Computer Data Communications

Chapter 9:Wireless ServicesPart 3: Cellular Wireless Networks Computer Data Communications

Introduction • Overview of Cellular System • Cellular Geometries • Frequency Reuse • Operations of Cellular System • Mobile Radio Propagation Effects • Design Factors • Multipath Propagation • Types of Fading • First Generation (1G) • Second Generation (2G) :- D-AMPS, GSM, IS-95 • Third Generation (3G)

Overview of Cellular System Figure 1: Cellular system

Overview of Cellular System • It is designed to provide communications between two moving units, called mobile stations (MSs) or between one mobile unit and one stationary unit is called a land unit. • A service provider must be able to locate and track a caller, assign a channel to the call, and transfer the channel from base station to base station as the caller moves out of range (See Figure 1).

Overview of Cellular System • Each cellular service area is divided into cells: • Each cell contains an antenna and is controlled by a solar or AC powered network station, called the base station (BS) • Each cell is allocated a band of frequencies and is served by a base station, consisting of transmitter, receiver and control. • Each base station is controlled by a switching office, called a mobile switching centre (MSC).

Overview Cellular Wireless Networks • Each cellular service area is divided into cells: • The MSC coordinates communication between all the base stations and the telephone central office. It is computerized centre that is responsible for connecting calls, recording call information and billing. • Adjacent cells are assigned different frequencies to avoid crosstalk or interference. • The transmission of each cell is kept low to prevent its signal from interfering with those of other cell.

Cellular Geometries Figure 2: Cellular Geometrics patterns

Cellular Geometries • The first design decision to make is the shape of cells to cover an area. • A matrix of square cells would be the simplest layout to define (Figure 2a). However this geometry is not ideal. The reason is because as a mobile user within a cell moves toward the cell’s boundaries, the adjacent antennas are not equidistant (central). It is best if all of the adjacent antennas are equidistant.

Cellular Geometries • A hexagonal pattern provides for equidistant antennas (Figure 2b). • The radius of a hexagon is defined to be the radius of the circle that circumscribes it (equivalently, the distance from the center to each vertex; also equal to the length of a side of a hexagon.

Cellular Geometries • For a cell radius R, the distance between the cell center and each adjacent cell center is d = R. In practice, a precise hexagonal pattern is not used. • Variations from the ideal are due to topographical limitations, local signal propagation conditions and practical limitation on sitting antennas.

Frequency Reuse • In general, neighbouring cells cannot use the same set of frequencies for communication because it may create interference for the users located near the cell boundaries. • However, the set of frequencies available is limited, and frequencies need to reuse. • It must manage reuse of frequencies. • The frequency reuse pattern is a configuration of N cells.

Frequency Reuse • N being the reuse factor, in which each cell uses a unique set of frequencies. When the pattern is repeated, the frequencies can be reused. • The numbers in the cells define the pattern. The cells with the same number in a pattern can use the same set of frequencies. • As Figure 3a shows in pattern with reuse factor 4, Figure 3b shows in pattern with reuse factor 7 and Figure 3c shows in pattern with reuse factor 19.

Frequency Reuse • The power of base transceiver is carefully controlled: • To allow communications within cell on given frequency • Limit the power at the frequency that escapes to adjacent cells • To allow re-use of frequencies in nearby cells, thus allowing the frequency to be used for multiple simultaneous conversations. • Typically 10 – 50 frequencies per cell • Example for Advanced Mobile Phone Service (AMPS) • N cells all using same number of frequencies • K total number of frequencies used in systems • each cell has K/N frequencies • K=395, N=7 giving 57 frequencies per cell on average

Frequency Reuse Figure 3: Frequency Reuse Patterns

Figure 4: Call Stages

Operations of Cellular System • Mobile unit initialization: • Mobile unit scans and selects the strongest setup control channel used for this system (Figure 4a). Then a handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell, to identify the user and register its location • Mobile-originated call: • A mobile unit originates a call by sending the number of the called unit on the preselected setup channel (Figure 4b).

Operations of Cellular System • Paging: • The MTSO then attempts to complete the connection to the called unit, sending a paging message to certain BSs depending on the called mobile number (Figure 4c). • Call accepted: • The called mobile unit recognizes its number on the setup channel being monitored and responds to that BS, which sends the response to the MTSO. The MTSO sets up a circuit between the calling and called BSs, and also selects an available traffic channel within each BS's cell and notifies each BS, which in turn notifies its mobile unit (Figure 4d).

Operations of Cellular System • Ongoing call: • While connection is maintained, the mobile units exchange voice or data signals, through respective BSs and MTSO (Figure 4e). • Handoff: • If a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell (Figure 4f).

Operations of Cellular System • Other functions performed by the system but not illustrated in Figure 4 include: • Call blocking: • During the mobile-initiated call stage, if all the traffic channels assigned to the nearest BS are busy, then the mobile unit makes a preconfigured number of repeated attempts. After a certain number of failed tries, a busy tone is returned to the user. • Call termination: • When one of the two users hangs up, the MTSO is informed and the traffic channels at the two BSs are released.

Operations of Cellular System • Call Drop: • During a connection, because of interference or weak signal spots in certain areas, if the BS cannot maintain the minimum required signal strength for a certain period of time, the traffic channel to the user is dropped and MTSO is informed. • Calls to/from fixed and remote mobile subscriber • MTSO connects mobile user and fixed line via PSTN • MTSO connects to remote MTSO via PSTN or dedicated lines

Mobile Radio Propagation Effects • Signal strength • The strength of signal between BS and mobile unit must be strong enough to maintain signal quality at the receiver • Must not too strong to create co-channel interference with channels in another cell using the same frequency • Must able to handle variations in noise • Fading • The term fading refers to the time variation of received signal caused by changes in transmission medium or path(s) • Even if signal strength in effective range, signal propagation effects may disrupt the signal

Design Factors • propagation effects • max transmit power level at BS and mobile units • typical height of mobile unit antenna • available height of the BS antenna • these factors determine size of individual cell • use model based on empirical data and apply that model to a given environment to develop guidelines for cell size. • eg. model by Okumura et al & refined by Hata • detailed analysis of tokyo area • produced path loss info for an urban environment • Hata's model is an empirical formulation

Multipath Propagation Figure 5: Sketch of Multipath Propagation Components: Refection (R), Scattering (S) and Diffraction (D)

Multipath Propagation • The multipath Propagation components consists of (see Figure 5 ): • Reflection (from the ground or large objects) • Diffraction (from edges and corners of terrain or buildings) • Scattering (from foliage and other small objects) • Reflection • Occurs when an electromagnetic signal encounters a surface that is large relative to the wavelength of the signal. • E.g. Suppose a ground-reflected wave near the mobile unit is received. Because the ground-reflected wave has a 180º phase shift after reflection, the ground wave and the line-of-sight (LOS) wave may tend to cancel, resulting in high signal loss.

Multipath Propagation • Diffraction • Occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges) . • When a radio wave encounters such an edge, waves propagate in different directions with the edge as the source. • Thus, signals can be received even when there is no unobstructed LOS (line of sight) from transmitter.

Multipath Propagation • Scattering • Occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength and the number of obstacles per unit volume is quite large. • An incoming signal is scattered into several weaker outgoing signals. • In practice the causes of scattering in cellular communication include foliage, street signs and lamp posts.

Types of Fading • Fast fading • Rapid changes in strength over half wavelength distances • Eg. At a frequency of 900MHz, which a typical for mobile cellular applications, a wavelength is 0.33 m. Changes of amplitude can be much as 20 or 30 dB (decibels) over a short distance. • Slow fading • Slower changes due to user passing different height buildings, gaps in buildings etc. • over longer distances than fast fading • Flat fading • Flat fading or non-selective fading, is that type of fading in which all frequency components of received signal fluctuate in the same proportions simultaneously. • Selective fading • Different frequency components affected differently

First Generation • AMPS • The original cellular telephone networks provided analog traffic channels and are now referred as first generation system. • Advanced Mobile Phone System (AMPS) is one of the leading analog cellular systems in North America. • It uses FDMA to separate channels in a link. • It uses two separate analog channels: • One for forward (base station to mobile station) communication • One of reverse (mobile station to base station) communication

First Generation • AMPS • Two 25-MHz bands are allocated to AMPS (see figure 6): • The band between 824 and 849 MHz carries reverse communication • The band between 869 and 894 carries forward communication • An operator is allocated only 12.5 MHz in each direction. • Each band is divided into 832channels. However, two providers can share an area, which means 416channels in each cell for each provider. • The channels are spaced 30kHz apart, which allows a total of 416 channels per operator .

First Generation • AMPS • 21 channels are allocated for control, leaving 395 to carry calls. • The control channels are data channels operating at 10 kbps. • It uses FM (frequency modulation) and FSK (Frequency Shift Keying) for modulation. • Figure 7 shows the transmission in reverse direction. • Voice channels are modulated using FM and the channels use FSK to create 30 kHz analog signals • It uses FDMA to divide each 25 MHz band into 30 kHz channels.

First Generation Figure 6: Cellular bands for AMPS

First Generation Figure 7: AMPS reverse communication band

Second Generation • To provide higher-quality (less noise-prone) mobile voice communications, the second generation of cellular network was developed. • The second generation was mainly designed for digitized voice. • Three major systems evolved in the second generation, as shown in Figure 8.

Second Generation Figure 8: Second generation cellular phone systems

Second Generation – D-AMPS • The product of the evolution of the analog AMPS into a digital system is digital AMPS (D-AMPS). • It was designed to backward-compatible with AMPS. • This means that in a cell, one telephone can use AMPS and another D-AMPS. • D-AMPS was first defined by IS-54 (Interim Standard 54) and later revised by IS-136 • Band: • D-AMPS uses the same bands and channels as AMPS.

Second Generation – D-AMPS • Transmission : • Each voice channel is digitized using a very complex PCM and compression • A voice channel is digitized to 7.95 kbps. • Three 7.95 kbps digital voice channels are combined using TDMA. • The result is 48.6 kbps of digital data. • As Figure 9 shows, the system sends 25 frames per second with 1944 bits per frame. • Each frame lasts 40 ms (1/25) and is divided into six slots shared by three digital channels; each channel is allotted two slots.

Second Generation – D-AMPS • Transmission : • Each slot holds 324 bits. • However only 159 bits comes from the digitized voice, 64 bits are for control and 101 bits are for error correction. • The resulting 48.6 kbps of digital data modulates a carrier using QPSK; the results is a 30 kHz analog system . • Finally, the 30 kHz analog signals share a 25 MHz band (FDMA). • D-AMPS has a frequency reuse factor of 7. • D-AMPS is a digital cellular phone system using TDMA and FDMA.

Second Generation – D-AMPS Figure 9: D-AMPS

Second Generation – GSM • The Global System for Mobile Communication (GSM) is European standard that was developed to provide a common second-generation technology for all Europe. • The aim was to replace a number of incompatible first-generation technologies. • Band: • GSM uses two bands for duplex communication. • Each band is 25 MHz in width, shifted toward 900 MHz • Each band is divided into 124 channels of 200 kHz separated by guard bands. (See Figure 10)

Second Generation – GSM Figure 10: GSM bands

Second Generation – GSM • Transmission : • Figure 11 shows a GSM system . • Each voice channel is digitized and compressed to 13 kbps digital signal . Each slot carriers 156.25 bits (see Figure 12) . • 26 frames also share a multiframe (TDMA). • To calculate the bit rate of each channel as follows • Channel data rate = (1/120 ms) x 26 x 8 x 156.25 = 270.8 kbps • Each 270.8 kbps digital channel modulates a carrier using a GMSK ( a form of FSK used mainly in European systems ); the result is 200 kHz analog signal.

Second Generation – GSM • Transmission : • Finally 124 analog channels of 200 kHz are combined using FDMA . • The result is 25 MHz band. • Figure 11 shows the user data and overhead in multi-frame components. • GSM is a digital cellular phone system using TDMA and FDMA

Second Generation – GSM Figure 11: GSM system

Second Generation – GSM Figure 12: GSM Multi-frame Components

Second Generation – IS-95 • Interim Standard 95 (IS-95) is one of the dominant second generation standards in North America • It is a digital cellular phone system using CDMA/DSSS and FDMA. • Bands and Channels • IS-95 uses two bands for duplex communications: • Traditional ISM 800-MHz • ISM 1900-MHz • Each band is divided into 20 channels of 1.228 MHz separated by guard bands. • Each service provider is allotted 10 channels . • It can be used in parallel with AMPS. • Each IS-95 channel is equivalent to 41 AMPS channels (41 x 30 kHz = 1.23 MHz)

Second Generation – IS-95 • Data Rate Sets • IS-95 defines two data rate sets, with four different rates in each: • The first set defines 9600, 4800, 2400 and 1200 bps. • The second set defines 14400 , 7200, 3600 and 1800 bps. • Frequency Re-use Factor • In an IS-95 system, the frequency-reuse factor is normally 1 because the interference from neighbouring cells cannot affect CDMA or DSSS Transmission.

Second Generation – IS-95 • Synchronization: • All base channels need to be synchronized to use CDMA. • To provide synchronization, the bases use the services of GPS (Global Positioning System, a satellite system. • It use two transmission structures : • Forward Transmission ( base to mobile direction ) • Reverse Transmission ( mobile to base direction ) • Forward Transmission : • In forward direction, communications between the base and all mobiles are synchronized; the base sends synchronized data to all mobiles.

Second Generation – IS-95 • Forward Transmission : • Figure 13 shows a simplified diagram for the forward direction. • Each voice channel is digitized, producing data at a basic rate of 9.6 kbps • After adding error-correcting and repeating bits, and interleaving , the result is a signal of 19.2 ksps (kilosignals per second) • The scrambling signal is produced from a long code generator that uses the electronic serial number (ESN) of mobile station and generate 2 42 pseudorandomly chips. • Each chip having 42 bits.

Second Generation – IS-95 • Forward Transmission : • The output of long code generator is fed to decimator which chooses 1 bit of 64 bits. • The output of decimator is used for scrambling. The scrambling is used to create privacy; the ESN is unique for each station. • The result of the scrambler is combined using CDMA. • For each traffic channel, one Walsh 64 x 64 row chip is selected. The result is a signal of 1.228 Mcps (megachips per seconds). • 19.2 ksps x 64 cps = 1.228 Mcps

Second Generation – IS-95 • Forward Transmission : • The signal is fed into a QPSK modulator to produce a signal of 1.228 MHz. The resulting bandwidth is shifted appropriately using FDMA. • An analog channel creates 64 digital channels, of which 55 channels are traffic channels (carrying digitized voice). 9 channels are used for control and synchronization: • Channel 0 is a pilot channel. This channel sends a continuous stream of 1s to mobile stations. The stream provides bit synchronization, serve as a phase reference for demodulation, allows the mobile station to compare the signal strength of neighbouring bases for handoff decisions.

Computer Data Communications