Page 1 Mobile Lab

TelecommunicationsCommunications Technologies

Mobile Communication Lab

By: Dr. Bessam Z. Hassan

A

TELECOMMUNICATIONSCOMMUNICATIONS TECHNOLOGIES

MOBILE COMMUNICATION LAB

by Dr. Bessam Z. Hassan

III

Table of ContentsIntroduction to GSM Specifications

Exercise Quadrature Phase Shift Keying(QPSK/DQPSK)

Exercise 1 QPSK Modulation

Exercise Quadrature Amplitude Modulation(QAM/DQAM)

Exercise 2 QAM Modulation

Exercise Digital Data on Cell Phone

Exercise 3 Signal Spectral Analysis

Exercise Code Division Multiple Access

Exercise 4 CDMA Simulation

Exercise Frequency Hopping

Exercise 5 Frequency Hopping Techniques

Exercise Multiple Input/Multiple Output

Exercise 6 MIMO System

Exercise GSM Protocols

Exercise 7 Protocols in Cellular Systems

Exercise Long Term Evolution

Exercise 8 LTE Simulation

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Introduction to Introduction to Introduction to Introduction to GSMGSMGSMGSM SpecificationsSpecificationsSpecificationsSpecifications

GSM (Global System for Mobile communications) is an open, digital cellular technology used for transmitting mobile voice and data services. GSM differs from first generation wireless systems in that it uses digital technology and Time Division Multiple Access (TDMA) transmission methods. GSM is a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time-slots. GSM operates in the 900 MHz and 1.8 GHz bands in Europe and the 1.9 GHz and 850 MHz bands in the US. The 850 MHz band is also used for GSM and 3GSM in Australia, Canada and many South American countries. GSM supports data transfer speeds of up to 9.6 kbit/s, allowing the transmission of basic data services such as SMS (Short Message Service). Another major benefit is its international roaming capability, allowing users to access the same services when travelling abroad as at home. This gives consumers seamless and same number connectivity in more than 210 countries. GSM satellite roaming has also extended service access to areas where terrestrial coverage is not available.

Global System for Mobile Communications is the first European digital standard, developed to establish cellular compatibility throughout Europe. It's success has spread to all parts of the world and over 80 GSM networks are now operational. It operates at 900 MHz.

The following is the technical specifications of the GSM

GSMGSMGSMGSM----900 and GSM900 and GSM900 and GSM900 and GSM----1800180018001800

GSM-900 and GSM-1800 are used in most parts of the world.

• GSM-900 uses 890 - 915 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 935 - 960 MHz for the other direction (downlink), providing 124 RF channels (channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used. In some countries the GSM-900 band has been extended to cover a larger frequency range. This 'extended GSM', E-GSM, uses frequency range 880 - 915 MHz (uplink) and 925 - 960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band.

• GSM-1800 uses 1710 - 1785 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 1805 - 1880 MHz for the other direction (downlink), providing 374 channels (channel numbers 512 to 885). Duplex spacing is 95 MHz.

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GSMGSMGSMGSM----850850850850

• GSM-850 and GSM-1900 are used in the United States, Canada, and many other countries in the Americas.

• GSM-850 uses 824 - 849 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 869 - 894 MHz for the other direction (downlink). Channel numbers 128 to 251.

Cellular is the term used to describe the 850 MHz band, as the original analog cellular mobile communication system was allocated in this spectrum. Providers commonly operate in one or both frequency ranges.

GSM GSM GSM GSM Pulse ShapePulse ShapePulse ShapePulse Shape is not rectangularis not rectangularis not rectangularis not rectangular: A Gaussian Pulse Shape is used

Constellation used by GSM:Constellation used by GSM:Constellation used by GSM:Constellation used by GSM: QPSK, 8PSK, 16-QAM.

The method of multiplexing chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.

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Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. It allows several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own timeslot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only the part of its bandwidth they require. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, and combat-net radio systems.

TDMA is a type of Time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers.

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SSSStructure of thetructure of thetructure of thetructure of the GSMGSMGSMGSM NetworkNetworkNetworkNetwork::::

The GSM system coordinates the communication between a mobile telephones (mobile stations), base stations (cell sites), and switching systems. Each GSM radio channel is 200 kHz wide channels that are further divided into frames that hold 8 time slots. GSM was originally named Groupe Special Mobile. The GSM system includes mobile telephones (mobile stations), radio towers (base stations), and interconnecting switching systems.

The figure below shows an overview of a GSM radio system. This diagram shows that the GSM system includes mobile communication devices that communicate through base stations (BS) and a mobile switching center (MSC) to connect to other mobile telephones, public telephones, or to the Internet. This diagram shows that the MSC connects to databases of customers. This example shows that the GSM system mobile devices can include mobile telephones or data communication devices such as laptop computers.

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The diagram below shows that the GSM system uses a single type of radio channel. Each radio channel has a frequency bandwidth of 200 kHz and a data transmission rate of approximately 270 kbps. The example shows that each radio communication channel is divided into 8 time slots (0 through 7). This diagram shows that a simultaneous two-way voice communication session requires at least one radio channel communicates from the base station to the mobile station (called the forward channel) and one channel communicates from the mobile station to the base station (called the reverse channel). This example also shows that some of the radio channel capacity is used to transfer voice (traffic) information and some of the capacity is used to transfer control messages.

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Key ElementsKey ElementsKey ElementsKey Elements

1. Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.

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2. Mobile Switching Centre

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjunction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signaling between functional entities in the Network Subsystem uses Signaling System Number 7 (SS7), used for trunk signaling in ISDN and widely used in current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile is typically in the form of the signaling address of the VLR associated with the mobile station. The actual routing procedure will be described later. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the signaling required. Note that the MSC contains no information about particular mobile stations --- this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel.

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3. Base Station Subsystem (BSS)

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the standardized Abis interface, allowing (as in the rest of the system) operation between components made by different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).

Quadrature Phase Shift Keying (QPSK/DQPSK) 9

When you have completed this exercise, you will be familiar with QPSK modulation, with the characteristics of QPSK signals and with the QPSK signal constellation. You will also be familiar with the Simulink software and the use of the virtual instruments.

The Discussion of this exercise covers the following points:

The QPSK waveform QPSK constellations A typical QPSK modulator Symbol rate and bandwidth

The QPSK waveform

With quadrature phase shift keying modulation (also called quaternary PSK, quadriphase PSK or 4-PSK), a sinusoidal waveform is varied in phase while keeping the amplitude* and frequency constant. The term quadrature indicates that there are four possible phases.

Equation (1) shows the general expression for a QPSK waveform.

[ ])(cos)( 0 ttAts ici ϕϕω ++= (1)

where si is the PSK signal waveform for phase i t is time A is the peak amplitude ωc is the carrier frequency in radians/s ( cc fπω 2= ) φ0 is the reference phase angle φi is phase i i ranges from 1 to 4

The instantaneous phase has discrete values equal to 4

20

iπϕ + , where i = 1, 2,

3, or 4.

QPSK constellations

The ideal PSK constellation has M equidistant phase states and constant amplitude, resulting in a circular symmetry. With QPSK, therefore, M = 4 and the phases are separated by 90°, as shown in Figure 4.

QPSK Modulation

Exercise 1

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

* Filters are usually used near the output of a QPSK modulator in order to restrict the bandwidth. This results in amplitude variations in the QPSK signal. Although these amplitude variations facilitate timing recovery in the demodulator, they do not convey data. Only the phase of the waveform conveys the data.

Exercise 1 – QPSK Modulation Discussion

10 Quadrature Phase Shift Keying (QPSK/DQPSK)

Figure 1 shows two common representations of the QPSK constellation. The constellation points are arbitrarily labeled A, B, C and D, each of which represents one of the four possible dibits 00, 01, 10, and 11. The mapping between the dibits and the constellation points depends on the modulator circuit.

Beside each constellation in Figure 1 is a plot showing all four phases (modulation symbols) as sinusoids. In each of these representations, the four phases are spaced 90° (π/2 radians) apart. The only difference between these representations is the choice of the reference phase angle (φ0 in Equation (1)).

Figure 1. QPSK constellation and waveforms.

A typical QPSK modulator

A QPSK signal can be generated by independently modulating two carriers in quadrature (cosωt and sinωt), as shown in Figure 2.

The Serial to Parallel Converter groups the incoming data into dibits (groups of two consecutive bits). Each time two bits have been clocked serially into its buffer, the Serial to Parallel Converter outputs one dibit in parallel at its two outputs.

One bit of each dibit is sent to the I channel of the modulator; the other bit is sent to the Q channel of the modulator. Each channel of the modulator works independently to processes the stream of bits it receives.

A B C D

A B C D

D

B

A

I

Q

a) QPSK (φ = π/4, 3π/4, 5π/4, 7π/4)

b) QPSK (φ = 0, π/2, π, 3π/2)

A B

C D

I

Q

C



The starting point for grouping bits into dibits is completely arbitrary. For educational purposes, the Serial to Parallel Converter in the QPSK application has a Drop 1 Bit button. Clicking this button causes the Serial to Parallel Converter to ignore one bit in the data sequence. This changes the grouping of all subsequent data bits into dibits.

The Level Converter in each channel converts the data into a (baseband) bipolar pulse stream that can be applied to one input of the mixer. To restrict the bandwidth of the QPSK signal, a Low-Pass Filter is usually used before the mixer in each channel of the modulator in order to provide the desired spectral shaping. In addition, a bandpass filter (not shown in Figure 2) may be used to filter the QPSK signal before transmission.

Figure 2. Simplified block diagram of a QPSK modulator.

The I- and Q-channel sinusoidal carriers cosωt and sinωt are in quadrature (90° out of phase). Each mixer performs modulation by multiplying the carrier by the bipolar data signal in order to produce a BPSK signal. The effect of the mixer is to shift the frequency spectrum of the baseband signal up to the frequency of the carrier.

The two BPSK signals are summed to produce the QPSK signal. Because these two BPSK signals are generated using two carriers in phase quadrature, the BPSK signals are orthogonal, and the QPSK demodulator will be able to demodulate them separately.

The output signal of the modulator is a sinusoidal carrier with four possible phases, each of which represents a two-bit symbol. This signal can be represented by Equation (2).

)sin()()cos()()( ttdttdts QI ωω += (2)

where s(t) is the QPSK signal waveform dI(t) is the I-channel bipolar pulse stream d0, d2, d4 … dQ(t) is the Q-channel bipolar pulse stream d1, d3, d5 … ω is the angular frequency

Figure 3 shows the waveforms present in a modulator with no filtering. The bipolar I-channel and Q-channel pulses have levels of +1 and -1. The QPSK signal shown in Figure 3 has discontinuities which increase the bandwidth of the signal. The Low-Pass Filters shown in Figure 2 smooth the discontinuities and decrease the effective bandwidth.

Orthogonal signals can be summed, transmitted in a channel and (theoretically) perfectly separated in the demodulator without any mutual interference.

If the levels of the bipolar pulses d0, d1, etc. are +1 and -1, the peak amplitude of the waveform repre-sented by Equation (2) is

2 . The factor 21 could be included in this equation to normalize the amplitude to unity.

Serial to Parallel

Converter

Low-Pass Filter

Low-Pass Filter

cosωt

sinωt

QPSK Signal

I Channel

Q Channel

Level Converter

Level Converter

Binary Data Input

Σ


12 Quadrature Phase Shift Keying (QPSK/DQPSK)

Note: Figure 3 shows all waveforms synchronized in time. In a real system, processing delays will cause successive signals to be slightly offset in time.

Figure 3 shows the dibit 11 mapped to the constellation point in the first quadrant. Other mappings are possible. Such mappings usually use a Gray code to ensure that only one bit changes between adjacent symbols. As a result, QPSK modulators map the dibits 00 and 11 to opposite quadrants.

0

1

−101

−101

Figure 3. QPSK signal generation from two BPSK signals.

Symbol rate and bandwidth

Because each symbol represents two bits, the rate that the symbols occur in the QPSK signal (the symbol rate) is one half the bit rate. Table 2 compares the symbol rates (and bandwidths) for BPSK and QPSK.

Table 2. Symbol rate and bandwidth for BPSK and QPSK.

Modulation Bits per symbol Symbol rate vs. Bit rate First-nulls bandwidth

BPSK 1 bs RR = bR2

QPSK 2 2b

sR

R = bR

11 1101

10 00

Data Bit Stream b0, b1, b2 …

I-Channel Pulses d0, d2, d4 … (even bits)

Q-Channel Pulses d1, d2, d5 … (odd bits)

I-Channel BPSK Signal

Q-Channel BPSK Signal

QPSK Signal

Constellation Points and Dibits (This is an example, other

mappings are possible)

b0 b1 b2 b3

d0 d2

d1 d3

b4 b5 b6 b7

d4 d6

d5 d7

b8 b9

d8

d9



The bandwidth of a modulated signal depends on the rate of change in the signal (i.e. the symbol rate) and not on the magnitude of each change. For this reason, using QPSK rather than BPSK will reduce by one-half the bandwidth required for a given bit rate. Alternatively, using QPSK can double the bit rate for a given signal bandwidth. This is illustrated in Figure 4, where fc is the carrier frequency.

fc − 2Rb fc − Rb fc fc + Rb fc + 2Rb

fc − 4Rs fc − 3Rs fc − 2Rs fc − Rs fc fc + Rs fc + 2Rs fc + 3Rs fc + 4Rs

Figure 4. BPSK and QPSK magnitude spectrum (for equal bit rates).

Figure 4 shows that the first-nulls bandwidth of a BPSK signal is 2Rb and that of

a QPSK signal with the same bit rate is 2Rs = Rb. Since 2b

sR

R = for QPSK, the

bandwidth efficiency of QPSK is twice that of BPSK.

The modulated signal is called a double-sideband suppressed-carrier signal, since its bandwidth is twice that of the baseband signal and there is no isolated signal at the carrier frequency.

BPSK

QPSK

Bandwidth

Bandwidth

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