Gsm Tdma Frame Slots And Bursts

GSM Design Library >Chapter 9: GSM Design Examples
Print version of this Book (PDF file)

GSM Traffic Channel Full Rate Speech Measurement

Measurement_TCH_prj Design Names

GSM_Measurement_TCH.dsn

Features

One TDMA frame with all time slots assigned to TCH/FS
Normal burst construction
Channel codec for TCH/FS
Gaussian minimum shift keying (GMSK, BT=0.3, B is 3db bandwidth for Gaussian filter, T is bit time) modulation
Radio frequency with 935.2 MHz in GSM band
GMSK spectrum
GMSK IQ constellations, eye diagram, phase trajectory and adjacent channel power ratio

Description

This example generates one TDMA frame signal for transmission at 270.8333 kbit/sec bit rate GMSK modulation for GSM measurement. It includes one TDMA frame of 8 time slots at carrier frequency of 935.2 MHz. The Training Sequence Code (TSC) value is set to 0.

The source of the system is a random bit source. Input data is convolutionally coded, with constraint length of 5 and rate 1/2, and interleaved before it is fed into a normal burst component. After the training sequence, tail bits and guard time bits are added by the normal burst construction component; eight normal bursts are combined into one TDMA frame; this frame is placed in the GMSK modulation model.

The baseband signal is then fed into the RF section that includes mixer, Butterworth filter and RF gain. The timed sink and spectrum analyzer is used to store the signal and GMSK spectrum.

The ACPR measurement can be displayed by opening the ACPR.dds file.

Figure 9-16 shows the schematic for this design.

Specifications
Symbol
Simulation Type
Unit
M
GMSK modulation sample rate
Agilent Ptolemy
2
N/A
TSC
training sequence code
Agilent Ptolemy
0
N/A
TStep
output time step for channel model
Agilent Ptolemy
3.692/16
sec

Notes

GMSK constellation diagram can be displayed by oversampled data (for example, 8 samples/symbol) or by one sample per symbol. When the output constellation is displayed by one sample per symbol, the downsample model is designed to get the optimum sample.
For ACPR and data, open dds files:
- Measurement_TCH_prjACPR.dds
- Measurement_TCH_prjGSM_Measurement_TCH.dds
M=0, 1, 2 indicates the GMSK modulation sample rate of 4, 8, 16 samples per symbol.
TStep must be related to M, that is TStep=3.69/Sampleratesec.

Simulation Results

I-Q Measurement (Figure 9-17)

By collecting all sample points, the ideal GMSK constellation would be a circle.

Figure 9-17. Constellation Diagram of ADS Simulation Results
(0.3 GMSK, 4 samples per symbol)

Eye Diagram (Figure 9-18)

The eye diagram of GMSK signal is ideal and consistent with the measurement results of the Agilent 89441 instrument.

Spectrum (Figure 9-19)

The spectrum of GMSK modulation at 935.2 MHz complies with the specification.

Figure 9-19. Magnitude and Spectrum of GMSK Modulation

**Table 9-1. Test Limits for Spectrum Due to Modulation**
Distance from Carrier (kHz)	0	100	200	400
Maximum Relative Level (dBc)	0	+0.5	-30	-60

ACPR (Figure 9-20)

ACPR calculation using resolution bandwidth. The power in the main channel is integrated over its bandwidth; the adjacent channel power is calculated at 30 kHz resolution bandwidth and zero span. The ACPR is expressed as a power ratio or dbc.

According to GSM Specification GSM 05.05, ACPR measure must be set to the following parameters:

Central reference channel frequency: 935.2 MHz
Integration bandwidth: 270.833 kHz
First adjacent channel offset: 100 kHz
First adjacent channel integration bandwidth: 30 kHz
Second adjacent channel offset: 200 kHz
Second adjacent channel integration bandwidth: 30 kHz
Third adjacent channel offset: 400 kHz
Third adjacent channel integration bandwidth: 30 kHz

Table 9-2. Summary of Agilent GSM-ESG Link ACPR Performance
versus ETSI GSM 05.05 Recommendations
Offset from carrier (kHz)	200	400	600	1200
ETSI spec (dBc)	-30	-60	-70	-73
Agilent Advanced Design System simulation results (dBc)	-35	-77	-95	-102

Benchmark

Hardware platform: Pentium Pro 200 MHz, 96 MB memory
Software platform: Windows NT 4.0 Workstation, Advanced Design System 1.1
Data points: 2496 frames
Simulation time: approximately 90 seconds

Global System for Mobile (GSM) David Tipper Associate Professor Graduate Program of Telecommunications and Networking University of Pittsburgh. GSM TDMA frame GSM time-slot (normal burst) 4.615 ms 546.5 µs 577 µs tail user data TrainingS guard space S user data tail guard space 3 bits 57 bits 26 bits1 1357 bits. Hence in GSM system, the basic radio resource is a time slot with duration of about 577 µs. This time slot carries 156.25 bits which leads to bit rate of 270.833 kbps. This is explained below in TDMA GSM frame structure. The GSM frame structure is designated as hyperframe, superframe, multiframe and frame. Figure 6 GSM TDMA frame, slots, and bursts There are three bits at the start and finish of each burst these are known as the 'tail' and are set to 0 so they can be used to enhance the receiver performance.

GSM primer includes:
GSM introductionNetwork architectureNetwork interfacesRF interface / slot & burstGSM framesPower classes & controlChannelsAudio codecs / vocodersHandover

The data frames and slots within 2G GSM are organised in a logical manner so that the system understands when particular types of data are to be transmitted.

Having the GSM frame structure enables the data to be organised in a logical fashion so that the system is able to handle the data correctly. This includes not only the voice data, but also the important signalling information as well.

The GSM frame structure provides the basis for the various physical channels used within GSM, and accordingly it is at the heart of the overall system.

GSM frame structure - the basics

The basic element in the GSM frame structure is the frame itself. This comprises the eight slots, each used for different users within the TDMA system. As mentioned in another page of the tutorial, the slots for transmission and reception for a given mobile are offset in time so that the mobile does not transmit and receive at the same time.

The basic GSM frame defines the structure upon which all the timing and structure of the GSM messaging and signalling is based. The fundamental unit of time is called a burst period and it lasts for approximately 0.577 ms (15/26 ms). Eight of these burst periods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.

In simplified terms the base station transmits two types of channel, namely traffic and control. Accordingly the channel structure is organised into two different types of frame, one for the traffic on the main traffic carrier frequency, and the other for the control on the beacon frequency.

Tdma Fdma Cdma

GSM multiframe

The GSM frames are grouped together to form multiframes and in this way it is possible to establish a time schedule for their operation and the network can be synchronised.

There are several GSM multiframe structures:

What Is Tdma

Traffic multiframe: The Traffic Channel frames are organised into multiframes consisting of 26 bursts and taking 120 ms. In a traffic multiframe, 24 bursts are used for traffic. These are numbered 0 to 11 and 13 to 24. One of the remaining bursts is then used to accommodate the SACCH, the remaining frame remaining free. The actual position used alternates between position 12 and 25.
Control multiframe: the Control Channel multiframe that comprises 51 bursts and occupies 235.4 ms. This always occurs on the beacon frequency in time slot zero and it may also occur within slots 2, 4 and 6 of the beacon frequency as well. This multiframe is subdivided into logical channels which are time-scheduled. These logical channels and functions include the following:
- Frequency correction burst
- Synchronisation burst
- Broadcast channel (BCH)
- Paging and Access Grant Channel (PACCH)
- Stand Alone Dedicated Control Channel (SDCCH)

Cellular Tdma

GSM Superframe

Multiframes are then constructed into superframes taking 6.12 seconds. These consist of 51 traffic multiframes or 26 control multiframes. As the traffic multiframes are 26 bursts long and the control multiframes are 51 bursts long, the different number of traffic and control multiframes within the superframe, brings them back into line again taking exactly the same interval.

GSM Hyperframe

Above this 2048 superframes (i.e. 2 to the power 11) are grouped to form one hyperframe which repeats every 3 hours 28 minutes 53.76 seconds. It is the largest time interval within the GSM frame structure.

Within the GSM hyperframe there is a counter and every time slot has a unique sequential number comprising the frame number and time slot number. This is used to maintain synchronisation of the different scheduled operations with the GSM frame structure. These include functions such as:

Frequency hopping: Frequency hopping is a feature that is optional within the GSM system. It can help reduce interference and fading issues, but for it to work, the transmitter and receiver must be synchronised so they hop to the same frequencies at the same time.
Encryption: The encryption process is synchronised over the GSM hyperframe period where a counter is used and the encryption process will repeat with each hyperframe. However, it is unlikely that the cellphone conversation will be over 3 hours and accordingly it is unlikely that security will be compromised as a result.

The slots and frames are handled in a very logical manner to enable the system to expect and accept the data that needs to be sent. Organising it in this logical fashion enables it to be handled in the most efficient manner.

Wireless & Wired Connectivity Topics:
Mobile Communications basics2G GSM3G UMTS4G LTE5GWiFiIEEE 802.15.4DECT cordless phonesNFC- Near Field CommunicationNetworking fundamentalsWhat is the CloudEthernetSerial dataUSBSigFoxLoRaVoIPSDNNFV
Return to Wireless & Wired Connectivity