A Glimps at Cellular Mobile Radio Communications Dr. Erhan A. İnce 28.03.2012
CELLULAR Cellular refers to communications systems that divide a geographic region into sections, called cells. The purpose of this division is to make the most use out of a limited number of transmission frequencies.
Each connection, or conversation, requires its own dedicated frequency, and the total number of available frequencies is about 1,000 [Advance Mobile Phone Service (AMPS)]. To support more than 1,000 simultaneous conversations, cellular systems allocate a set number of frequencies for each cell. Two cells can use the same frequency for different conversations so long as the cells are not adjacent to each other.
MOBILE Cellular phones allow a person to make or receive a call from almost anywhere. Likewise, a person is allowed to continue the phone conversation while on the move. Hence MOBILE. Cellular communications is supported by an infrastructure called a cellular network, which integrates cellular phones into the public switched telephone network.
Early Mobile Radio Large Coverage Area using: Single High Power Transmitter Antenna Mounted on a tall Tower Good Coverage Difficult to reuse the same frequencies throughout the system due to significant interference (No spectrum sharing a lot of bandwidth is dedicated to a single call)
The Cellular Concept developed by Bell Labs 1960 s-70 s areas divided into cells Each cell is served by a base station with lower power transmitter Each cell gets portion of total number of channels Neighboring cells assigned different groups of channels, to minimize interference The available channels can be reused as many times as necessary as long as the interference between co-channel stations is kept below acceptable levels Cells using the same channels should be spaced enough to reduce co-channel interference
First-generation: Analog cellular systems (450-900 MHz) Frequency shift keying for signaling FDMA for spectrum sharing NMT (Europe), AMPS (US) NMT: Nordic Mobile Telephone, AMPS: Advanced Mobile Phone System Second-generation: Digital cellular systems (900, 1800 MHz) TDMA/CDMA for spectrum sharing Circuit switching GSM (Europe), IS-136 (US), PDC (Japan) GSM: Global System for Mobile Communications PDC: Personal Digital Cellular 2.5G: Packet switching extensions Digital: GSM to GPRS (General Packet Radio Service) Analog: AMPS to CDPD (Cellular Digital Packet Data ) 3G: High speed, data and Internet services IMT-2000 (International Mobile Telecommunications for the year 2000 )
In a cellular system the hand-sets carried by the users are called Mobile Stations (MS). The Mobile Stations communicate to the Base Stations (BS) through a pair of frequency channels, one for up-link and another for downlink. All the base stations of a Cellular systems are controlled by a central switching station called Mobile Switching Center (MSC) or Mobile Telephone Switching Office (MTSO). The MSC is responsible for all kinds of network management functions such as channel allocations, Handoffs, billing, power control etc. The MSC is also connected to the Public Subscriber Telephone Network (PSTN) or Public Land Mobile Network (PLMN) so as to allow the MS to talk to a Land Line telephone or vice versa.
CAPACITY IN a CELLULAR SYSTEM Capacity in a cell is basicly the number of users the operator can give service to. Capacity is a concern in any wireless communications system. High demand for cellular service, especially in large urban markets, has created a need to serve a greater number of users in a limited amount of frequency space. Cellular system operators are looking for new ways to fit more users into their increasingly crowded network
FREQUENCY REUSE
In hexagonal geometry, there are six neighbors of each cell and the line joining the centers of any cell and each of its neighbors are separated by 60 degrees. This restricts the number of usable cluster sizes and their layouts. In order to tessellate-to connect cells without gap-the number of cells per cluster, N, can only have values, which satisfy the following equation: where i and j are non-negative integers. N=1,3,4,7,9,12,13,16,19,21
CLUSTER SIZE
Cluster Size vs Capacity We know that Cell capacity and Reuse are interlinked N C N: Cluster size C: Capacity Improving Capacity & Coverage in Cellular Systems Power Control to reduce interference Cell Splitting Sectoring Microcell Zone Concept Repeaters for Range Extension
For hexagonal cells Reuse dıstance D = 3N R Reuse factor is q = D/R = If cellular system has S duplex channels Each cell is allocated k channels And there are N cells S = k.n If a cluster of N cells is replicated M times in the system Total # of duplex channels = C = M. k. N = M. S If cluster size N while the cell size remains fixed, more clusters are needed to cover the area of interest. M C Much higher CCI since reuse distance goes down.
To find the nearest co-channel neighbors of a particular cell, one must do the following: 1. Move i cells along any chain of hexagons. 2. Turn 60 degrees counter-clockwise and move j cells. Figure below illustrates this process with i = 3, j = 2, and N = 19.
Power Control to Reduce Interference In practical systems, power level of every subscriber is under constant control by the serving BS Power control not only reduces interference levels but also prolongs battery life In CDMA spread spectrum systems, power control is a key feature to ensure maximal utilization of system capacity Reduced Interference leads to high capacity
Improving Capacity As demand for service increases, system designers have to provide more channel per unit coverage area Common Techniques are: Cell Splitting, Sectoring and Microcell Zoning Cell Splitting increases the number of BS deployed and allows an orderly growth of the cellular system Sectoring uses directional antennas and frequency reuse to further control interference Micro cell Zoning distributes the coverage of cell and extends the cell boundary to hard-to-reach areas i.e. basement of buildings, tunnels, valleys etc
Cell Splitting Cell splitting is the process of subdividing a congested cell into smaller cells with their own BS a corresponding reduction in antenna height a corresponding reduction in transmit power Splitting the cell reduces the cell size and thus more number of cells have to be used For the new cells to be smaller in size the transmit power of these cells must be reduced. More number of cells more number of clusters more channels high capacity
Cell Splitting Cells are split to add channels with no new spectrum usage
Picocells : 50mW to 1W
Cell Splitting-Power Issues Suppose the cell radius (R) of cells is reduced by half then what is the required transmit power for these new cells? Pr[at old cell boundary]=p t1 R -n Pr[at new cell boundary]= P t2 (R/2 )-n where P t1 and P t2 are the transmit powers of the larger and smaller cell base stations respectively, and n is the path loss exponent. So, P t2 = P t1 /2 n
n: path loss exponent For n =3 P T2 = P T1 /2 3 = P T1 /8 9dB lower transmit power Interference will be mitigated (lowered)
Cell Splitting Suppose a congested service area is originally covered by 5 cells where each cell has 80 channels. If cell splitting is used how much more capacity do we get? Capacity_orig = 5 x 80 = 400 After cell splitting R new = R/2 We now have 24 new cells Capacity_new = 24 x 80 = 19200 For n=4,transmit Power of New BS is 12 db lower than original No change in frequency use! Cost is that more base stations and more handoffs.
Illustration of handoff scenario at cell boundary
In practice not all the cells are split at the same time. This means that different size cells will exist simultaneously. In such situations, special care needs to be taken to keep the distance between co-channel cells at the required minimum, and hence channel assignments become more complicated.
Cell Splitting When there are two cell sizes in the same region, one cannot use original transmit power for all new cells or the new transmit power for all original cells Larger transmit power for all->some channels used by smaller cells would not be sufficiently separated from co-channel cells Smaller transmit power for all->some parts of larger cells left un-served Channels in the old cell must be broken down into two channel groups, one for smaller cell and other for larger cell The larger cell is usually dedicated to high speed traffic so that handoffs occur less frequently
Practical Handoff Considerations
Two channel group sizes depend on the stage of splitting process At the beginning of splitting process, there will be fewer channels in small power groups With increasing demand,smaller groups will require more groups Splitting continues until all channels in area are used in lower power group Entire system by that time is rescaled to have smaller radius per cell Antenna down tilting is used to focus energy from BS toward ground, to limit radio coverage of newly formed microcells
Sectoring As opposed to cell splitting, where D/R is kept constant while decreasing R, sectoring keeps R untouched and reduces the D/R Capacity improvement is achieved by reducing the number of cells per cluster, thus increasing frequency reuse In this approach first SIR is improved using directional antennas
Sectoring The CCI may be decreased by replacing the single omni-directional antenna by several directional antennas, each radiating within a specified sector
Sectoring
Sectoring A directional antenna transmits to and receives from only a fraction of total of the co-channel cells. Thus CCI is reduced
Problems with Sectoring Increases the number of antennas at each BS Decrease in trunking efficiency due to sectoring(dividing the bigger pool of channels into smaller groups) Increase number of handoffs(sector-to sector) Good news:many modern BS support sectoring and related handoff without help of MSC
TRUNKING EFFICIENCY It is a measure of the number of users which can be offered a particular Grade Of Service (GOS) using fixed number of channels 10 channels trunked system has higher trunking efficiency than two 5 channel trunked systems because it can support 60 % more traffic
Microcell Zone Concept The Problems of sectoring can be addressed by Microcell Zone Concept A cell is conceptually divided into micro cells or zones Each microcell(zone) is connected to the same base station(fiber/microwave link) Doing something from both cell splitting and sectoring by extracting good points of both Each zone uses a directional antenna Each zone radiates power into the cell. MS is served by strongest zone As mobile travels from one zone to another, it retains the same channel, i.e no hand off The BS simply switches the channel to the next zone site
The Micro Zone Cell Concept
Microcell Zone Concept Advantages: Reduced Interference (Zone radius is small and directional antennas are used). No loss in trunking efficiency (all channels are used by all cells). No extra handoffs. Increase in capacity (since smaller cluster size can be used).
Illustration of extent of Capacity Increase by an example Suppose the desired S/I = 18 db with path loss exponent of n = 4 For a system of N=7,a D/R of 4.6 was shown to achieve this How much capacity increase can occur if we use Microcell Zoning of 3 zones/cell??? In zone microcell system, transmission at any instant is confined to one zone Therefore, Dz/R z = 4.6 Each hexagon represents a zone and 3 hexagons represent a cell Zone radius=one hexagon radius Capacity of system related to distance between co-channel cells and not zones. (Shown as D)
Value of co channel reuse is 3 D/R = 3 corresponds to N=3 Reduction in cluster size from N=7 to N=3 Increase in capacity is 7/3=2.33 times If previously 100 users can be supported now 233 will be supported.
Repeaters for Range Extension Useful for hard to reach areas Buildings Tunnels Valleys Radio transmitters called Repeaters can be used to provide coverage in these area Repeaters are bi-directional Rx signals from BS Amplify the signals Reradiate the signals Received noise and interference is also reradiated.
REPEATERS
OMNIDIRECTIONAL Antennas Sectorized Omnidirectional
DIRECTIONAL Antennas
2.4 GHz Sector Antenna
Turkish mobile network operator Avea is using the advantages of both solar and wind energy to power a base station, which they claim is the first of its kind in the country
Mobile Phone Base Station Turkcell s Mobile BS
3 x Andrews GSM/UTMS 1200mm dual-band panel antennas 1 x 30cm TMW backhaul link with covered dish
Microwave Backhaul Mobile or wireless backhaul is the portion of a wireless network that connects information traveling from a wireless tower to a mobile switching center. Information that travels across these sub networks includes phone calls, texts, gaming and Web browsing.
Multi System Operator (MSO) is an operator of multiple systems. A cable company that serves multiple communities is an MSO