Cellular Networks

1. Cellular Networks

2. Cellular Networks • It is an infrastructure network that exploits the frequency reuse concept, first developed by Bell Labs in the 70s. • Frequency reuse: use the same spectrum to support multiple users separated by a distance.

3. Cellular Networks (contd.) • An area is divided into a number of cells. • Each cell has a base station which has a low-power transmitter of 100W or less covering a limited area. The power of the transmitter is carefully controlled to limit the power leaked into neighboring cells. • Adjacent cells are assigned different frequencies to avoid interference (crosstalk). • A CDMA network is somewhat different, as discussed later.

4. Frequency Reuse • A number of frequencies (more precisely, chunks of small frequency bands, or channels) are assigned to each cell. • Cells are grouped into clusters. Each cluster uses the entire available radio spectrum, but adjacent cells, whether within a cluster or in different clusters, use different frequencies, and thus they don’t interfere with each other. • Therefore, the number of clusters is the number of times the entire spectrum can be used. This is called frequency reuse. • In a CDMA network, adjacent cells may use the same frequency band. It will be discussed later.

5. Example 1—FDMA • Assume we have a spectrum for 36 voice channels to cover a 100 square km area. • Scenario 1: A single high power transmitter is used to cover the entire area. • Scenario 2: Divide the area into seven cells each covering a 14.3 square km area and having 12 channels. • Scenario 1 supports 36 users and Scenario 2, 84 channels, a 2.3 time capacity increase.

6. Example 1—FDMA (contd.) • Cells 1 and 7 are far apart and therefore can use the same frequency. So do pair 2 and 5 and pair 6 and 4. • Thus cells 1, 2 and 6 form a cluster, and cells 7, 5 and 4 form another. Cell 3 belongs to a third cluster. • The smaller the cells (thus more cells in the area to be covered) the larger the capacity of the system (the more users it can support, as explained later), and the smaller the power required for phones, very important feature. • But smaller cells will increase the installation and maintenance costs, the complexity of the system due to too many handoffs, etc.

7. Example 1—FDMA (contd.) • If we divide the area into 15 cells, and each group (cluster) of 3 cells can use the entire spectrum, then the spectrum will be used for 5 times. • Each time the entire spectrum is used, 36 users can be supported. All together 36x5=180 users will be supported. It’s a five time increase of the capacity.

8. Example 2—FDMA • We have a total bandwidth of 25 MHz and each user requires 30 KHz for voice communication. (This is how spectrum was used for 1G cellular systems.) • Scenario 1: one high power antenna to cover the entire town. We can support 25MHz/30 KHz = 833 simultaneous users. • Scenario 2: 20 low power antennas are used. That is, to divide the area into 20 cells. We divide the entire frequency band into 4 sub-bands and assign one to each cell, which has a spectrum of 25 MHz/4 = 6.25 MHz.

9. Example 2—FDMA (contd.) • The number of users supported by each cell is 6.25 MHz/30 KHz = 208. • In this example, 4 cells form a cluster. Since there are total of 20 cells, the town is covered by 5 clusters (20/4=5). • Each cluster will use the entire frequency band, so the number of users per cluster is 833, as calculated earlier, and the total number of simultaneous users for 5 clusters is 833x5 = 4,165. • The capacity is now 5 times the capacity with a single antenna.

10. Capacity calculation—FDMA • n: capacity (number of total users) • m: number of cells to cover the area • N: frequency reuse factor (# cells/cluster) • B: bandwidth per user • W: total available bandwidth (spectrum)

11. Capacity calculation—FDMA (contd.) In the previous example, • m=20, • W=25 MHz, • N=4, and • B=30 KHz.

12. Capacity calculation—TDMA • n: capacity (number of total users) • m: number of cells to cover the area • N: frequency reuse factor (# cells/cluster) • B: bandwidth per user • W: total available bandwidth (spectrum) • Nu: number of time slots per carrier

13. Capacity calculation—TDMA (contd.) Assuming again, • m=20, • W=25 MHz, • N=4, • B=200 KHz, • Nu=4.

14. Cell sizes • The sizes of cells can be different. • Cells designed to cover suburban areas have antennas on tall towers and cover a large area. • In urban areas antennas are low and transmitting powers are also low. Therefore the coverage areas are small for two reasons. • Since the population density is high and the number of users per cell is limited, the cell size has to be smaller. • Buildings may block radio wave transmission, therefore more cells may needed to cover an area in a city.

15. Capacity of CDMA • In CDMA users are separated by different codes but not by frequencies or time slots as in TDMA and FDMA. In CDMA many users can share the same frequency band and communicate at the same time. • A channel in TDMA or FDMA is a frequency and a time slot. There is only a limited number of channels, which restrict the number of simultaneous users. In CDMA a channel is a code. There is an almost unlimited number of codes, and thus channels, but it doesn’t mean an unlimited capacity. • Each user is a source of noise to the receivers of other users (recall the discussion we had on DSSS) or to the receiver in the base station. This will limit the number of users. • The number of user per cell (the capacity) is determined by the signal to noise ratio. If there are too many users, the noise will be high, the S/N (signal to noise) ratio will be low and reception quality will be poor. • This is different from TDMA/FDMA, where the capacity is determined by the number of available channels.

16. Capacity of CDMA (contd.) n: number of users W: total bandwidth R: data rate Sr: signal to noise ratio

17. Capacity per cell (CDMA) Assume: W=1.25MHz=1,250,000 Hz R=9600 bps Sr should be larger than 3dB => 2 times

18. Capacity comparison: TDMA Number of users per cell for IS-136 (a TDMA standard used in the US) Assume: W=1.25 MHz, B=30 kHz, N=4 Number of users per carrier (number of time slots per frequency) Nu=3

19. Capacity comparison: GSM W=1.25 MHz, B=200 kHz, N=3 Number of users per carrier Nu=8

20. Capacity expansion techniques • Obtain additional spectrum. Can be very expensive. • Cell splitting, cell sectoring using directional antennas • Dynamically assigning channels to cells, instead of fixed assignment of channels. If the demand for a cell is high, more channels are assigned.

21. Cell sectoring • Several antennas are used in one cell. • Each antenna, which is directional, only covers a section of the cell. • The interference will be reduced and thus a lower frequency reuse factor can be used. • Using three- and six-sector cells the frequency reuse factor can be reduced from 7 to 4 or even 3, which means a capacity increase of 1.67 and 2.3, respectively.

22. How Does Cellular System Work • When a phone is turned on, it scans and selects the strongest (control channel) signal sent by adjacent BSs. • Then a handshaking process takes place between the mobile unit and the MTSO (Mobile Telecommunications Switching Office) to identify the user and register its location. This procedure is repeated periodically as long as the mobile unit is on to monitor the location of the MS.

23. How is a call established • Mobile unit initializes calls. The MS sends the number it wants to reach to the BS, which sends the request to the MTSO. • The MTSO sends a paging message to certain BSs depending on the called number. Each BS sends the paging signal on its own setup channel. • The called MS responds to the BS, which sends the response to the MTSO. • The MTSO sets up a circuit between the calling and called BSs. • The MTSO selects an available channel within each BS’s cell and notifies each BS, which in turn notifies its mobile units. The two MS tune to their respective assigned channels.

24. Location Management Tracking the MS in order to deliver data to it. • Locate the MS. • Establish a dedicated channel before the data change starts. • If the MS moves from one cell to another during the data change, handoff (disconnect the MS from the current BS and connect it to a new BS) may be required.

25. Handoff • Handoff: if a MS moves out of the range of one cell and into the range of another, the MTSO assign another channel to the MS. • Soft handoff (for CDMA only): An MS moving towards the edge of a cell will maintain communication with two or more BSs for a short while before decide which BS to select as its point of attachment. • Soft handoff makes the transition from one cell to the next smooth.

26. Location Management (contd.) • Periodic location updates by the MS • Paging the MS in a group of cells the MS may be located in. (The location updates are periodic but not continuous. Therefore we may not know the exact location.) • Location information dissemination: store and distribute the location information.

27. 1-G system: Advanced Mobile Phone Service (AMPS)

28. 2G systems

29. 3G systems • International standard IMT-2000 (International Mobile Telecommunications beyond 2000) • A spectrum of 230 MHz is assigned globally to IMT-2000 • Combine voice and data services • Increase the quality of the voice, capacity of the network, and data rate (384 kbps everywhere and 2 Mbps indoor). • W-CDMA (GSM) and CDMA2000 (IS-95) are competing proposals.