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T E K nsultTelecom Engineering and Consulting

GPRS Training

EGPRS

1

ORASCOM Telecom Tunisia

Tunis 2004

EGPRS

Contents1 Enhanced Data Rates for GSM Evolution (EDGE) 31.1 Basics 41.2 Modulation 62 EGPRS 92.1 Channel Coding 102.2 Link Quality Control 14

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Copyright TEKonsult Munich

All rights reserved

November 2004

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1 Enhanced Data Rates for GSMEvolution(EDGE)

EDGE –

Enhanced Data Ratesfor GSM Evolution

ECSD –

Circuit-

switched

EGPRS –

Packet-

switched

Fig. 1

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1.1 Basics

Enhanced Data Rates for GSM Evolution (EDGE) is another high-speed mobiledata standard. It allows data transmission speeds of 384 kbps to be achieved whenall eight timeslots are used. In fact, EDGE was formerly called GSM384. Thismeans a bit rate of 48 kbps per timeslot. Even higher speeds may be available ingood radio conditions.

EDGE was developed (by Ericsson initially) for mobile network operators who failto win Universal Mobile Telephone System (UMTS) spectrum. EDGE givesincumbent GSM operators the opportunity to offer data services at speeds that are

near to those available on UMTS networks.

EDGE also provides an evolutionary migration path from GPRS to UMTS byimplementing now the changes in modulation that will be necessary forimplementing UMTS later. The idea behind EDGE is to eke out even higher datarates on the current 200 kHz GSM carrier by changing the type of modulationused.

High Speed Circuit Switched Data (HSCSD) and GPRS are both based onsomething called Gaussian minimum-shift keying (GMSK) which only yields a

moderate increase in data bit rates per time slot. It is the modulation scheme thatGPRS uses that limited its data speeds to 115.2 kbps.

EDGE on the other hand is based on a new modulation scheme that allows a muchhigher bit rate across the air interface. This modulation technique is called eight-phase-shift keying (8 PSK). It automatically adapts to radio circumstances andthereby offers its highest rates in good propagation conditions close to the site ofbase stations. This shift in modulation from GMSK to 8 PSK is the central changewith EDGE which prepares the GSM world (and TDMA in general) for UMTS.

Implementation of EDGE by network operators has been designed to be simple.No new operator licenses are needed for EDGE and existing spectrum, carrierprocesses and cell planning can be used. Only one EDGE transceiver unit will needto be added to each cell. Software upgrades to the BSC and Base Stations can becarried out remotely. The new EDGE capable transceiver can also handle standardGSM traffic and will automatically switch to EDGE mode when needed.

EDGE capable terminals will also be needed- existing GSM terminals do notsupport the new modulation techniques and will need to be upgraded to useEDGE network functionality. Some EDGE capable terminals are expected to

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support high data rates in the downlink receiver only, whilst others will access

EDGE in both uplink and downlinks and will therefore need greater terminalmodifications to both the receiver and the transmitter parts.

In addition, the TDMA industry association, the Universal WirelessCommunications Corporation, has introduced what it calls EDGE Compact. This aspectrum efficient version of EDGE that will support the 384 kbits mandatedpacket data rates but will require only minimum spectral clearing and thereforecould work for network operators with limited spectrum allocations. In fact, as aresult of this, EDGE has been renamed Enhanced Data Rates for GSM and TDMAEvolution.

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I

Q

0

1

Gaussian Minimum

Shift Keying

1 bit per symbol 3 bits per symbol

I

Q

8-Phase Shif t

Keying

(0,1,0)

(0,1,1)

(1,1,1)

(1,1,0)

(1,0,0)

(1,0,1)

(0,0,1)

(0,0,0)

Fig. 2 GMSK and 8-PSK

… 7 0 1 2 3 4 5 6 7 0 …

3 58 sym 26 sym 58 sym 3 8.25

Burst: 0.577 msec

GSM Frame: 4.615 msec

Tail Data Training Sequence Tail GuardPeriod

Data

Fig. 3 Burst Structure

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1.2 Modulation

The EDGE standardization leads to changes in the physical layer. In addition toexisting GMSK modulation used in GSM/GPRS, EDGE uses 3π/8-shifted 8PSKmodulation.Through the 3π/8-shifted approach, the EDGE modulator will rotate the phase ofeach modulating symbol by multiples of 3π/8 during every symbol period. 8PSKmodulation has eight distinguishable symbols and each symbol represents 3 bits ofinformation. Alternatively, GMSK modulation contains one bit of information persymbol, so EDGE increases by three the transmission bit rate over GPRS.

EGPRS GSM

Modulation 8-PSK (3 bit/sym) GMSK (1bit/sym)

Symbol rate 271 ksym/s 271 ksym/s

Payload / burst 346 bit 114 bit

Gross rate / time slot 69.6 kbit/s 22.8 kbit/s

Max. netto rate / time slot 59.2 kbit/s (MCS-9)

43.2 kbit/s (TCS-3) 21.4 kbit/s (CS-4)

14.4 kbit/s

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I0

Q0

(0,1,1)

(1,1,1)

(1,1,0)(1,0,1)

(0,0,0)

I 1

Q1

(1,1,1)

(0,1,1)

(1,1,0)

(1,0,0)

(1,0,1)

(1,0,0)(0,0,1)

(0,0,0)

(0,0,1)

(0,1,0)

(0,1,1)

Fig. 4 8-PSK in EDGE

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Unlike GMSK modulation, which has a constant envelope signal, 8PSK

modulations use linear modulation techniques with amplitude varying signal. AnEDGE 8PSK-modulated signal has a peak-to-average ratio (PAR) of 3.2 dB and apeak-to-minimum signal ratio of 17 dB.

One of the challenges in EDGE design is to implement a cost effective, highlylinear modulator and power amplifier that can handle the peak to average powerratio. Insufficient linearity in the modulator and power amplifier designs willresult in a loss of EDGE throughput due to the necessity to operate at reducedamplifier output power.

In a practical deployment, where EDGE is situated on the same TDMA signal asthe base control channel (BCCH), a reduced 8PSK power would result inundesirable mobile performances and adversely impact features such as cellselection / re-selection. It is therefore important to closely match the averagepower for both the GMSK and 8PSK modulation.

2 EGPRS

EGPRS is another high-speed mobile data standard, which enables GSM/GPRS

networks to offer high data rates, either as a complement to 3G services, or as asolution for operators not as yet licensed for 3G frequencies. Based on newmodulation and coding schemes (MCS-1 to MSC-9), EGPRS improves the raw datarates over GSM/GPRS, achieving up to three times the data rates per channel. Inaddition, Incremental Redundancy and Link Adaptation improve effectivethroughput, directly impacting the user. No less important, EGPRS enables farbetter utilization of the spectrum than GPRS. This translates as either freeing upchannels for voice that had been allocated for data services, or offering a betteruser experience with the current allocated resources. EGPRS is a cost-effectivesolution, enabling the provision of 3G-like services at only a fraction of the cost

and complexity of actual 3G.The main difference between GPRS and EGPRS lies in the different modulationschemes; GPRS uses Guassian Minimum Shift Keying (GMSK), while EGPRS usesboth GMSK and 3p/8 rotated 8-Phase Shift Keying (8PSK).

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EGPRS

• New channel coding• Link quality control:

Link adaptation

Incremental

Redundancy

EGPRS

• New channel coding

• Link quality control:

Link adaptation

Incremental Redundancy

Fig. 5 EGPRS

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2.1 Channel Coding

As it was mentioned in the precedent paragraph, 9 new Modulation and Coding

Schemes MCS have been developed for EGPRS. The following table compares thecoding schemes for GPRS and EGPRS:

CodingScheme

Modulation User data

rate(kbit/s)

Code rate Usefulbits

Family

CS-1 GMSK 9.05 0.5 181 n. a.

CS-2 GMSK 13.4 ~ 2/3 268 n. a.

CS-3 GMSK 15.6 ~ 3/4 312 n. a.

CS-4 GMSK 21.4 1.0 428 n. a.

MCS-1 GMSK 8.8 0.53 176 C

MCS-2 GMSK 11.2 0.66 224 B

MCS-3 GMSK 13.614.8

0.80 272+24296

A(padding)A

MCS-4 GMSK 17.6 1.00 352 C

MCS-5 8PSK 22.4 0.37 448 B

MCS-6 8PSK 27.2

29.6

0.49 544+48592

A(padding)A

MCS-7 8PSK 44.8 0.76 2x448 B

MCS-8 8PSK 54.4 0.92 2x544 A(padding)

MCS-9 8PSK 59.2 1.00 2x592 A

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MCS-1

CS-1

MCS-2

CS-2

MCS-3

CS-3

MCS-4

CS-4

MCS-5

MCS-6

MCS-7

MCS-8

MCS-9

Data Rate (per time slot)

G M S K

8 - P S K

GPRS

Family A

Family B

Family C

~ 9 kbit/s

~ 2 2 k b i t / s

~ 5 9 k b i t / s

Fig. 6 Channel Coding Schemes in GPRS and EGPRS

Max. Data Rate

R e l i a b i l i t y

L o w

H i g

h

9 kbit/s 59 kbit/s

MCS-1

MCS-2

MCS-3

MCS-4

MCS-5

MCS-6

MCS-7

MCS-8

MCS-9

Fig. 7 Reliability vs. Data Rate for different MCS

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Families

As explained in the table above, all the mentioned coding schemes are grouped indifferent families. Several types of modulations (GMSK and 3p/8 rotated 8-PhaseShift Keying ) can be used within the same family.

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37 octets 37 octets 37 octets 37 octetsFamily A

MCS-3

MCS-6MCS-9

34 octets 34 octets 34 octets 34 octets

Family A

pading

MCS-3

MCS-6

MCS-8

34 + 3 octets 34 + 3 octets

28 octets 28 octets 28 octets 28 octets

MCS-5

MCS-7

MCS-1

MCS-4

22 octets 22 octets

Family B

Family C

MCS-2

Fig. 8 Families of Coding Schemes

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2.2 Link Quality Control

EGPRS introduces two techniques to improve throughput and reduce latenciesrelated to the air-interface: Link Adaptation (LA) and Incremental Redundancy(IR).Link Adaptation allows dynamic variation of the modulation and coding schemes,adapting them according to the changing signal quality. Thus EGPRS adapts betterto the changing conditions of the channel, choosing more protection when lowersignal quality is detected, or higher throughput when the signal is of better quality.GPRS does not support changing the coding scheme and once set, it is not changedfor the length of the session.

Changes of coding schemes are only possible within the same family (even if thetype of modulation is different).Incremental redundancy utilizes Hybrid Automatic Request (ARQ) II schemes toimprove the efficiency of packet-data transfer. Backward error correction (ARQ) isused when received blocks are corrupted, and the retransmissions are sent withmore protection-bits and a different puncturing scheme. The retransmitted blocksare then combined with the original corrupted blocks, improving the quality ofdecoding and thus lowering the error-rates.

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hroughput in kbit/s

60

50

40

30

20

10

0

305 10 15 20 25 C/I in dB

Link Adaptation

50

40

3050

MCS-2MCS-1 (C)

5040

50

Fig. 9 Link Adaptation (example: TU50)

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