amount of energy lost. The figure normally assumed for radio planning purposes is 3dB.

50


Typical Body Loss – 3dB

Fig. 23 – Body Loss © Informa Telecoms & Media

51

LTE Link Budgets

Uplink and Downlink Noise Rise Noise rise occurs in TDMA/FDMA systems when the same frequency and time resources are used simultaneously in neighbouring cells. This will be a key factor for implementing LTE networks, the eNBs will communicate across the X2 interface regarding resource allocation either warning of potential noise or simply indicating what resources are currently being used. In lightly loaded systems the noise rise should be kept to a minimum by the interference coordination between the base stations, however when the system becomes loaded the noise rise is likely to have a greater impact on overall system performance.

52


Factors affecting Noise Rise •

UL and DL system load

•

Number of RB used by victim UE

•

Number of RBs used by aggressor systems

•

Average pathloss between aggressor and victim BS Affected resources Pathloss

eNB eNB Nrb_victim Pue_victim

Nrb_aggressor Prb_aggressor

Typical Margin for Noise rise = 1 – 10 dB

Fig. 24 – Uplink Noise Rise © Informa Telecoms & Media

53

LTE Link Budgets

Propagation Modelling Propagation modelling or prediction is the science of predicting the pathloss of a particular radio frequency when some of the system attributes are know, typically the radio frequency, tower and UE heights and distance are the information required, however more complex models can use the average height of buildings or terrain, relative angle of roads, antenna tilts etc to produce more accurate results. The model shown opposite is at the heart of this science. This mode ls the theoretical wave front from an isotropic radiator and predicts the field strength at a given distance. If a value for the receive antenna attributes is included it is possible to derive the Free Space Pathloss model. In the free space pathloss model energy radiated from the source decays in proportion to the square of the distance, a doubling of distance will increase the path loss by a factor of 4.

54


Sphere

sarea =

4πr2 1 meter

Isotropic source r (meters)

Ls (dB) = 10.log10 (4πr2 )

Put simply – Power density at distance r =

Power transmitted 4πr2

Watts/m2

As 4 and π are constants we can state: Path loss is proportional to 1/r2 or 1/distance2

Fig. 25a – The Isotropic Radiator and Power Density © Informa Telecoms & Media

55

LTE Link Budgets

56


Pt λ 2 Pr = x 2 4πd 4π

Isotropic radiator Pt

Pr

d

Effective apperture in m2

This formula, converted to decibel format is: LF (dB) = 20 log10[d (kM)] + 20 log10[f (MHz)] + 32.45 (Add 60 dB to the 32.45 dB to use GHz instead of MHz)

Fig. 25b – The Free Space Pathloss Model © Informa Telecoms & Media

57

LTE Link Budgets

Coverage from Link Budget Having calculate the MAPL above it is now possible to convert the pathloss into a nominal cell range using an appropriate propagation model. The results will vary according to the model used. There are many different kinds of model, the classical empirical models such as Okamura-Hata, Walfisch-Ikegami and those used by RF planning models. It is important to select the correct model and some model tuning is required to obtain theoretical results that reflect the actual loss or distance likely to be experienced in the field. The following is a list of empirical models can be used in the preliminar y stages of planning.

Empirical models Power law Okumura – Hata Lee COST 231 Hata Walfish – Ikagami IEEE 802.16 (SUI) • • • • • •

Physical models Free space Free space + RMD TIREM Longley-Rice Anderson 2D • • • • •

Pathloss models, Ok-Hata, dedicated RF models.

58


Empirical models •

Power law

•

Okumura – Hata

•

Lee

•

COST 231 Hata

•

Walfish – Ikagami

•

IEEE 802.16 (SUI)

Physical models •

Free space

•

Free space + RMD

•

TIREM

•

Longley-Rice

•

Anderson 2D

Fig. 26 – Typical Propagation Loss Models © Informa Telecoms & Media

59

LTE Link Budgets

Comparison of Models There are of course many different models that can be used under different circumstances, the choice of model will depend on system design parameters such as the frequency band used, LOS or NLOS systems, antennas above or below rooftop height etc. The table on the page opposite shows some of the standard models in common use and the range of frequencies over which the model will return sensible results. Some of the models are empirical models which means that they are also dependant on the circumstances under which they were developed. In many cases different models will return different pathloss results for the same set of inputs (frequency, tower height, link distance etc) therefore several models may need to be test to see which model returns the most accurate results for the are being designed. Many RF planning tools will allow you to select different propagation models in order that comparisons can be made, in addition the RF software development companies will offer their own models that use a combination of empirical and physical models to predicate the pathloss.

60


Some Common Propagation Models 1.5GHz

Okumura hata model 2

2GHz

ITU530 microwave model

800MHz

70GHz

ITU452 microwave model

800MHz

70GHz

800MHz

Walkish Ikegami model

150MHz

Okumura hata model 1

2GHz 1.5GHz

Longley rice model

30MHz

Aeronautical model

30MHz

Flat earth model

30MHz

10GHz

Egli urban model

30MHz

10GHz

HCM model

30MHz

ITU370 model

30MHz

1GHz

ITU567 model

30MHz

1GHz

CEPT model

30MHz 3MHz

ITU533 shortwave model 150KHz

Sky wave model

30GHz

2GHz

250MHz 30MHz

1.7MHz

10KHz

Ground wave model

40GHz

30MHz

Free space model 0

3 Hz

30 Hz

300 Hz

3 KHz

30 KHz

VLF

LF

300 KHz

3 MHz

MF

30 MHz

300 MHz

3 GHz

30 GHz

300 GHz

HF VHF UHF SHF EHF

Fig. 27a – Some Common Propagation Models and Frequency Ranges © Informa Telecoms & Media

61

LTE Link Budgets

62


Comparison of Coverage Models Loss (dB) 180

160

140

120

100

80

0

2

4

6

8

10

12

14

Distance (km)

CCIR

Hata

Walfsch et al

Fig. 27b – Comparison of Pathloss Models © Informa Telecoms & Media

63

LTE Link Budgets

COST 231 Propagation Model The model shown on the opposite page is the COST 231 model which is an adaptation of the well known Okamura-Hata model. The COST 231 is an empirical model designed to model NLOS radio systems in the frequency range 1.5GHz to 2GHz making it suitable for cellular systems such as GSM1800, UMTS and even Mobile WiMAX technologies. This a baseline model which can be used to make comparisons of other empirical and custom designed models.

64


L = 46.3 + 33.9 .log f c – 13.82.log hb – a(hm ) + (44.9 – 6.55.log hb )log R + Cm Where: Fc = operating frequency (1500-2000 MHz) hb = BS antenna height (30-200 m) hm = mobile antenna height (1-10 m) R = distance between BS and mobile unit (1-20 km) Correction factors:

Medium city and suburban a(hm ) = (1.1.log f c – 0.7)hm – (1.56.log f c – 0.8) Large city a(hm ) = 3.2(log(11.75.hm ))2 – 4.97 Cm =

0 dB medium city and suburban areas 3 dB metropolitan centres

Fig. 28 – COST 231 Propagation Model © Informa Telecoms & Media

65

LTE Link Budgets

The WINNER Model The WINNER model had been developed by Information Society Technologies (IST) for predication for indoor and outdoor systems. The novel features of the WINNER models are its parameterisation, using of the same modelling approach for both indoor and outdoor environments, new scenarios like outdoor-to-indoor and indoor-to outdoor, elevation in indoor scenarios, smooth time (and space) evolution of large-scale and small-scale channel parameters (including cross-correlations), and scenariodependent polarisation modelling. The models are scalable from a single single-input-singleoutput (SISO) or multiple-input-multiple-output (MIMO) link to a multi-link MIMO scenario including polarisation among other radio channel dimensions. WINNER II channel models can be used in link level and system level performance evaluation of wireless systems, as well as comparison of different algorithms, technologies and products. The models can be applied not only to WINNER II system, but also any other wireless system operating in 2 – 6 GHz frequency range with up to 100 MHz RF bandwidth. The models supports multi-antenna technologies, polarisation, multi-user, multi-cell, and multi-hop networks.

66


PL = Alog10(d[m]) + B + Clog10

(

fc[GHz] + X 5.0

)

PLfree = 20log10(d) + 46.4 + 20log 10(f c /5.0)

Scenario

Definition

Environment LOS/NLOS

Mobility (km/h)

A1

Indoor offie

LA

LOS/NLOS

0-5

B1

Urban microcell

LA, MA

LOS/NLOS

1-70

B4

Outdoor to indoor microcell

MA

NLOS

0-5

B5

LOS stationery feeder

MA

LOS

0

C1

Suburban

WA

LOS/NLOS

0-120

C2

Typical urban macrocell

MA, WA

LOS/NLOS

0-120

D1

Rural macrocell

WA

LOS/NLOS

0-200

D2

(a) Moving networks: BS-MRS*, rural

WA

LOS

0-350

Very large Doppler variability

LOS/LOS**/ NLOS

0-5

Same as A1 NLOS

(b) Moving LA networks: MRS*, UE, rural

Notes

Below rooftop to street level

* MRS: Mobile Relay Station. ** OLOS: Obstructed Line-of-sight.

Fig. 29 – The WINNER Model © Informa Telecoms & Media

67

Scenario

A1

Path loss (dB)

Shadow Applicability range, fading std antenna height default (dB) values

LOS

A=18.7, B=46.8, C=20

σ =

3

3m < d < 100m, hBS=hMS=1…2.5m

NLOS1

A=36.8, B=43.8, C=20 and X=5(nw-1) (light walls) or X=12 (nw-1) (heavy walls)

σ =

4

Same as A1 LOS, nw is the number of walls between the BS and the MS (nw > 0 for NLOS)

A=20, B=46.4, C=20, X=5nw

σ =

6

A=20, B=46.4, C=20, X=12nw

σ =

8

Same as A1 LOS, nw is the number of walls between BS and MS

NLOS2 light walls heavy walls FL

For any of the cases above, add the floor loss (FL), if the BS and MS are in different floors: FL=17+4(nf -1), nf >0

NLOS

PL=PL3+PLtw+PLin PL3=PLB1 (dout+din ) PLin=14+15(1-cos( θ ))2 PLin=0.5din

σ =

7

3m < dout+din < 1000m, hBS=3(nn-1)+2m hMS=1.5, See 3 for explanation of parameters

A=22.7, B=41.0, 20

σ =

3

10m < d1 < d’BP4

PL-40.0log10(d1 )+9.45– 17.3log10(h’as )–17.3log10(h’as ) +2.7log10(f c /5.0)

σ =

3

d’BP < d1 < 5km hBS=10m, hMS=1.5m

NLOS

PL=min(PL(d1, d2 ), PL(d2, d1 ) where PL(dk , d1 )= PLLOS(dk )+20–12.5n, +10, log10 (d1 )+3logn(f c /5.0) and nf =max(2.8–0.0024dk , 1.84), PLLOS is the path loss of B1 LOS scenario and k, l (1, 2)

σ =

4

10m < d1 5km, w/2 < d2 < 2km5 w=20m (street width) hBS=10m, hMS=1.5m when 0 < d2 < w/2, the LOS PL is applied

NLOS

Same as B1

σ =

4

LOS

A=13.9, B=64.4, C=20

σ =

3

5m < d < 100m, hBS=6m, hMS=1.5m

NLOS

A=37.8, B=36.5, C=23

σ =

4

Same as B3 LOS

B4

NLOS

Same as A2, except antenna heights

B5a

LOS

A=23.5, B=42.5, C=20

σ =

4

30m < d < 8km hBS=25m, hRS=25m

B5c

LOS

Same as B1 LOS, except antenna heights (hRS is the relay antenna height)

σ =

3

10m < d < 2000m hBS=10m, hMS(= hRS )=5m

B5f

NLOS

A=23.5, B=57.5, C=23

σ =

8

30m < d < 1.5km hBS=25m, hRS=15m

A2

LOS

B1

B2

{

nf is the number of floors between the BS and the MS (nf > 0)

B3

3m < dout+din < 1000m, hBS=10m, hMS=3(nn-1)+1.5m

Fig. 30 – WINNER Model Parameters 68


Scenario

Path loss (dB)

Shadow Applicability range, fading std antenna height default (dB) values

A=23.8, B=41.2, C=20

σ =

4

30m < d < d BP

PL=40.0log10(d)+11.65–16.2log10(hBS ) –16.2log10(hMS )+3.8log10(f c /5.0)

σ =

6

dBP < d < 5km, hBS=25m, hMS=1.5m

NLOS

PL=(44.9–6.55log10(hBS ))log10(d)+ 31.46+5.83log10(hBS )+23log10(f c /5.0)

σ =

8

50m < d < 5km, hBS=25m, hMS=1.5m

LOS

A=26, B=39, C=20

σ =

4

10m < d < d’ BP4


σ =

6

d’BP < d < 5km hBS=25m, hMS=1.5m

NLOS

PL=(44.9–6.55log10(hBS ))log10(d)+ 34.46+5.83log10(hBS )+23log10(f c /5.0)

σ =

8

Same as C1 NLOS

C3

NLOS

Same as C2 NLOS

C4

NLOS

PL=PLC2(dout+din )+17.4+0.5dn–0.8hMS where PLC2 is the path loss function of C2 LOS/NLOS scenario. (Use LOS, if BS to wall connection is LOS, otherwise use NLOS.)

σ =

10

Same as C2 NLOS See 3 for explanation of parameters hBS=25m, hMS=3nF1+1.5m

LOS

A=21.5, B=44.2, C=20

σ =

4

30m < d < d BP6


σ =

6

dBP < d < 10km, hBS=32m, hMS=1.5m

NLOS

PL=(25.11log10(d)+55.3 –0.13log10(hBS –25))log10(d/100) –0.9log10(hMS –1.5)+21.3log10(f c /5.0)

σ =

8

50m < d < 5km hBS=32m, hMS=1.5m

LOS

Same as D1 LOS

LOS C1

C2

D1

D2a

Same as C2 NLOS

Fig. 30 – WINNER Model Parameters (cont) © Informa Telecoms & Media

69

LTE Link Budgets

Cell Range Calculations from MAPL The link budgets calculations done previously can now be used with the propagation models to determine the nominal cell range based on the equipment performance assumptions. The pathloss models require some transposition to derive distance rather than pathloss, this is best done by modelling within spreadsheets or other software models.

70


EiRP IRLreq

eNB

•

MAPL >>Cell Radius

Link Budget Calculations can be used to determine cell radius using an appropriate propagation model

Fig. 31 – Determining Cell Radius © Informa Telecoms & Media

71

Service Coverage Provision

UE Radio Measurements The 3GPP standards define three key radio measurements that may be perform ed and reported by the UE for the evaluation of cell reselection and handovers. Reference Signal Received Power (RSRP) is a measurement of the power in the reference signal from the selected or serving cell. Received Signal Strength Indicator (RSSI) is a measurement of the total receive signal strength, which includes all transmissions from the selected or serving cell plus all signals received in the measurement bandwidth from neighbour cells. Reference Signal Received Quality (RSRQ) is calculated as the ratio (the difference in terms of values expressed in dB) between the RSRP and RSSI. It is therefore a measure of the signal-to-noise ratio for the reference signals.

28


UE Definition Measurement

Value Range

RSRP The linear average over the power contributi ons (in watts) of the (Reference Signal resource elements that carr y cell-specific reference signals within Received Power) the considered measurement frequency bandwidth.

IE Value: 0…97 (maps to –140…–44 dBm with 1 dB resolution)

RSSI (Received Signal Strength Indicator)

The linear average of the total received power (in watts) obser ved only –140…–44 dBm in OFDM symbols containing reference symbols for antenna port 0, (nominal) in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise, etc. If higher-layer signalling indicates certain subframes for performing RSRQ measurements, then RSSI is measured over all OFDM symbols in the indicated subframes.

RSRQ The ratio N×RSRP/(E-UTRA carrier RSSI), where N is the number (Reference Signal of RBs of the E-UTRA carrier RSSI measurement bandwidth. Received Quality) The measurements in the numerator and denominator shall be made over the same set of resource blocks.

IE Value: 0…34 (maps to –19.5…–3 dBm with 0.5 dB resolution)

Fig. 13 – UE Radio Measurements © Informa Telecoms & Media

29


Limitations of RSRP and RSRQ for Optimisation There are obvious parallels between the LTE measurement values RSRP and RSRQ and the UMTS values CPICH RSCP and CPICH Ec/No, and perhaps even GSM RSSI and RXQUAL measurements. However, there are some important differences that must be borne in mind when using these measurements for performance evaluation, coverage estimation or interference analysis. This arises chiefly because both measurements are based solely on reference signal performance and that cannot be assumed to map directly to the performance allocated channel resources. RSRP is an averaged measurement of the power in a single resource element modulated with the cell’s reference signal PCI. However, a user’s data will be transmitted over multiple resource elements in one or multiple RBs using one of many modulation and coding schemes. They are therefore not like-for-like quantities and there is no direct relationship between measured RSRP and channel performance for any given service. However, it possible for an optimiser to develop one or more rule-of-thumb guide measurements for RSRP that will indicate that different required service types are likely to function acceptably in an tested area of interest. Note also that the power available for one resource element is likely to be affected by the configured transmission bandwidth and that therefore the expected value of measured RSRP could vary dependent on cell configuration. In simple terms, doubling the bandwidth through the same total transmit power should result in a reduction in expected RSRP of 3 dB unless power offsets are used between subcarrier types. Similar consideration must be given to RSRQ when using it to assess performance, because it is calculated from RSRP.

30


RSSI is measured across the whole configured transmission bandwidth RSRP and RSRQ relate to the performance of a single reference signal resource element

User data is transmitted over multiple resource elements in one or multiple RBs using one of many different modulation and coding schemes

Fig. 14 – Limitations of RSRP and RSRQ © Informa Telecoms & Media

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Formulation of RSRP and RSRQ Performance Targets The diagram shows a suggested starting point for RSRP and RSRQ targets. However, it is very important to note that these will be subject to considerable variation dependent on the physical implementation of a network, its location, the features supported and the particular service type being considered. More specific targets need to be set as part of the optimisation process through testing of a network at the earliest opportunity in the rollout phase. This targets should be reviewed from time to time as more accurate service profiles become available. In order to make RSRQ a meaningful ratio of RSRP and RSSI they must be modified such that the measurement bandwidth is the same. The 3GPP standards define that this is done by multiplying RSRP by the number of resource blocks in the measurement bandwidth for RSSI. The result is the equivalent of considering the individual SINR for one reference si gnal resource element within one RB. Thus the best case occurs when there is no data being transmitted or external interference source measured with the RB. In this case RSRP is the power in one resource element and RSSI is the power in two resource elements per RB; the resultant RSRQ is –3 dB. In the case where data is transmitted on all resource elements with the same power but there is no external interference source, then the ratio becomes 1/12 and the resultant RSRQ is –10.8 dB. Anything worse than this suggests the presence of external interference.

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Data Performance

Measurement

Suggested Starting Range

Good

RSRP

> –75 dBm

RSRQ

> –9 dB

RSRP

–75 dBm to –95 dBm

RSRQ

–9 dB to –12 dB

RSRP

< –95 dBm

RSRQ

< –12 dB

Acceptable Poor

Note: 1. Best case for RSRQ is when no user data is transmitted and no external interference is transmitted. In this case RSRQ is –3 dB. 2. When full data transmission takes place at equal power per resource element and no external interference is present RSRQ is –10.8 dB. 3. RSRQ below –10.8 dB suggests the presence of external interference.

Fig. 15 – RSRP and RSRQ Targets © Informa Telecoms & Media

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LTE Measurement Tools Protocol analysis performed on the air interface and in the radio access network has been a fundamental approach for the optimiser for many years. This remains the case for LTE and includes drive test tools and interface probes, as shown in the diagram. Ideally. however, protocol analysis needs to be performed at both ends of the radio link. Unlike UMTS and GSM, where air interface signalling activity can be monitored within the RAN on the A-bis or Iur interface, in LTE air interface signalling occurs only on the air interface itself. This means that some form of tool will be required to capture the eNB end of air interface activity from within. This may involve a proprietary solution, although a standard for a digital interface between the signal processing part of the eNB and the antenna referred to as Common Public Radio Interface (CPRI) is available. For equipment conforming to this standard generalised monitoring may be possible. There are a number of drawbacks to any approach based only on ai r interface protocol analysis of this type, including lack of visibility of multiple users on the same cell and lack of visibility of scheduling efficiency. Another approach would be a direct capture from the air inter face itself. This is more commonly used in the context of a lab, but it can be applied in the field and is one way to view the cell as a whole and performance for multiple users. Tools of this type may also be able to decode scheduling information and display the results in a graphic format. Some method of analysing the performance of the scheduling algorithm is certainly required for effective optimisation. Analysis of KPIs from the OMC is also essential in order to gauge overall per formance against defined metrics. It may also provide some insight into the effectiveness of the scheduling algorithm. Finally, the optimiser should have access to information regarding the complete network in order to assess and identify the root cause of any performance issues.

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IP interface probe

Drive test tool

eNB

PDN

HSS

MME

UE EPC

SGW

E-UTRAN

P-GW eNB

Off-air monitoring OMC

Fig. 16 – LTE Measurement Tools © Informa Telecoms & Media

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The EPS Bearer Concept An EPS bearer is defined in terms of its end points, an APN at the PDN-GW and an IP address at the UE, and the applied QoS (Quality of Service) for packets carried in the EPS bearer. The IP address may be predefined or dynamically allocated and may be in either an IPv4 or IPv6 format. Similarly, an APN could be dynamically allocated, although in the typical case it will be predefined. APNs may provide access to either public or private PDNs. An EPS bearer is defined by one APN and one IP address. QoS is defined in terms of the four key characteristics shown in the diagram: QCI, ARP, GBR and MBR. Multiple data flows may share a single EPS bearer; however, an EPS bearer has only one QoS profile and this is applied to all data packets using the bearer. An EPS bearer is created through the concatenation of a Data Radio Bearer (DRB) between the UE and the eNB, an S1 Bearer between the eNB and the SGW and an S5/S8 Bearer between the SGW and the P-GW. Note that air interface testing and optimisation conside r only the characteristics of the DRB, yet the service performance will also depend on factors relating to the S1 Bearer and the S5/S8 Be arer, as well as the external PDN, the end-point server and an application’s behaviour.

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APN

QoS

(Access Point Name)

• QCI – QoS Class Identifier • ARP – Allocation and Retention Priority • GBR – Guaranteed Bit Rate • MBR – Maximum Bit Rate

IP Address

(v4 or v6)

EPS Bearer

S5/S8 Bearer

S1 Bearer

Data Radio Bearer

PDN P-GW E-UTRAN SGW E-UTRAN eNB

UE

Air interface testing and optimisation focuses only on the DRB, but service performance is affected by the complete system

Fig. 17 – EPS Bearer Concept © Informa Telecoms & Media

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ICLTERPOP Section 3_LTE Link Budgets.pdf

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