chapter 16 - edge enhanced data rates for gsm and tdma136 evolution

8/20/2019 Chapter 16 - EDGE Enhanced Data Rates for GSM and TDMA136 Evolution

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32 EDGE:ENH NCED DATA RATESFORGSMANDTDMA/136

EVOLUTION

operation, but since both high speed circuit switched data (HSCSD) and General Packet

Radio Service (GPRS) are based on the original Gaussian-filtered minimum shift keying

(GMSK) modulation, the increase

of

bit rates is modest [2-4]. For TDMA/136 evolution,

similar standardization activities are ongoing. In IS

136+

the combination of multislot

operation and the introduction of 8PSK based on the 30-kHz carrier bandwidth enables

data rates approximately four times higher than today [5].

EDGE provides an evolutionary path from existing standards for delivering third-

generation services in existing spectrum bands. The advantages

of

EDGE include fast

availability, reuse of existing GSM and TDMA/136 infrastructure, as well as support for

gradual introduction. For example, as a

t

frequency reuse overlay to TDMA/136, EDGE

can be deployed using as little as 600 kHz of total bandwidth. In GSM, EDGE can be

introduced using a minimum

of

only one time slot per base station. EDGE was first

proposed to ETSI as an evolution of GSM in the beginning of 1997. During 1997, a

feasibility study was completed and approved by ETSI, making way for the currently

ongoing standardization [6]. Although EDGE reuses the GSM carrier bandwidth and time

slot structure, it is by no means restricted to use within GSM cellular systems. Instead it

can be seen as a generic air interface for efficiently providing high bit rates, facilitating an

evolution of existing cellular systems toward third-generation capabilities.

After evaluating a number

of

different proposals, EDGE was adopted by the Universal

Wireless Communications Consortium (UWCC) in January 1998 as the outdoor compo-

nent

of

136 High Speed (136HS) to provide 384-kbps data services. One

of

the arguments

in favor of this approach was leveraging the technology evolution for both GSM and

TDMA/136 systems, also leading to opportunities for global roaming. Consequently,

EDGE was included in the UWC-136 IMT-2000proposal. UWC-136 was adopted by TR-

45 in February 1998 and submitted by the U.S. delegation to ITU as a Radio Transmission

Technology candidate for IMT-2000 [7]. Since then, EDGE development has been

concurrently carried out in ETSI and UWCC to guarantee a high degree

of synergy

with both GSM and TDMA/136. The standardization roadmap for EDGE is based on two

phases. In the first phase the emphasis has been placed on EGPRS (enhanced GPRS) and

ECSD (enhanced circuit-switched data). Both were targeted in ETSI for standards release

1999 with products to follow shortly afterwards. The second phase

of

EDGE is concerned

with the improvements for multimedia and real-time services.

In speech planned TDMA/136 or GSM network there is typically a distribution

of

user

signal to interference ratio (SIR), where almost all users have an SIR above an operating

point. Speech users do not normally gain from being above this threshold. The principle of

EDGE is to utilize this excessive SIR to increase bit rates and spectral efficiency. This is

accomplished by the use

of

higher order modulation (8PSK) in combination with a control

mechanism for adapting the bit rate to the channel conditions. This control mechanism is

called link quality control (LQC).

The EDGE concept and various aspects of its link and system performance have been

described in the literature [8-16]. Although EDGE phase 1 supports both circuit-switched

and packet-switched services, this chapter focuses on the packet-switched part, enhanced

GPRS (EGPRS), which is based as much as possible on GPRS.

16 3 PHYSICAL LAYER

The EDGE air interface is based on the air interface

of

GSM. Higher order modulation,

8PSK, is introduced with as few changes of the parameters as possible.


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16.3 PHYSICALLAYER

321

6 3

TDMA Format and Modulation

The GSM carrier spacing is 200 kHz, and each carrier is divided into eight time slots ,

according to Figure 16.1. Within each time slot a burst is transmitted, consisting of payload

symbols, training symbols, and tail symbols according to Figure 16.2. The symbol rate is

13

/48

MHz

~

271 kHz. The bursts in GSM are modulated with binary GMSK, and hence

one symbol corresponds to one bit. Each burst contains 2 x 58 bits, and the gross bit rate is

23.2kbps.

In EDGE linear 8PSK is introduced using the same burst format, thus giving

3 x 2 x 58 = 348 payload bits per burst. The gross bit rate becomes 69.6kbps, which

is three times the gross bit rate

of

GSM. Since 8PSK is less robust than GMSK, EDGE

adapts the modulation (GMSK or 8PSK) to the current radio and interference situation. As

will be shown later, the amount of channel coding applied is also adapted to suit the

channel conditions. The 8PSK symbol constellation is shown in Figure 16.3. Three bits are

Gray mapped to one symbol.

To keep the 200-kHz carrier spacing the modulation is partial response, that is,

intersymbol interference (lSI) is introduced on the transmitter side. The pulse shape

used is a linearized GMSK pulse [12] (see Figure 16.4), which gives approximately the

same spectrum and lSI in a receiver as normal GMSK.

However, nonideal power amplifiers will distort the spectrum more for 8PSK than for

GMSK, and hence the spectrum requirements are slightly relaxed compared to the GMSK

_1...-......I.---: ..L-_..J....:=--I...-......I._...J...._One carrier

r-


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322 EDGE: ENHANCED DATA RATES FOR GSM AND TDMA/136 EVOLUTION

0.9

0.8

0.7

0.6

OJ

0

:J

E

0.5

E


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GMSK

Training

Symbols

Q

8PSK

Training

Symbols

16.4 LINK LAYER 323

Q

IGUR

16.5 Trainingsymbols for 8PSK.

dynamically and often. To signal such a change to the receiver in advance is undesired.

Instead the receiver can exploit the fact that the same training sequences are used, but with

a different constellation rotation. If the receiver makes a conventional linearized approx-

imation

of

the GMSK modulation, the corresponding binary phase shift keying (BPSK)

constellation will be rotated with

n/2

radians, while the 8PSK constellation is rotated with

3n

/8

radians. This rotation factor can be identified during channel estimation, and thus the

modulation format is detected by the receiver before the actual equalization. This is called

blind detection ofmodulation, and the performance

of

this procedure depends on the cross

correlation between a training sequence with the different rotations. For the specific GSM

training sequences 3n/8 rotation for 8PSK gives better performance than any other

kn

/8

rotation where k is odd.

16 3 3 Channel Coding

Although channel coding is a part

of

the physical layer, it is also related to the link layer for

EDGE, and therefore it is described in the link layer section below. Basically the channel

coding for EGPRS is based on punctured convolutional codes, where the puncturing is

used to adapt the code rate to the channel quality. Enhanced circuit-switched data (ECSD)

also utilizes Reed Solomon codes.

16 3 4 Physical Layer Performance

For GMSK it is common to use a full state equalizer, that is, a MLSE or a MAP receiver.

However, for 8PSK this is unfeasible . Instead suboptimal receivers must be used. Since

8PSK is more sensitive to residual interference due to lSI not covered by the equalizer, the

equalizing window for 8PSK needs to be larger than for GMSK, even if the symbol rate is

the same. In Figure 16.6 the performance for the two different modulations, with an

exemplary receiver for 8PSK, is shown. The receiver used in these simulations is low-

complex and straightforward, and the performance of commercial receivers is expected to

be significantly better.

16 4 LINK LAYER

The link layer contains automatic repeat request (ARQ) procedures and ways of adapting

the bit rate to the channel quality, that is, link quality control LQc) . Both functions

depend on the service type (e.g., packet-switched bearers or circuit-switched bearers) and


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324 EDGE : ENH NCED DATAR TES FORGSMANDTDMA/136

EVOLUTION

-:

.

- GMSK

- - 8PSK , simple receiver

OJ

Cil

a:

100

r-- - - -

r

- - - - - - - - -

- -

-

- - - - -

: .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

'

r;

30

5

05

0

10.

4

L ' ' ---L -- '-_::>..- -----.J

5

C/I [dB]

IGUR

16.6 Performance for GMSK and 8PSK. The figures show the uncoded bit error rate for

a low dispersive channel (typical urban profile).

are therefore described separately for these two cases. The channel coding is furthermore

connected to the LQC, especially for EGPRS, and is therefore also described in the

following sections.

6 4

Enhanced GPR5 EGPRS

EGPRS is a natural extension

of

GPRS, providing the packet switching

of

GPRS, but with

higher data rates. Since the 8PSK modulation is more susceptible to noise and interference

than GMSK, there is a need to adapt the transmission scheme used to the interference

situation. This is essential for providing to each user the maximum throughput that the

rapidly changing conditions allow at the moment. The LQC is also the main reason why

the EDGE RLC protocol is somewhat different from the corresponding GPRS protocol.

EGPRS uses a combination of two methods: link adaptation (LA) and incremental

redundancy (IR) for link quality. In short IR provides better performance than LA in most

cases but is also more complex to implement, which is more elaborated in Eriksson [11].

The two methods and how they are used for EGPRS are described in the following

sections.


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16.4 LINK LAYER 325

Link

Adaptation

A pure LA scheme [18,19] uses a set of type I hybrid ARQ schemes

with different coding rates R and modulations, in the sequel called modulation

and

coding

schemes

(MCSs). A type I hybrid ARQ uses a forward error correcting (FEC) code to

correct errors in blockwise encoded data, and additionally an ARQ mechanism to

retransmit remaining erroneous blocks, detected by a frame check sequence (FCS). The

channel quality is estimated continuously, and the MCS maximizing the link bit rate at the

moment is chosen. Link adaptation in this way is introduced already in GPRS.

Incremental Redundancy In a pure IR scheme [20], a fixed type II hybrid ARQ

scheme is used. The type II hybrid ARQ scheme first encodes a block of data with some

low rate FEC code. Only a part of this codeword (a subblock) is transmitted initially,

yielding some initial code rate R

1

(possibly, R

1 ==

1). For erroneously decoded blocks,

detected by an FCS, transmission of additional redundancy subblocks from the same

codeword is requested, received, and combined with the first subblock, yielding a lower

code rate

R

1

+2'

This procedure is repeated until decoding succeeds, giving a stepwise

increment of the amount of redundancy, or, equivalently, a decrement of the code rate

R

I

+

+

i

·

Link

Quality Control LQC

fo r

EGPRS

A flexible LQC solution has been chosen

for EGPRS, enabling pure LA, but also IR with different initial rates, and dynamic

adaptation between all modes. The scheme enables a range of solutions with different

trade-offs between complexity and performance [11]. The solution is as follows:

Nine MCSs are used, five using 8PSK and four using GMSK, each of which can be

used in both LA and IR mode. The maximum bit rate (i.e., the bit rate after channel

decoding without errors) ranges from 8.4 to 59.2kbps. Some parameters for the MCSs are

listed in Table 16.1.

For eachMCS, an

R

== t convolutionally encoded data block is divided into

n

subblocks

(where

n

is .either 2 or 3) by puncturing with

n

puncturing patterns,

PI'

. ..

P

(Figure

16.7). Initially, the subblock SI corresponding to PI is transmitted. On retransmission, one

additional subblock

S,

corresponding to PJ is transmitted, where

i ==

2,

. . .

,

n,

1, 2, . . . .

Since each subblock for a given MCS is by itself a decodeable codeword, with the rate

TABLE 16.1 Parameters for MCS-1 to MCS-9 of the EGPRS LQC Scheme

Maximum

Scheme Modulation

Rate (kbps)

R

1

R

1

R

2

R

R2 R3

MCS-9 8PSK 59.2 1.0 0.5

0.33

MCS-8 54.4

0.92 0.46

31

G

MCS-7 44.8 0.76 0.38

0.25

G

MCS-6 29.6 0.49

0.24

G

MCS-5

22.4 0.37

0.19

a

MCS-4

GMSK 16.8

1.0

0.5

0.33

MCS-3 14.8

0.85 0.42

0.28

G

MCS-2 11.2

0.66

0.33

MCS-1 8.4 0.53

0.26

a

a

Code rates less than

t

are obtained by repetition.


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326

EDGE: ENHANCED DATA RATES FOR GSM AND TDMA/136 EVOLUTION

J

:

'

:

'

:

:

'

'

:

.

R==1 3

convolutional code

Code

word

puncturing

,

'

.

'

\

\

\

\

\

'

FIGURE 16.7 Encoding and subblock puncturing for the EGPRS LQC scheme.

R

1

...

R

n

, the receiver can either discard or keep old subblocks when requesting

retransmissions, thereby utilizing type I or type II hybrid ARQ.

Always altering in a cyclic manner among the subblocks Si for an MCS enables the

receiver to switch between combining and noncombining mode without notifying the

transmitter. Thus,

if

the receiver temporarily enters noncombining mode due to lack

of

memory, IR operation will be possible as soon as memory is available again.

The network controls the choice of MCS in both uplink and downlink, based on the

channel quality measured by the receivers.

If

IR combining is used in the receiver, this

choice can be more aggressive, that is, less robust schemes can be used for a given channel

quality.

The quality

of

the downlink is periodically reported to the network by the mobile. The

short-term variations are typically faster than the reporting period. Therefore, there is a

need to average the measures over time.

16 4 2 Enhanced Circuit Switched Data

Enhanced circuit-switched data (ECSD) is a continuation

of

GSM's HSCSD. ECSD

provides higher data rates per timeslot than HSCSD by utilizing the 8PSK modulation. In

EDGE phase I, no extra ECSD service will be introduced compared to HSCSD, but by

utilizing ECSD, the same data rates as in HSCSD could be achieved while using fewer

time slots.

TABLE 16.2 Enhanced circuit-switched schemes

Radio Interference

Scheme Modulation Rate (kbps)

Code Rate

ECSD TCS-l 8PSK

29.0

0.419

ECSD TCS-2 32.0 0.462

ECSD TCS-3 43.5 0.629

Service

Type

Q

NT/T

T

NT

a

T means transparent service and NT denotes nontransparent service.


9/12

16.5 EGPRS PERFORMANCE

327

The set of new coding schemes (Table 16.2) introduced in ECSD covers both

transparent and nontransparent services. For the nontransparent services, the same ARQ

mechanism as for HSCSD applies also for ECSD.

The new coding schemes introduced in ECSD all utilize the 8PSK modulation and

make use of the same convolutional code polynomials as EGPRS. On top of that, the

lowest ECSD code rates have been provided with a Reed Solomon code on top of the

convolutional code. In a similar way as for GPRS and EGPRS, link adaptation is included

in the ECSD concept to provide for usage of the most efficient coding scheme.

16 5 EGPRS PERFORMANCE

The downlink performance in a multiple cell network with dynamic packet traffic is

evaluated by means of simulations. A standard three-sector frequency reuse pattern is used,

using only three carriers in total. The time step of the simulator is 5ms (corresponding to

one burst) and the users produce packets according to a measurement-based Worldwide

Web (WWW) traffic model. Queuing in the system is modeled. Finally,multipath fading is

modeled on system level. More assumptions and details about the results can be found in

Furuskar et al. [10]. Three different scenarios have been studied. First, EGPRS using the

incremental redundancy and link adaptation mode is compared. Then, as a reference, a

comparison to standard GPRS is also made.

0.5

r

-

- - - --- - - - -,---- -,------ -- -

- ---,

0.8.7

....44..

0.6

40

24

0.3 0.4 0.5

spectra l efficiency [bps/Hz/site]

16

Standard GPRS

. .

. . . . . .

.

.

. . . . . .

0.2

.. 12 .

0.1

oL-_ _ -'-- -'--__--'- -'--_ _ ---'

-'-

-=-'

o

0.45

0.05

0.35

c

OJ

OJ

0.3

o

0>

'

i

0

]j

0.2

o

'

.

0

. 0.15

Cii

E

g 0.1

FIGURE 16 8

System capacity in terms

of

spectral efficiency vs. packet quality in terms of

normalized delay (the offered load is also given as number of user per sector).


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328

EDGE: ENHANCED DATA RATES FOR GSM AND TDMA

/136

EVOLUTION

100 i 'T

- - - - --- =

==:: ?1

90

80

70

60

~

u,

50

ci

0

40

30

20

10

----L

----L

----.J

o

10 80) 20 160) 30 240) 40 320) 50 400) 60 480)

Average packet bitrate per user per timeslot bitrate per 8 timeslots in brackets) [kbps]

FIGURE

16.9 Average packet bit rate per user distributions at load limits. The bit rate figures are

per time slot, supplemented

by

corresponding figures per eight time slots.

Used performance measures are normalized delay (i.e., the average packet delay in

seconds per kilobit) and average packet bit rate (i.e., the average bit rate per user) .

Furthermore system load in terms of spectral efficiency (bps/Hz /site) is used. The more

spectrum-efficient a system is, the higher system load is possible at a certain user

performance (delay or throughput) or vice versa.

Figure 16.8 shows the spectral efficiency plotted against the 90th percentile of the

normalized delay.

It

is seen that considerably higher spectral efficiencies are achieved

using the incremental redundancy mode than when using the link adaptation mode.

Assuming a delay requirement of 0.15 s/kbit at the 90th percentile (90 of

the packets

having a total delay of less than 0.15 kbps), a spectral efficiency of 0.60 bps /Hz /si te (one

site comprises three sectors) is obtained in the incremental redundancy mode. This

corresponds to a 70 gain over the link adaptation mode achieving 0.35 bps /Hz /site.

Even higher spectral efficiencies can be achieved if higher normalized delays can be

accepted for the worst packets. At 0.4 kbps a spectral efficiency

of

0.70 bps/Hz /si te is

reached, corresponding to a gain of 55 over the link adaptation mode. Compared to

standard GPRS, the EGPRS spectral efficiency for the same delay requirement is

approximately tripled.

The distribution of average packet bit rate per user per 1 (8) time slots is plotted in

Figure 16.9. First, a case with offered loads that result in normalized delays just around

0.15 kbps is studied. Notice the significant increase in packet bit rate when EDGE is

introduced compared to standard GPRS. For the link adaptation case, at the studied offered


11/12

REFERENCES

329

load (LA 24 curve, 24 being the number of users per sector), it is seen that approximately

50 0

of

the users achieve a packet bit rate exceeding 48 kbps per time slot (384 kbps per 8

time slots), and that 84 of the users achieve a packet bit rate exceeding 18kbps per time

slot (144 kbps per 8 time slots). Using incremental redundancy, at the higher load limit

of

40 users per sector, it is seen that (IR 40 curve), 10 of the users achieve a packet bit rate

exceeding 48 kbps per time slot (384 kbps per 8 time slots), and that 86 of the users

achieve a packet bit rate exceeding 18kbps per time slot (144 kbps per 8 time slots).

It is also interesting to investigate how incremental redundancy affects the user data

rates for the same offered load. The IR 24 curve shows the user data rates achieved at the

load limit for the link adaptation case. It is seen that considerably higher rates are achieved:

48 (384)kbps is now reached by 20 of the users, whereas 95 reach 18 (144)kbps. Also

notice the steeper cummulative distribution function (CDF) of the incremental redundancy

operation, indicating a more fair system behavior.

16 6 CONCLUSIONS

EDGE is a common evolution of GSM and IS/136, providing third-generation services.

Both packet-switched services (EGPRS) and circuit-switched services (ECSD) are

provided. Depending on how the link layer protocol is used, the spectral efficiency of

an EDGE packet data service can be twice or three times that of GPRS.

knowledgements The author wishes to thank Christer Edholm, Stefan Eriksson,

Anders Furuskar, Sara Mazur, Frank Muller, and Hakan Olofsson for their large

contributions to this chapter.

REFERENCES

1. E. Dahlman et al., UMTS/IMT-2000 Based on Wideband CDMA, IEEE Commun. Mag.,

1998.

2. ETSI. TS 101 038 V5.0.1 (1997-04), Digital Cellular Telecommunications System (Phase

2+);

High Speed Circuit Switched Data (HSCSD)-Stage 2 (GSM 03.34), version 0.4, 1997.

3. ETSI. TS 03 64 V5.1.0 (1997-11), Digital Cellular Telecommunications System (Phase 2+);

General Packet Radio Service (GPRS); Overall Description of the GPRS Radio Interface; Stage 2

(GSM 03.64 version 5.1.0), version 11, 1997.

4.

1.

Cai and D. Goodman, General Packet Radio Service in GSM,

IEEE Commun. Mag.,

Oct.

1997.

5. S. Labonte, A Proposal for the Evolution ofIS-136,

Proc. IEEE VTC'98.

6. ETSI. Tdoc SMG2 95/97. EDGE Feasibility Study, Work Item 184; Improved Data Rates

through Optimised Modulation, version 0.3, Dec. 1997.

7. The UWCC-136 RTT Candidate Submission.

8. A. Furuskar, S.Mazur,

F.

Miiller, and H. Olofsson, EDGE, Enhanced Data Rates for GSM and

TDMA/136 Evolution, IEEE Commun. Mag., Vol. 6, No.3, June 1999.

9. A. Furuskar, D. Bladsjo, S. Eriksson, M. Frodigh, S. Javerbring, and H. Olofsson, System

Performance of the EDGE Concept for Enhanced Data Rates in GSM and TDMA/136, Proc.

IEEE WCNC'99.


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10. A. Furuskar, M. Hook, S. Javerbring, H. Olofsson, and

1

Skold, Capacity Evaluation of the

EDGE

Concept for Enhanced Data Rates in GSM and TDMA/136, Proc. IEEE VTC'99 Spring.

11. S. Eriksson, A. Furuskar, M. Hook, S. Javerbring, H. Olofsson, and

1.

Skold, Comparison of

Link Quality Control Strategies for Packet Data Services in EDGE, Proc. IEEE VTC'99 Spring.

12. Schramm, H. Andreasson, C. Edholm, N. Edvardsson, M. Hook, S. Javerbring,

F

Muller, and

1.

Skold,

Radio

Interface Performance ofEDGE, a Proposal for Enhanced Data Rates in Existing

Digital Cellular Systems, Proc. IEEE VTC'98.

13. A. Furuskar, M. Frodigh, H. Olofsson, and 1 Skold,

System

Performance ofEDGE, a Proposal

for Enhanced Data Rates in Existing Digital Cellular Systems, Proc. IEEE VTC'98.

14. K. Zangi, A. Furuskar, and M. Hook, EDGE: Enhanced Data Rates for Global Evolution of

GSM and IS-136, Proc. Multi-Dimensional Mobile Communications

1998

MDMC'98 .

15. A. Furuskar, M. Hook, C. Johansson, S. Javerbring, and K. Zangi, EDGE-Enhanced Data Rates

for Global Evolution, Proc. Nordic Radio Symposium

1998

NRS'98 .

16. H. Olofsson and A. Furuskar, Aspects of Introducing EDGE in Existing GSM Networks, Proc.

IEEE ICUPC'98.

17. P Jung, Laurent's Representation

of

Binary Digital Continuous Phase Modulated Signals with

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Revisited,

IEEE

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Feb./Mar./Apr.1994.

18.

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Dunlop et aI., Estimation

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19. A. Goldsmith and S. G. Chua, Adaptive Coded Modulation for Fading Channels, IEEE Trans.

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