Some Design Issues in Mobile Cellular Communication

1. Some Design Issues in Mobile Cellular Communication P. P. Bhattacharya URSI & DST Young Scientist Awardee Department of Electronics and Communication Engineering, Netaji Subhash Engineering College, Techno City, Garia, Kolkata – 700 152, India Phone – 91-33-2436 1285, Fax – 91-33-2436 1286 E – mail :partha_p_b@yahoo.com

2. Partha Pratim Bhattacharya meeting President of India in Rashtrapati Bhavan

3. Contents – • Selection of cluster size • Channel assignment strategies • Call handover strategies • Some newly proposed call handover strategies • Cellular system capacity • Improving capacity in cellular systems • Radio design for a cellular network • Base station antennas • Cell phone antennas • Second generation (2G) cellular networks • Global System for Mobile (GSM) • Introduction to CDMA digital cellular standard • Evolution for TDMA and CDMA standards • Third generation (3G) cellular networks • Towards 4G

5. Selection of Cluster Size - The ratio D/R is called co-channel reuse ratio (Q) and may be approximated as Q = D/R = (3N)1/2 Low N -> Low D/R -> Low D -> More interference High N-> High D/R -> Low R -> More number of cells The carrier to interference ratio can be approximately given as C / I = (D/R)n / 6, n=4 (n can be anything from 2 to 6).

6. Channel Assignment Strategies – • Fixed Channel Assignment (FCA) Strategy • Channels are preallocated during planning.. If all the channels in a cell are occupied, the call is blocked and the subscriber does not receive service. • Dynamic Channel Assignment (DCA) Strategy • In this case, voice channels are not allocated permanently to the cells. Each time a call request is made, the serving BS requests a channel from the MSC. MSC then allocates a channel to the requested cell in fashion which accounts the chance of future blocking within the cell, the frequency of use of the candidate channel, reuse distance of the channel and other cost functions. Therefore, the MSC only allocates a given frequency if the frequency is not presently in use in the cell or any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference. Dynamic channel allocation reduces the probability ofblocking. • DCA scheme performs well under nonuniform and low traffic density. • FCA performs well under high and uniform traffic.

7. Hybrid Channel Assignment (HCA) - • Some channels are permanently assigned to BS as in FCA • Other channels are kept in a central pool for borrowing • Borrowing strategy.- • Combination of FCA & DCA. • In this scheme, a cell is allowed to borrow channels from a neighbouring cell if all of its own channels are already occupied. • Better performance than FCA under light and moderate traffic load. • Borrowing with channel ordering (BCO) – First channel has the highest priority to be assigned to the next call and the last channel has the highest priority to be assigned to neighbouring cell. • Borrowing with directional channel locking (BDCL) – After a channel is borrowed it is locked in the cochannel cell within the channel reuse distance of the borrowing cell. • Borrowing without channel locking (BWCL) – Overcomes disadvantages of other schemes. Channel can be borrowed only from adjacent BS and used with reduced transmitted power.

8. Call Handover Strategies – Handover procedure involves measurement, decision and execution. Handover may be based on measurement such as – Signal strength ·        Bit error rate ·        Traffic load ·        Carrier to interference ratio etc. Signal strength based algorithm is simple and effective 1st generation systems – Signal strength was measured by base stations 2nd generation systems – Signal strength is measured by mobile stations (Mobile Assisted Handoff)

9. * Relative signal strength , the handover will occur at position A. * Relative signal strength with threshold allows a user to hand over only if the current signal is sufficiently weak (less than a threshold) and the other is the stronger of the two.

10. The effect of the threshold depends on its value compared to the signal strengths of the two base stations at the point at which they are equal. If the threshold is higher than this value, say T1, this scheme performs exactly like the relative signal strength scheme, so the handover occurs at position A. If the threshold is lower than this value, say T2, the mobile will delay handover until the current signal level crosses the threshold at position B. In the case of T3, the delay may be so long that the mobile drifts far into the new cell. This reduces the quality of the communication link and may result in a dropped call. * Relative signal strength with hysteresis allows a user to hand over only if the new base station is sufficiently stronger (by a hysteresis margin, h) than the current one. In this case the handover will occur at point C. This technique prevents the so-called ping-pong effect, the repeated handover between two base stations caused by rapid fluctuations in the received signal strengths from both base stations.

11. * Relative signal strength with hysteresis and threshold hands a user over to a new base only if the current signal level drops below a threshold and the target base station is stronger than the current one by a given hysteresis margin. The handover will occur at point C if the threshold is either T1 or T2, and will occur at point D if the threshold is T3. * Prediction techniques base the handover decision on the expected future value of the received signal strength. Prioritizing handoff – 1) Guard channel concept 2) Queuing of handoff calls 

12. Some Newly Proposed Call Handover Strategies - Fuzzy Logic Based Handover Algorithm Advantages of Fuzzy Logic – Multivalued logic, many input parameters can be considered, less number of fluctuations Input parameters – 1. Distance from base station which is defined as very near, near, medium, far and very far 2. Signal strength difference which is defined as very high, high, medium, low and very low Handover state - No handover, Wait, Be careful, Handover, Sure handover

13. Handover response -

14. Characterization of Velocity Dependence of Call Handover - Handover characteristics depend on velocity. To describe the effect of user mobility a parameter α is considered and defined as where Tm is the mean call duration. The interval tmc, between the time a user starts a call in a cell and the time the user reaches the cell boundary is where L is the distance which the user transits. Thus a handover occurs if tmc < td where td is the call duration time.

15. The probability that a call in the current cell produces a handover towards a neighbouring cell Average number of handovers per call attempt ηh where Pba is the blocking probability for new call attempts and Pbh is the handover failure probability.

16. Probability that a call is dropped by an unsuccessful handover where Pba - Probability of new call blocking Pdrop - Probability of handover call dropping 1/η - Mean residual time (taken to be 2 minutes) 1/μ - Mean call holding time (taken to be 3 minutes) The probability of call completion or throughput

21. Velocity Dependent Variable Hysteresis Margin based Algorithm - Recently proposed variable hysteresis margin based scheme – h  exp [ -  / 6], where  is the path loss exponent and varies from 2 to 6. Velocity dependent variable hysteresis margin based scheme – h = H exp [ -  / 6] / h where H is a constant hysteresis margin, his the average number of handover anddepends on mobile velocity. H = 5 dBm (Optimum response)

22. Handover response -

23. Umbrella cell approach to accommodate wide range of velocities -

24. Fuzzy Logic Based Velocity Dependent Call Handover Algorithm - Input parameters – 1. Distance from base station which is defined as very near, near, far and very far 2. Signal strength difference which is defined as very high, high, low and very low 3. Velocity of mobile user which is definedas very low, low, high and very high Handover state - No handover, Wait, Be careful, Handover Membership function – Bell shaped

25. Handover response -

26. Cellular System Capacity – Let, a cellular system has a total of S duplex channels available for use and each cell is allocated a group of K channels. If the S channels are divided among N cells (cluster) such that each cell has the same number of channels, the total number of radio channels can be expressed as S = KN The N cells which use the complete set of available frequencies is called a cluster. If a cluster is replicated M times, then the total number of duplex channels (C) can be used as a measure of capacity and given by C = MKN = MS Thus, capacity of a cellular system is directly proportional to the number of times a cluster is repeated in a service area. N is called cluster size and is typically equal to 4, 7 or 12.

27. Improving Capacity In Cellular Systems – 1) Conventional Microcell Approach – In this case, much smaller cells compared to the normal cells are used to accommodate more users. It does not provide intelligence because when the cell size becomes smaller, the control of interference among the cells becomes harder. Handoffs may not have enough time to complete. 2) Cell Splitting – Cell splitting is the process of subdividing a congested cell into smaller cells such that each smaller cell has its own base station with reduced antenna height and transmitter power. It increases the capacity of a cellular system since number of times channels are reused increases.

28. C D E D G B E F D F C A F C G E C B D E G B G F Fig: Cell splitting The increased number of cells would increase the number of clusters over the coverage region, which again would increase the number of channels and thus capacity. The distance between co-channel cells also reduces to half as the cell radius is reduced to half. Thus the co-channel reuse ratio remains same. Splitting – Static, Dynamic

29. 2 2 3 3 1 1 4 4 6 6 5 5 (b) Fig: (a) 120 sector and (b) 60 sector 1 1 2 2 3 3 (a) 3) Sectoring – The co-channel interference in a cellular system may be reduced by replacing a single omni-directional antenna at the base station by several directional antennas radiating within specified sectors. A cell is normally partitioned in three 120 sectors or six 60 sectors. A given cell will receive interference and transmit with only a fraction of the available co-channel cells. In the sectoring scheme, the co-channel interference is reduced and thus system capacity is improved. Co-channel interference is reduced because the number of interferer gets reduced.

30. D D C E C E A A B F B F G D G D C E D C E A E C A F B A B G F F B G D G D E C C E A A B F B F G G Fig: Only 2 co-channel cells out of 6 interfere with the center cell

32. Zone splitter Base station Tx / Rx Tx / Rx Microwave or fiber optic link Tx / Rx Fig: Novel microcell zone concept 4) Novel Microcell Zone Concept – In case of sectoring, number of handoffs increases which results in an increased load on the system. As a solution of this problem, novel microcell zone concept was proposed in which each cell is divided into three or more zone sites, which are connected to a single base station.

33. –20 –30 Average signal power Received power (dBm) – 40 Small-scale fading – 50 – 60 – 70 – 80 0 50 100 150 400 200 300 350 250 Distance from transmitter (metres) Fig: Small-scale fading • Radio design for a cellular network – • 1. Radio link design - • BS density and corresponding radio coverage is determined. • Gain of a mobile antenna is assumed to be 0 dBi. In reality, it can be as low as –6 to –8 dBi. 2. Coverage planning – • Propagation model – Large scale model, Small scale model

34. Accurate field measurement should be made in urban area and the measured data can be used in the planning process. • Loss = Path loss + shadow fading + multipath fading + penetration loss for buildings and vehicles + vegetation loss etc. • Path loss exponent - 2 to 6 • Shadow fading – 8 to 12 dB • Multipath fading – + / - 20 dB • Penetration loss – 10 to 30 dB in a building, 3 to 6 dB in • a car, 10 to 12 dB in a bus. • Propagation models – • Lee model • Walfish – Ikegami model • Okumara model • Hata model • Walfish – Bertoni model

35. Some new models – 1. Indoor office environment – L = 37+30log R+18.3 n[(n+2)/(n+1)-0.46] dB, where R is the distance (m) and n is the number of floors. 2. Outdoor to outdoor and pedestrian environment – L = 40 log R + 30 log f + 49 dB, valid for NLOS link, average building penetration loss of 18 dB, log normal shadowing of 10 – 12 dB. 3. Vehicular environment – L = 40 (1-0.04h)log R-18log h+21 log f+80 dB, h is the BS antenna height measured from the rooftop level (m) and building penetration loss of 18 dB.

36. Base Station Antennas -

37. Fig: Nonsmart-antenna system Fig. Beam forming smart antenna

38. Compared with traditional omni-directional and sectorized antennas, smart-antenna systems can provide: • Greater coverage area for each cell site • Better rejection of co-channel interference • Reduced multipath interference via increased directionality • Reduced delay spread as fewer scatterers are allowed into the beam • Increased frequency reuse with fewer base stations • Higher range in rural areas • Improved building penetration • Location information for emergency situations • Increased data rates and overall system capacity • Reduction in dropped calls

39. Cell Phone Antennas – • A cellular handset antenna is the small cylindrical stub sticking out of the top of the hand set case. • In some cases antennas are embedded in the handset case and are not visible to the user. • Since a cell phone user is constantly changing his or her position and moving from cell to cell, the mobile phone requires an antenna that transmits and receives equally well in all directions. • Another major concern in cellular antenna design is the potential radiation hazards posed to humans. As the use of cell phones increases, there are growing concerns about what affect the cell phone’s radio waves have on human health. Various antenna designs have been promoted to minimize the amount of energy that is radiated into the skull of the user.

40. Embedded antennas may make a handset more durable but having the antenna inside the device can impede performance because other electrical components inside the phone can cause interference. • CDMA handsets tend to have more external antennas than GSM handsets because on CDMA networks operators can squeeze in more users per base station. GSM systems don't have the same advantage. • Today there are four leading antenna architectures that are commonly used in embedded applications: microstrip, patch, Planar Inverted 'F' Antenna (PIFA) and Meander Line Antenna (MLA). • Microstrip lines are an extension of the monopole, only laying it down on a two-dimensional surface. It can be easily fabricated by etching a copper strip of 1/2- or 1/4-wavelength onto the radio circuit board. While very inexpensive to make, its performance is limited to the extent that surrounding metallic sections of the circuit board severely interfere with its radiation efficiency. Furthermore, it is a single-frequency solution and most wireless devices today implement more than one mode of communication, usually in different frequency bands.

41. Patch antennas have been around for a long time and are a good choice for a system that requires a beam pattern focused in a certain direction. They are typically used in single frequency applications requiring the directed beam pattern, such as a GPS receiver or a wall-mounted access point. • The PIFA antenna literally looks like the letter 'F' lying on its side with the two shorter sections providing feed and ground points and the 'tail' providing the radiating surface. PIFAs make good embedded antennas in that they exhibit a somewhat omnidirectional pattern and can be made to radiate in more than one frequency band. But, their efficiency is only average and it can be difficult to properly match the device to the transmitting circuitry at both operating frequencies. • The MLA is made from a combination of a loop antenna and frequency tuning meander lines. This is more efficient for its size than many competitive antennas used in wireless applications. In addition, MLAs can be designed to exhibit broadband capabilities that allow operation on several frequency bands, such as AMPS, PCS, and GPS bands simultaneously. However, initial MLA designs are slightly more expensive than the previous antenna options.

43. Second Generation (2G) Cellular Networks – Since 1990, most of the cellular networks use second generation or 2G digital technology. Unlike first generation (1G) which relied exclusively on FDMA/FDD and FM, 2G technology use digital modulation formats and TDMA/FDD and CDMA/FDD multiple access techniques. The most popular second generation TDMA standards include Global System for Mobile Communication (GSM), Interim Standard 136 (IS-136), Pacific Digital Cellular (PDC). The popular 2G CDMA standard is Interim Standard 95 Code Division Multiple Access (IS-95) also called cdmaone.

44. BTS BTS BSC VLR AUC HLR BTS MS BTS PSTN BTS BTS MSC BSC ISDN BTS OMC Operation support subsystem (OSS) Data network Base Station Subsystem (BSS) Network Switching Subsystem (NSS) Public networks Fig: GSM system architecture Global System for Mobile (GSM) -

45. GSM Air Interface Specifications – Reverse Channel Frequency 890  915 MHz Forward Channel Frequency 935  960 MHz ARFCN Number 0 to 124 and 975 to 1023 TX/RX Frequency spacing 45 MHz Modulation data rate 270.833 Kbps Frame period 4.615 ms users per frame 8 Time slot period 576.9 μs Bit period 3.692 μs Modulation 0.3 GMSK ARFCN channel spacing 200 KHz

46. GSM channels - Basically there are two types of GSM logical channels  traffic channels (TCH) and control channel (CCH). GSM Traffic channels – Full rate TCH The different full rate speech and data channels are discussed below. a) Full rate speech channel (TCH/FS) The full rate speech channel carries user speech, digitized at a raw data rate of 13 Kbps. With GSM channel coding it becomes 22.8 Kbps. b) Full Rate Data channel for 9.6 Kbps (TCH/F9.6) The full rate traffic data channel carries raw user data, sent at 9.6 Kbps. After forward error connection, it is sent at 22.8 Kbps. c) Full Rate Data channel for 4.8 Kbps (TCH/F4.8) The full rate traffic data channel carries raw user data sent at 4.8 Kbps. The data are ultimately sent at 22.8 Kbps with additional forward error connection. d) Full Rate Data Channel for 2.4 Kbps (TCH/F 2.4) It carries user data sent at 2.4 Kbps which with additional forward error correction is sent at 22.8 Kbps.

47. Half rate TCH Different half rate speech and data channels are discussed below. a) Half Rate speech channel (TCH/HS) This is used to carry digitised speech which is sampled at a rate half that of the full rate channel (6.5 Kbps). With GSM channel coding added, half rate speech channel carries 11.4 Kbps. b) Half Rate Data channel for 4.8 Kbps (TCH/H 4.8) This half rate data channel carries raw user data sent at 4.8 Kbps. With additional error correction coding it is sent at 11.4 Kbps. c) Half Rate Data channel for 2.4 Kbps (TCH/H2.4) This half rate channel carries user data which is sent at 2.4 Kbps. It is sent at 11.4 Kbps after adding forward error correction as per GSM standard.

48. Broadcast Control Channel (BCCH) Broadcast Channel (BCH) Frequency Correction Channel (FCCH) Synchronization Channel (SCH) Paging Channel (PCH) Common Control Channel (CCCH) GSM Control Channels Random Access Channel (RACH) Access Grant Channel (AGCH) Stand-alone Dedicated Control Channel (SDCCH) Dedicated Control Channel (DCCH) Slow Associated Control Channel (SACCH) Fast Associated Control Channel (FACCH) Fig: Different GSM Control Channels GSM Control Channels -

49. Introduction to CDMA digital cellular standard – In code division multiple access (CDMA) systems, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise (PN) code sequence that has a chip rate which is few orders of magnitudes greater than the message data rate. In this system, each cell is a cluster (cluster size N = 1) and shares the same bandwidth. All users use the same carrier frequency and may transmit simultaneously. Each user has its own PN code which is almost orthogonal to all other codewords. The receiver needs to know the code used by the transmitter for detection of the message signal. IS - 95 uses 824  849 MHz band for reverse channel and 869  894 MHz for forward channel. A forward and reverse channel pair is separated by 45 MHz. In India, CDMA service provides offer two services – WLL (M) and fixed wireless terminal (FWT).

50. Comparison between GSM and CDMA -