GPRS / EDGE

Sometimes it is more cost effective to simply augment the existing system

rather than completely creating a newer one. This is the case with GSM and

GPRS.

GPRS was only an extension to the older GSM technology.

As the demand for data services increased, GPRS was developed to support

packet switching.

A feature that used GPRS technology was the Multimedia Messaging System

or MMS. It allowed subscribers to send videos, pictures, or sound clips to

each other just like text messages.

GPRS also gave mobile phones the ability to surf the internet at dial-up

speeds through WAP enabled sites.

WHY GPRS IS USED ?

Difference between circuit switched

and packet switched

A

B C

D

Circuit-Switched Communication:

Packet-Switched Communication:

A

B C

D

Info

Gs

SMSC

BTSMTTE

BSC/TRC

GWMSC

ISDN

PLMN

PSTN MSC/VLR

HLR

AUC

SGSN GGSN

PLMN

X.25

Network

Corporate

LAN

InternetGd

Gp

Gf

Gr

Gb

Gi

Gn

IP Backbone network

BSC/TRC + PCU

EIR

Signaling

Traffic

GPRS ARCHITECTURE

GSM Network

Element

Modification or Upgrade Required for

GPRS

Terminal Equipment

(TE)

A totally new subscriber terminal is required to access GPRS services.

These new terminals will be backward compatible with GSM for voice calls.

BTS A software upgrade is required in the existing base transceiver site (BTS).

BSC The base station controller (BSC) will also require a software upgrade, as

well as the installation of a new piece of hardware called a packet control unit

(PCU). The PCU directs the data traffic to the GPRS network and can be a

separate hardware element associated with the BSC.

Core Network The deployment of GPRS requires the installation of new core network

elements called the Serving GPRS Support Node (SGSN) and Gateway

GPRS Support Node (GGSN).

Databases

(VLR, HLR, etc.)

All the databases involved in the network will require software upgrades to

handle the new call models and functions introduced by GPRS.

Serving GPRS Support Node (SGSN)

The SGSN forwards incoming and outgoing IP packets addressed to/from a mobile station that is attached

within the SGSN service area.

The SGSN provides:

• Packet routing and transfer to and from the SGSN service area.

• Ciphering and authentication.

• Mobility management

• Output of charging data, the SGSN collects charging information for each MS related to the radio

network usage.

Gateway GPRS Support Node (GGSN)

The GGSN provides:

• The interface towards the external IP packet networks.

• From the external IP network’s point of view, the GGSN acts as a router for the IP

addresses of all subscribers served by the GPRS network.

• Functionality for associating the subscribers to the right SGSN

• Output of charging data, the GGSN collects charging information for each MS, related to

the external data network usage.

LIMITATIONS OF GSM DATA SERVICES

Enhancements of GSM data rates: HSCSD

Which allows the assignment of maximum 4 circuit switched time slots to the

same user over the air interface.

Thus the rate of 4 x 9.6(GSM data rate) = 38.6 kbps is achievable

Enhancements of GSM data rates: Coding Schemes

Coding Scheme 1 (CS1) was the first CS to be used and it adds a large number

of coding bits causing the user rate to be low.

Rate of data over GSM using this CS is 9.6 Kbps

To increase the rate more data will be sent Instead of strong error correction.

This makes the link less reliable but increases the rate.

Coding Scheme 2 (CS2) uses a less number of coding bits allowing the user rate

to reach 14.4 Kbps.

LIMITATIONS OF GSM DATA SERVICES

Enhancements of GSM data rates: HSCSD + Coding

Schemes

Combining the effect of the HSCSD and CS2 will jump with the rate to reach

14.4 x 4 = 57.6.

Yet, this bit rate is still low for some applications and consumes large number of

resources, the service will be expensive to the user.

Channel coding

in GPRS

Coding schemes

And

The Corresponding data rates

CHANNEL CODING IN GPRS

Channel coding in a wireless cellular network is how the digital data

(either voice or data for non-voice applications) from the mobile or base

station is formatted to deal with the problem of transmitting information

across a radio channel.

Channel coding includes parity generation, convolution coding, puncturing

and interleaving. These processes are structured so that either the mobile

or the base station can receive a stream of data bits with corrupted values

and still have a high likelihood of decoding the bit stream correctly.

The GPRS data is protected by four different channel protection schemes:

CS1, CS2, CS3, and CS4.

The main difference between the four coding schemes is the

level of protection from transmission errors that they offer

and the maximum throughput that can be obtained.

The GPRS system dynamically chooses the coding scheme

best suited for the transmission conditions at hand.

Coding scheme -1Maximum protection to

error

Lowest throughput

Coding scheme -4Maximum throughput

Lowest protection to

error

GPRS CODING SCHEMES

Four coding schemes, CS-1 to CS-4, are used for the GPRS

PDTCHs. They offer different levels of protection, and the CS

to be used is chosen by the network according to the radio

environment

Scheme Code rate USF Pre-coded USF

Radio Block excl. USF and BCS (Payload)

BCS Tail Coded bits

Punctured bits

Data rate kbps

CS-1 1/2 3 3 181 40 4 456 0 9.05

CS-2 2/3 3 6 268 16 4 588 132 13.4

CS-3 3/4 3 6 312 16 4 676 220 15.6

CS-4 1 3 12 428 16 - 456 - 21.4

Block Check Coding

USF

pre-coding

add

tail bits

convolution

codingpuncturing

data in

Data out

290 bits

294 bits

588 bits 456 bits

287 bits

CS-2

+3 bits

+3 bits

+4 bits

x2

-132 bits

Add USF

268 bit

block

+16 bits

284 bits

GPRS Data Encoding

Coding schemes performance

GPRS Modulation

Modulation scheme used in GPRS is GMSKNow the question here is why GMSK?

• Power efficiency.

• Bandwidth efficiency.

• Cost and the complexity of the receiver is low.

• Good BER performance.

• Lower values of the C/I ratio (Carrier-to-Interference ratio).

• A higher traffic-carrying capacity.

• Efficient utilization of available dc power using a class C power amplifier.

In view of this, the MSK and GMSK are good choices. It must not only be able to withstand

the severe multipath fading but also the BER should at least be <10^-2 or better. (minimum

acceptable BER for speech communication).

MSK (Minimum shift keying):Is a special case of coherent FSK modulation where modulation index is equal to 0.5.

But Unfortunately, the main lobe of MSK is wide and it has a lot of side lobes

(unwanted) so it is not suitable for narrowband application.

GMSK (Gaussian Minimum Shift keying):GMSK is the MSK derivative.

In GMSK the base band binary data is passed on a Gaussian filter first (reduces the main

lobe width and the side lobe power). Then the output is passed on FM Modulator.

The performance of GMSK is measured by the 3 dB bandwidth-bit duration product of

the Gaussian filter (BT) where as BT is lowered the amount of inter-symbol interference

introduced decrease.

As BT decreases bandwidth increases.

In GPRS we use BT= 0.3 which is the best compromise between increased bandwidth

occupancy and resistance to ISI.

GMSK Performance:

EDGE

EDGE system is quite similar to the GPRS system but with the capability for higher data

rates.

The most important change is the new modulation scheme.

In GSM and GPRS-> the GMSK modulation scheme -> one bit per symbol is

used + constant amplitude modulation.

In an EDGE network-> 8-PSK modulation-> three bits per symbol + variation

in modulation amplitude.

Notes:

• 3 Bits per symbol enables a data rate of 59.2 kbps per radio time slot.

• Modulation amplitude variation changes the radio performance

characteristics, so hardware changes in the base stations are mandatory.

Diagram Showing EDGE modulation benefits

Another improvement that has been made to the EGPRS standard is the ability to

retransmit a packet that has not been decoded properly with a more robust coding scheme,

IN GPRS-> no re-segmentation i.e. once packets have been sent, they must be

retransmitted using the original coding scheme even if the radio environment has

changed.

IN EGPRS-> re-segmentation i.e. Packets sent with little error protection can be

retransmitted with more error protection, if required by the new radio environment.

Conclusion:In EDGE, the rapidly changing radio environment has a much smaller effect on the

problem of choosing the wrong coding scheme for the next sequence of radio blocks

because re-segmentation is possible.

EDGE Radio Network Planning:

Coding Schemes:There are nine modulation and coding schemes (MCS-1 to MCS-9) that provide

different Throughputs. The MCS scheme carries data from 8.8 kbps to 59.2 kbps.

For coding schemes MCS-1 to MCS-4, modulation is still GMSK; for MCS-5 to

MCS-9 it is 8-PSK.

Another advantage in EDGE networks is that the switching between different

coding schemes can take place easily, which was not possible in a GPRS network.

When data transmission takes place in a GPRS coding scheme, it is not

possible to switch the coding scheme on reception failure, so the re-transmission

takes place with exactly the same protection as for its initial transmission.

In EGPRS, it is possible to change the MCS, i.e. the data block can be sent

again but with better protection than for its initial transmission. This is done

through a process called link adaptation.

Advantages of using link adaptation (LA):

It leads to the highest throughput possible with the lowest amount of delay. This

gives better link quality and makes EDGE a more efficient system.