8. Cellular Systems References 1. Bell System Technical Journal, Vol. 58, no. 1, Jan 1979. 2. R. Steele, Mobile Communications, Pentech House, 1992. 3. G. Calhoun, Digital Cellular Radio, Artech House, 1988. (Not that technical but makes quite interesting reading) The primary objective of any mobile communication system is to provide coverage for a large number of users scattered over a wide geographic area, using the limited resources (like spectrum, transmitted power) available. Example: Consider a hypothetical system serving the greater Vancouver area with a single base station and an allocated bandwidth of 25 MHz. Each active user in the system requires 25 KHz of transmission bandwidth for voice communication. The number of simultaneous users this system can support is 1000, or approximately 0.05% of the population of the lower mainland. Assuming 1% of the subscribers are active at the same time, this means the system can accommodate at most 100,000 subscribers. The system will be costly because of its relatively low capacity and the need to use high-power handsets. In addition, the health issue would become a major concern. 8-1
8.1 Basic Cellular Concepts The main concept in cellular communication is to replace a single, highpower transmitter (base-station) by many low-power transmitters, each providing coverage to only a small area or cell. The base-stations are connected to Mobile Telephone Switching Office (MTSO) via landlines. The MTSO in turn are connected to the central office (CO) of the public switched telephone network (PSTN). A cellular user communicates with another cellular user or a user on the PSTN through its MTSO. There is no direct mobile-to-mobile communication. One MTSO typically handles about 100,000 subscribers. The pairing of base-station and user is based on signal strength (or distance, in most cases). 8-2
As the signal strength/distance changes because of the motion of the user, a new pairing may have to be established. The transition from an old pairing to a new pairing is known as hand-off. Subsribers locations are traced and constantly updated in the subscriber database. Cells are grouped into clusters. The entire set of frequencies, Ω, allocated to the system is re-used in every cluster. Cells in the same cluster use different subsets of frequencies. The size of a cluster, N, is the number of cells it contains. The frequency reuse factor is 1/ N. 8-3
Cells in different clusters that use the same frequencies are known as cochannel cells. Transmissions in co-channel cells will interfere with each other. This is known as co-channel interference (CCI) Fig: A cellular system with hexagonal cells and a cluster size of N=7. The cell size can be made small enough so that there are enough channels to go around for the active users in each cell. If the traffic density increases over time, a large cell can be split into smaller cells; see diagram next page. So in principle, we can achieve unlimited coverage with a limited set of frequencies. 8-4
Summary: four essential principles of the cellular architecture are: 1. Low power transmitters and small coverage zones or cells; 2. Frequency reuse; 3. Cell-splitting to increase capacity; 4. Hand-off and central control. 8.1.1 Cell Geometry The actual radio coverage of a cell is known as the cell s footprint. With omni-directional antennas, the coverage contour is circular. 8-5
We can thus divide the coverage area into circular cells, each of radius R. For simplicity, consider the radius R as the maximum separation that is allowed between a mobile and its base (say from the SNR point of view). In actual system design and planning, R depends on other issues such as infrastructure cost per unit area, frequency of handoff, capacity per unit area etc. Unfortunately, circular cells create void areas without coverage. R R R R R R Uniform area polygons can be used to tesselate the plane without overlap or missed areas. The hexagon is usually chosen as the cell shape because it maximizes area coverage. 8-6
It the received signal strength is the same monotonically decreasing function of distance irrespective of location, then a mobile will always be paired up with the base-station closest to it. In this case, the cell shape can indeed be represented accurately by a hexagon. In a real operating environment, terrain variations, obstacles, etc can greatly alter the received signal strength. This leads to an irregular (or even disjoint) coverage contour and a mobile will not always be paired with the closet base station. In this case, the concept of cells becomes a bit fuzzy. In other word, a hexagonal cell is just a deterministic model for a probabilistic world. It does, however, help the system engineers to know roughly where the radio resources should be deployed. 8.1.2 Cluster Size With hexagonal cells, the center p! of any cell from the reference cell can be specified by a pair of integers ( i, j ), where i and j are respectively the displacements, in number of cells, along the U and V axes shown in the following diagram. 8-7
Note that the angular spacing between the 2 axes is 0 60. The distance between the two cell centers is 2 2 D = i + j + ij 3R In the context of cell planning, the integers i and j are called shift parameters. We use them to identify the first-tier of cochannel cells; see diagram below. There are altogether 6 co-channel interferers in the first tier. The cluster size is 2 2 N = i + j + ij You can arrive at this result by dividing the area of the clusters by the area of the cells; see diagram below for example. 8-8
Examples for N = 3, 4, 7, and 9 are shown below The cluster size N is dictated by the CCI generated by first tier of interferers. 8-9
- The distance between the mobile and the interfering base stations is 2 2 D = i + j + ij 3R = 3N R; approximately ( ) - The distance between the mobile and its own base is at most R; - The path loss exponent is α ; - All base-stations transmit at the same power level of P o ; - The worst case signal-to-interference ratio (SIR) is SIR α R Po 1 D 1 = α = 6D P 6 R 6 o α ( 3N ) α where 6 is the number of first-tier co-channel interferers. Example: When 4 α =, cluster sizes of 3, 4, 7, and 9 yield SIRs of 11.3, 13.8, 18.7, and 20.8 db respectively. 8-10
The SIR shown above must be above a certain threshold SIR o. Although using a very large cluster size N will allow you to meet the requirement, an unnecessarily large N means too few channels are available per cell. 8.1.3 How many channels per cell? Cellular systems rely on truncking to accommodate a large number of users with a limited number of channels. This is achieved by exploiting the callers statistical characteristics and behaviors. How many channels we need in each cell is determined by a number of factors: 1. the callers density β, in callers/sq. km, 2. the average activity factor per user ρ, in Erlang, and 3. the probability of a blocked call, PB If each user, on average, use the phone for 2 minutes/hour, then the activity factor is ρ = 2 / 60 = 0.03333 Erlangs The number of users/cell is M = 2.6 2 R β 8-11
and the traffic intensity per cell is thus 2 λ Mρ 2.6R βρ = = Erlang. With C channels, the blocking probability is P B C C k λ λ = C! k = 0 k! 1 This equation is known as the Erlang B formula. 8-12
The truncking efficiency is η = λ C Example: Assuming a hexagonal cell with a radius of 1 Km and a user density of 1000/sq.km. Each user talks on average for 1.5 minutes/hour. How many channels are needed to support a blocking probability of no more than 0.02? Solution - The number of users in a cell is M = 2.6 1000 = 2600 ; - On average, these users used up M 1.5 = 3900 minutes of air-time per hour, or a traffic intensity of 3900 λ = = 65 Erlangs; 60 - From the graph on the previous page, the required number of channels is C=80. This gives a truncking efficiency of η = ( 65/ 80) 100% = 81% 8.2 The 1 st Generation Cellular System in North America the AMPS AMPS stands for the Advanced Mobile Phone Service. Deployed in the early 80s. 8-13
An analog-voice only system. Uses analog FM (with a peak frequency deviation of 5 KHz) and frequency division multiple access (FDMA). Frequency division duplexing (FDD) is used for simultaneous two-way conversation. - Outbound transmission (base to mobile) occupies the band 870-890 MHz. - Inbound transmission (mobile to base) occupies the band 825-845 MHz. There are 666 channels of 30 KHz bandwidth in each band. Of these 666 channels, 42 are control channels. Once a user is assigned a channel (frequency), no other users in the same cell will be able to use that channel (frequency) until the owner releases it at the end of the call. The cluster size is N = 7. This is based on a SIR threshold of SIR o = 18 db and a path loss exponent of α = 4. So each cell has approximately (666-42)/7=89 channels. The frequency assignment is tabulated below. 8-14
8.3 2-G Cellular System in North America IS-54, and beyond IS-54 is a digital cellular technology first deployed around 1994. Its existence was short and it served more like a transitional technology from AMPS to the so-called IS-136. The digital speech codec (coder and decoder) used in IS-54 is called VSELP (vector sum excited linear prediction) and it operates at 8 Kbps. Convolutional coding (of the more significant bits) is used to protect the digital speech signal from transmission error. The combined rate of speech and channel coding is 13 Kbps. 8-15
Another 3 Kbps is used for transmitting system overhead. So the transmission rate of each user is 16 Kbps. Hybrid FDMA/TDMA (time division multiple access) is used in IS54. - The allocated spectrum is identical to AMPS; - Each radio channel is still 30 KHz; - The same frequency plan as AMPS; - IS-54 phones are dual-mode, meaning they support AMPS as well. Each radio channel in IS54 is time-shared by 3 users using TDMA. Each time frame is 20 ms and is divided into 3 slots as shown below. Thus a channel in IS-54 is specified not only by frequency, but also by the time slot as well. 1 2 3 20 ms Given that each user generates a bit stream of 16 Kbps, the transmission rate in IS-54 is 48 kbps. π / 4 DQPSK modulation with a symbol rate of 1/ T = 24 KSym/s is used. This means the roll-off factor β of the SQRC pulse satisfies ( β ) ( T ) 1+ 1/ = 30 KHz, 8-16
or a β of ¼. Symbol spacing is approximately 40µ s. A typical delay spread is 5µ s. So the channel is not very frequency selective. Can get away without equalization. IS-54 evolves into IS-136. A key component in IS-136 is the Digital Control Channel (DCCH). It provides new system functionality and supports enhanced features like - supporting multiple vocoders to take advantage of improved speech coding technology; - the ability to acquire the same services in both the cellular (800 MHz) as well as the 1900 MHz PCS (personal communication service) band; - the transmission of short alpha numeric messages to the cell phones. There are approximately 90 million subscribers (as of 2001) of IS-54/IS- 136 in North America. Another 2 nd generation cellular technology is IS-95. It uses code-division multiple access (CDMA). You may be able to find out more about this standard from the graduate course offered by Prof. Dong In KIM. There are approximately 90 million subscribers (as of 2001) of CDMA in North America. By far, the most popular 2 nd generation cellular technology worldwide is GSM. There are approximately ½ a billion subscribers scattered over the world. - GSM is also a TDMA based technology; - It uses Gaussian MSK (with a BT=0.3) as the modulation format; 8-17
- The transmission rate is 270 Kbps and the channel spacing is 200 KHz; - There are 8 slots per frame and the frame duration is 4.615 ms; - Because of the short symbol duration, GSM requires complex equalization. Third Generation (3G) systems, or PCS, are all based on CDMA (some may argue though that IS-136 is a 3 rd generation technology). They will not only support voice, but also broadband multi-media services. Unfortunately, the original schedule of 3G did not meet the perceived demand of new services. That s why we have interim services like GPRS (General Packet Radio System) and EDGE (Enhanced Data rates for GSM Evolution) which are classified as 2.5 G. 8-18