Page Cellular Coverage Concepts Student Notes Global Wireless Education Consortium Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039. GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
After completing this module and all of its activities, you will be able to: Explain how cell splitting and channel grouping are achieved in a cellular network. Explain how to calculate a C/I ratio. Calculate a D/R ratio. Determine the C/I interference for a given D/R ratio. Draw a diagram of a cellular network architecture.
This module is based on the initial analog cellular offering, also called FDMA (Frequency Division Multiple Access). Today, there are other access methods, all digital, in use: TDMA (Time Division Multiple Access) – Allows up to three users to use a channel that only one analog user could otherwise access. Requires the same radio frequency (RF) engineering techniques as analog. CDMA (Code Division Multiple Access) – Theoretically allows unlimited number of users per channel, however, real world is somewhere between 12 and 20 users depending on interference. This is the easiest of the access methods to do an RF design around because it utilizes only one channel and is used in every cell. It is also the most costly in terms of hardware due to the high number of antenna towers required. GSM (Global System for Mobile Communications) – Employs TDMA technology, but is more sophisticated; allows up to six users per channel. Requires same RF engineering techniques as TDMA. These methods offer greater capacity and more features than analog. Due to insufficient time to cover them in this lesson, they will not be discussed. More information can be obtained from references listed at the end of this module or in the GWEC modules AI- TDMA, AI-CDMA, and AI-GSM .
In order to understand the why of cell splitting, it is important to understand the roots of cellular communication. What we recognize as “radio” has only been around 100 years! The first communication by radio with any serious purpose or consequences was in the merchant marine and the military as World War I began. Soon after the war, the 1920s saw the birth of amplitude modulated (AM) radio broadcasting. AM broadcasting grew to be a big business in the 1930s. In 1922, the first “mobile” use of radio was a simplex (one-way only) broadcast to police cars in Detroit, Michigan. This was short lived for three reasons: Base station did not know if the call was received. Radios were large and bulky, and took up the entire trunk of the car. Early radios were all tubes, resistors, capacitors and wires, making them very susceptible to breakage from being jarred around. In the 1940s, war again intruded, forcing rapid development of radio techniques. Equipment was built smaller and in more reliable packages. For the first time, radio links were built with enough power output and gain to hear their delayed signal “echoes” from passive objects. Radar was born and barely saw its first practical use, giving about 50-minutes’ warning of incoming enemy aircraft during the Battle of Britain. Television was a lab curiosity during the late 1930s, but the war prevented its successful commercial introduction until the early 1950s. Color television was introduced commercially in the early 1960s. First attempts at public mobile telephony also began in the late 1940s and early 1950s. “Mobile Telephone Service” required a trunk full of vacuum-tube equipment and an operator manually dialed and placed calls for the few dozen users on a single radio channel. Things have come a long way since then!
True duplex (two-way communication) mobile radio began in the public safety arena and then expanded to commercial use. It still faced problems. Coverage was restricted to a very small geographical area and coverage was as effective as the strength of the broadcast signal. Once you left your limited coverage area, you did not have availability unless you brought signals into other coverage areas. There was only a very limited number of frequencies available. Two frequencies were required for each channel, one for transmit and one for receive. Due to the interference that could result, the carrier frequencies had to be separated by great distances before they could be reused.
A wise person once said that necessity was the mother of invention. The necessity here was the need to expand the coverage, both by area covered and number of subscribers serviced. Thus, a brilliant idea was conceived by Bell Laboratories in the 1960s. Reduce the size of each coverage area. Create more coverage areas. Reuse the frequencies more often. Create a means to hand off the call from one coverage area to another without dropping the call. Not everyone was “thrilled” by this idea. First of all, AT&T, who owned Bell Laboratories, was dismayed when the Federal Communications Commission (FCC), decided wisely to go along with the idea and provide frequency spectrum, but with restrictions: AT&T would be only one of several provider companies. No provider company would have a monopoly in a given area. (This topic will be further discussed later in this module). Another group that was not pleased were the traffic engineers. Where were all these phones going to be at any given moment? How easy will it be to accommodate changes in capacity needs?
Unlike landline telephones, cellular companies cannot pinpoint where every telephone will be located. Once installed, a landline phone does not move across town for a couple of hours and then return back. What this meant for cellular engineers is that they would have to develop a frequency reuse plan based on a number of estimates. An impactful variable is the mobility of the subscribers and roamers (subscribers from another network who have “roamed” into your network). A change in the driving patterns of a number of people could change the requirements in a specific area very rapidly. Delivery personnel, salespersons, retired people, field technicians, various local government officials, and vacationers are all examples of people on the move that will cellular phones daily and without specific driving patterns. As the requirement for more service increased, it became apparent that the current frequency reuse plan was not sufficient to accommodate it. Redesigning the entire network to accommodate this change would be neither practical nor cost efficient. Therefore new cellular design techniques were devised. Among them was a concept called cell splitting .
The diagram above represents a basic N=7 cellular layout . (N=7 will be explained later in this module.) It represents a way to group available channels for the best balance between coverage and interference. Each color and number represent a group of channels where each channel is assigned a pair of frequencies. In order to keep the assigned cellular channels from interfering with each other, an attempt must be made to to minimize co-channel (same channel used twice) and adjacent-channel (one channel up or down) assignments . In the diagram above, note there are two adjacent channel assignments and no co-channel assignments. Because an N=7 plan was chosen, a cell cluster of seven cells was created. This cluster can then be repeated as many times as needed to fill a coverage area. In the diagram above, the basic cluster is comprised of the seven cells in the middle (cells 1 through 6, centered around cell 7). In this particular scenario, only two more cells were needed. Due to the location for the extra cells, groups 6 and 2 are duplicated. This allows the channels in each of these groups be reused at a distance sufficient enough to reduce or eliminate any unwanted interference. Having them closer could cause any of the following situations: Cross talk (multiple conversations heard simultaneously) Static Dropped call
In the previous slide, a basic seven-cell cluster was built and two of the cells were reused when more coverage was needed. In reality, you probably will never find a coverage area so small that a single cluster will be sufficient. In the diagram above, the basic cluster is duplicated in its entirety, as many times as necessary to ensure coverage is sufficient. When needed, the cluster arrangement can be modified to minimize both the co-channel and adjacent channel interference. (For more information on this topic, see Wireless Communications by Theodore S. Rappaport, chapter 2.) Sometimes, even the best planned clusters will be insufficient because of changes that require an increase in capacity in small geographical areas inside of existing cells. There may not be sufficient channels left in the affected cell to accommodate this need. A practical way to increase the capacity in this geographical area is to “split” the cells with the highest amount of call traffic and reuse channels from another group.
Cell splitting is the process of subdividing an already existing cell into smaller cells, with each one having its own antenna system and radios. In this way, call capacity of the network can be increased without violating the original frequency reuse plan. The new subcells are located between the existing cells in such a way as to cover the area of identified call traffic increases. In the plan above, notice that the call traffic at the junction where cells groups 2, 5, and 7 intersect has increased significantly beyond the original calculations. This could be due to a number of reasons such as: New roads or highways New shopping mall New office buildings Subcells can be inserted at reduced power to accommodate the increase without violating the original plan. It is assumed that there are insufficient spare channels in cell groups 2, 5, and 7, to meet the need, so reuse channels will be used from cell groups 3, 6, and 1. Large cells 1, 6, and 2 face the same problem and are supplemented with channels from cells 3, 5, and 4 respectively. Notice that channels are being used from cells the as far away as possible. They should be positioned in a manner that will provide the lowest amount of interference.
In order to understand channel grouping, it is important to understand how the channels were allocated. This scenario focuses on the 800 MHz spectrum. Because the first cellular phones operated in the analog mode, rules governing frequency allocation were designed around analog. AMPS (Advanced Mobile Phone System) is the set of rules that was developed to cover North American Cellular. (For additional information on AMPS, refer to Essentials of Wireless Communications by W.C.Y. Lee, chapter 1; Wireless Communications by Theodore S. Rappaport, chapter 10; or the GWEC module AI-AMPS. ) These AMPS rules stated the following: Each channel must have a pair of frequencies, one for the uplink and one for the downlink. The uplink frequency (also called the reverse channel or reverse frequency ) is the frequency that is transmitted by the mobile and received by the cell. The downlink frequency (also called the forward channel or forward frequency ) is the frequency that is transmitted by the cell and received by the mobile. Using the two frequencies allows true duplex operation, that is, it accurately emulates a landline phone conversation where both parties can talk at the same time. Each channel must have a bandwidth of 30 kHz. The FCC ruled that no one company could have a monopoly (like landline companies) in a given area, so they divided the allocated frequencies into two bands: A Band – Originally reserved for companies that were solely in the cellular business. B Band – Originally reserved for existing landline companies to let them into the cellular business.
Originally, the allotted frequency spectrum allowed for a total of 666 channels of 30 kHz bandwidth. 333 channels for each band. 21 control channels for each band. When not engaged by the user in a conversation, the mobile uses control channels to communicate with the cellular system and let it know its availability and location. The mobile does not communicate with the control channel during an active call. 312 voice channels for each band. Later, 166 more channels were allocated to the cellular community. 83 voice channels for each band.
The diagram above shows how the channel numbers are distributed across the spectrum. The original voice channels are commonly referred to as non-extended spectrum and the additional channels are referred to as extended spectrum . Notice that the A band has extended spectrum both above the B band and below the original band A. The reason for this is that most of the new frequencies came from above the original allocated frequencies. Therefore, in order for the A band to enjoy the same benefit as the B band, it has to accept some frequencies at the higher end.
Each designated channel in the 800 MHz spectrum has two frequencies assigned to it that are always separated by 45 MHz. In other frequency spectrums, this will be different. Therefore, channel 1 will have two frequencies assigned: 825 MHz for the uplink or reverse link. 849 MHz for the downlink or forward link. Channel 2 will be 30 kHz away: 825.030 MHz for the uplink or reverse link. 849.030 MHz for the downlink or forward link. Notice that the downlink or forward link is always the higher of the two frequencies. This pattern continues all the way through, is important for the channel grouping, and is controlled by the AMPS rules.
Frequency reuse grouping is the process of taking all of the available channels and dividing them by the number of different groups that are needed to satisfy your needs. Example: N=1 is where all of the available channels are in a single group. N=4 is where all of the available channels are divided into four separate groups. And so forth. It sounds simple enough. However, there are some restrictions that keep us from just simply dividing all the channels. Remember the 30 kHz bandwidth. Because the bandwidth is determined by a sloping filter (i. e., the edges are not perpendicular), there is some overlap between the channels. This means that there cannot be adjacent channels in the same cell (channels 1 and 2 are adjacent, channels 456, 457, and 458 are adjacent, etc.). So, at the very least, we must make sure that the adjacent channels are in different groups. There are other restrictions as well. Cell sites employ a device called a combiner-tuner network . This device allows the use of a single antenna to transmit multiple frequencies. Without this device, a separate antenna would be needed for each channel. The inherent nature of the device requires a 21-channel separation to avoid signal degradation. Over the years, many RF engineers worked to find optimal groupings. The results are mixed, but most RF engineers agree that for most omnidirectional applications, the N=7 plan is best. The N=4 and N=9 plans are becoming more common as the industry makes more use of cell sectorization.
This chart shows an N=7 plan. Notice there are 21 total groups in the plan, one for each of the FCC mandated control channels. There are variations of this chart, but they all contain the same channels grouped together. In this particular chart, you will see additional group designations: A1 through G1 A2 through G2 A3 through G3 Groups A1, A2, and A3 are considered compatible groups, as are B1, B2, and B3. This holds true all the way through G1, G2 and G3. This is called an N=7 plan because there are seven basic groups with four subgroupings for a total of 21 groups. When laying out a plan, the RF engineer tries to group all A groups (A1, A2 and A3) together and all B groups together, etc. Look at group 18 (D3). You will find channel 1 here on this particular chart. Notice that the next channel in the same column is channel 22, twenty-one channels away. This pattern will hold true all the way through this chart. An N=4 chart divides the channels into four basic groups with each basic group having six subsets. This has a total of 24 groups and allows us to maintain at least a 21-channel separation in each subgroup. The N=9 chart uses a total of 27 groups, that is, 9 basic groups with 3 subsets each.
This chart is set up identical to the previous one, the only difference being that it is designed to accommodate the B band channels.
The typical cell in North America uses from 14 to 22 radio channels. This is well within the compatible group allocations. We will use the clusters from a previous section to deploy the radio channels. Assume that the network is a band A, or A band system. We will use an N=7 reuse plan. The numbers in parenthesis are the number of radio channels that are needed in that particular cell. Note that the same cell numbers were kept, however, a letter and a number was added after them to identify the subgroup being used. These come from the Channel Set chart. Where a cell number (cells 6 and 2) is reused, we will try to use a different compatible subgroup of radio channels. This greatly reduces the likelihood of causing co-channel interference. We will do the same thing when there are chances of adjacent channel interference. Notice where we used 6F1 and 6F2, and 2B1 and 2B2. When clusters are duplicated, as discussed earlier in this module, we are very careful to watch for co-channel and adjacent-channel interference. When it cannot be avoided due to capacity, another option is sectorizing.
An alternative to omnidirectional cells are sectorized cells . In a sectorized cell, the transmitted signal is focused in a single direction rather than sent out in all directions. This is accomplished by using directional antennas. There are many advantages to this that will be discussed in another topic. Notice that this configuration calls for three sectors in each cell. Look back at the channel groups and notice how convenient the N=7 plan is for this kind of layout. Now, instead of talking about groups 1 through 21, we will take a look at groups A1 through G3. We will group all the A groups together, the D groups, etc. An advantage of using sectorized cells is that the same channels can be reused more often that can be in an omni configuration. Look at the two cells with F1, F2, and F3 groups in them. By using directional antennas, sector F3 can be radiating to the left and slightly up with very little to no radiation to the right. (Refer to the the diagram above.) That means the other “F” cell can be brought closer without suffering interference. The closer cells are brought together, the more often they can be reused. The more they can be reused, the greater the number of users that can be handled at one time. Again, the biggest concern is interference. This is measured by something called the carrier to interference ratio, or C/I ratio .
In AMPS cellular, there will be good reception if the desired signal is at least 18 dB stronger than any other signal in the background on our channel. This is referred to as an 18 dB C/I ratio (carrier-to-interference ratio). Suppose two cells use the same channels (co-channels). There will be a region close to each cell where its signal is at least 18 dB stronger than the signal of the other. In the middle between the two cells, there is a region where neither signal is at least 18 dB stronger than the other, and serious interference exists. The place where that 18 dB separation occurs is the outside limit of the cell before it will have interference from the next cell that is using the same channel group. In the 800 MHz mobile environment, the transmitted signal strength decays at a nominal rate of 40 dB/decade. What that means is that if you measure the signal strength at a certain distance from the antenna, then measure it again at a distance ten times (decade) further, you should see approximately a 40 dB loss. Because we know these facts about C/I, we can graph out the signal to find where that 18 dB separation will occur and thus ensure co-channel interference is minimized.
Co-channel interference can occur independently on the uplink and the downlink, from completely different sources. This and the next slide explore these situations. In this scenario, the interfering mobile is on the same channel as the interfered-with mobile. This problem occurs usually when both mobiles are at a point in their respective cells that puts them as close as they can get. It can also occur when one mobile is on a higher point than was anticipated in the original design. This could be due to a new overpass or a high-rise parking garage.
On the downlink, interference occurs when insufficient distance is planned between cells using the same channel groups.
The D/R ratio formula is used to determine the frequency reuse plan that will fit your network best. It is the ratio of the cell radius to the tower distance. In the formula, D represents the distance between two sites using the same channel group. R is the radius of the coverage area in cell. This is the point where the signal becomes so weak it cannot be received by the mobile.
Cellular engineers recognized early that frequencies are typically distributed in a repeating cluster pattern in cellular systems. Often the clusters follow an N = 7 plan. It is useful to understand why this is so in order to draw conclusions about the level of performance that can be expected from such an arrangement. A relationship is needed between the reuse factor, N, and resulting values of co-channel distance D and coverage distance R, such that the minimum empirically-derived C/I of 18 is achieved. There is pressure to keep N as small as possible since the number of channels distributed in the cells will be 395 divided by N. The smaller the N, the greater the number of channels per cell, and the greater the amount of traffic that can be served by an individual cell. Thus, what is needed is the minimum integer N that yields a C/I of 18. The relationship of N to D/R is repeated in numerous cellular texts. (For more information, see Wireless Communications. Principles and Practices. by Theodore S. Rappaport.) Now that there is a relationship between D/R and N, it continues to relate D/R to C/I. This should be fairly easy, since we know that C drops off at 40 dB/decade with respect to increasing R, and likewise, I drops at 40 dB/decade with respect to increasing D. We know that we need a C/I of 18, which historically is the “bingo” figure for interference-free communication for FM radios on the cellular band. It is interesting to note that 18 dB is a power ratio of about 63. We need to have our desired signal get no closer than 63 times as strong as the sum of the interferers.
As indicated previously, the number of cells in the reuse cluster (commonly referred to simply as “N”) is very important. The table above gives D/R for various values of N. Once the D/R ratio is determined, locate it in the table and identify the reuse plan to which it is closest. Example: Cell radius equals 3 miles Distance to the co-channel tower is 14 miles D/R = 114/3 = 4.667 The N it is closest to is N=7 Remember, as distance increases, so does N. When N increases, the number of radio channels available in each group decreases. There needs to be a balance between sufficient distance and available channels in each group. The next few slides examine various reuse patterns.
Suppose we have selected N to be 3. How would we assign channels to the cells? Imagine all the channels are in order, similar to a deck of cards. Assigning channels is like dealing cards to all the cells in the cluster. We deal until the deck is completely gone. The “hand” of channels that one cell receives is called a channel set . Usually, channel sets are named in ascending order of frequency, i.e., the lowest in frequency being 1, the next 2, etc. Some engineers like to use letter designations such as A1, A2, A3, B1, B2, B3, etc. Charts showing common channel set designations for North American cellular are shown at the end of this module. Notice that regardless of the value of N, the highest channel set is frequency-adjacent to the lowest channel set. In the example above, channel set 1 is adjacent to channel set 3. Why? Because, for example, channel 3 (in channel set 3) is adjacent to channel 4 (in channel set 1).
Let’s go on a “tour” of the various AMPS frequency plans might be uses for omni cells. We will start with N=1; i.e., we are using the same channels in every cell. This would be really great for capacity. We could have 395 channels in each cell! Unfortunately, co-channel interference would destroy over 60% of the coverage. Each cell would have a little island of interference-free service surrounded by a sea of destructive interference. In addition, both of the adjacent channels would be active in the same cell and every one of the neighboring cells would be an adjacent-channel cell. Clearly N=1 is not a survivable experience in AMPS. N=2 still has two co-channel neighboring cells producing destructive interference. Four of the six neighboring cells are also adjacent-channel.
N=3 is better, but all of the neighbor cells are adjacent in frequency. Co-channel neighbors are farther away, but still too close to allow calls to survive. N=4 is somewhat better, but there are still four adjacent-channel neighbors and the co-channel cells are still too close to be effective.
N=5, N=6 -- better, but they still do not meet the objectives.
N=7 is the first arrangement that works in typical terrain, yielding an acceptable co-channel C/I. Still, two of the six neighbor cells are on adjacent channels, and no matter how channels are assigned in the cluster, this will remain inescapably true. Overall, this is a workable situation in most markets although adjacent channel relationships must be carefully managed. There is not much “slack” available in the co-channel situation either, so we must watch out for sneaky propagation exceptions that might deliver interference to specific areas. N=8 is better, but not significantly better than N=7.
N=9 is a significant improvement over N=7, both in co-channel characteristics and adjacent channel characteristics. If time permits, experiment with other channel assignment schemes in basic cluster of 9 cells to see if you can reduce the number of adjacent-channel relationships. N=10 is not sufficiently better than N=9 to warrant use.
N=11 and N=12 are little-used frequency plans. If traffic needs can be met with this few of channels, you may be better off to do ad hoc channel assignment and gain even more separation between co-channel sites.
The goal of cellular network architecture is to build a cluster of cells that can be duplicated to fill the designated coverage area. Since the size of each cell has already been determined, cells need to be laid out to maximize coverage while minimizing the interference. Using the appropriate Frequency Plan chart, determine which frequency groups you want to place where. It is unlikely that you will need every channel in every cell. Therefore, you can choose which groups to use in order to get the best coverage with the least interference. Assume the following requirements have been identified: Cell A needs 7 radio channels Cell B needs 12 radio channels Cell C needs 23 radio channels Cell D needs 16 radio channels Cell E needs 3 radio channels Cell F needs 11 radio channels Cell G needs 8 radio channels Based on this, the network can be accommodated by assigning the following groups: Cell A = A1 Cell B = B1 Cell C = C1 and C2 Cell D = D3 Cell E = E 1 Cell F = F 1 Cell G = G1 Note: D3 is used for cell D. This keeps cell D from having channels adjacent to cells C and E, and minimizes interference from one cell to another.
This is a simplified version of how a network is diagrammed. In the real world, there are a great deal of trade-offs as you “test drive” your network and fine tune it.
Supervisory audio tone (SAT) is an audio tone that rides on the carrier just outside of the audible range of the cell phone. This tone is used to distinguish between co-channels and helps reduce the chances of misdirecting a call. There are three SAT codes: 5970 Hz (SAT 1) 6000 Hz (SAT 2) 6030 Hz (SAT 3) SAT assignment is performed with two objectives in mind: Adjacent cells should have different SATs. Co-channel deployments in adjacent clusters should have different SATs. These are accomplished by the method shown above. In the frequency grid, every cell has an SAT that is different from its adjacent cells. When propagated throughout the system, this will ensure that co-channel deployment in adjacent clusters have different SATs. This plan breaks down, of course, when the full grid is not built out. A check should be made to ensure that partial build-outs maintain the rules mentioned above. A poor SAT plan leads to call processing problems.
In this module on cellular coverage, we saw how cellular has emerged as a natural outgrowth of wireless communication that started in the early part of the 20th century. The movement toward individual mobile (cellular) telephones has met a demand in our society to provide communication wherever we are, whenever we want it. This has also driven all of the advances ever since then. By utilizing the various techniques described in this module, we can help ensure that mobile telephony will be provided wherever and whenever, and that it will be provided with the least amount of interference possible. We saw that as demand increased, or driving patterns changed, it was possible to meet these demands without major re-engineering by utilizing a technique called cell splitting . This allows for small cells to be inserted into larger ones at strategic places, without interfering with the larger cell. One of the most difficult tasks to perform is the assignment of radio channels to cells in such a manner that they will not interfere with other cells. In the discussion on channel grouping , we saw a set of tables, developed many years ago, that are designed to assist in this endeavor. Using these tables correctly, will ensure that minimal interference will occur. Another tool we looked at was C/I ratio With this tool we can determine the maximum distance from the cell tower that will provide the minimum acceptable signal without co-channel interference.
The D/R ratio , used in conjunction with the C/I ratio, helps determine how close two cells can be with the same radio channels (co-channel). The section on network architecture brought all these pieces together to show how to design a network of cells, minimize the interference, and distinguish between cells containing the same radio channels by employing a supervisory audio tone (SAT). You now have received a “taste” of what an RF engineer has to deal with. For more information on these topics, refer to books listed in the references or other modules in the GWEC Wireless Curriculum on related topics. Learn from those that have gone ahead!
3. FRP-CCC <ul><li>Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039. </li></ul><ul><li>GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use. </li></ul>
4. Table of Contents <ul><li>Overview 5 </li></ul><ul><li>Learning Objectives 6 </li></ul><ul><li>History of Wireless Communication 7 </li></ul><ul><li>Cell Splitting 12 </li></ul><ul><li>Channel Grouping 16 </li></ul><ul><li>Carrier to Interference (C/I) Ratio 26 </li></ul><ul><li>Tower Distance to Cell Radius (D/R) Ratio 30 </li></ul><ul><li>Cellular Network Architecture 41 </li></ul><ul><li>Summary 45 </li></ul><ul><li>Contributors 48 </li></ul>
5. Overview <ul><li>In this module, you will learn about about cellular coverage concepts that affect the ability to provide the best cellular coverage in the most efficient manner. </li></ul>
6. Learning Objectives <ul><li>After completing this module, you will be able to: </li></ul><ul><li>Explain how cell splitting and channel grouping are achieved in a cellular network </li></ul><ul><li>Explain how to calculate a C/I ratio </li></ul><ul><li>Calculate a D/R ratio </li></ul><ul><li>Determine the C/I interference for a given D/R ratio </li></ul><ul><li>Draw a diagram of a cellular network architecture </li></ul>
7. History of Wireless Communication
8. History of Wireless Communications <ul><li>About 100 years of Radio... </li></ul><ul><li>1914 Radio first used for practical communication in WWI </li></ul><ul><li>1920s First radio broadcasting and simplex mobile broadcast </li></ul><ul><li>1940s Radar first used in WWII </li></ul><ul><li>1950s First commercial television </li></ul><ul><li>1950s First mobile telephony </li></ul>
9. Specialized Mobile Radio (SMR) <ul><li>SMR was used primarily for commercial use </li></ul><ul><li>Two way over air communication between two or more mobile units </li></ul><ul><li>Coverage limited to area the signal could reach </li></ul><ul><li>If signal got too weak, call was dropped </li></ul><ul><li>No handoffs </li></ul><ul><li>Calls to the PSTN through a provided dial pad </li></ul>Public Service Telephone Network (PSTN) SMR Provider
10. Cellular Telephone Arrives <ul><li>Make coverage areas smaller, but more of them </li></ul><ul><li>Develop a means to hand call from one coverage area to another </li></ul><ul><li>Call these coverage areas cells </li></ul>Delicious Catering Base Station Digital Mobile Switching Center Delicious Delicious Delicious Public Switched Telephone Network (PSTN)
11. Cells Become Crowded <ul><li>Number of users outgrew number of channels available </li></ul><ul><li>More efficient reuse plan had to be developed </li></ul>Digital Mobile Switching Center Public Switched Telephone Network (PSTN)
12. Cell Splitting
13. Cell Splitting Basic N=7 frequency reuse plan 3 6 6 2 2 5 7 4 1
15. Cell Splitting 3 6 6 2 2 5 7 Basic N=7 frequency reuse plan with split cells 1 1 4 1 1 6 3 4 5 3
16. Channel Grouping
17. AMPS Cellular Bands <ul><li>Cellular telephony provides full-duplex communications </li></ul><ul><ul><li>Two-way simultaneous conversation requires simultaneous voice paths in both directions </li></ul></ul><ul><ul><li>25 MHz band of frequencies used for mobile transmission (uplink) </li></ul></ul><ul><ul><li>25 MHz band of frequencies used for cell site transmission (downlink) </li></ul></ul><ul><li>Cellular bands divided equally between two competing operators </li></ul><ul><ul><li>A operator </li></ul></ul><ul><ul><li>B operator </li></ul></ul>824 835 845 870 880 894 869 849 846.5 825 890 891.5 Uplink Downlink Paired Bands Frequency ( MHz) Uplink Downlink
18. 800 MHz Channel Assignments by Band
19. 800 MHz Telecommunications Band A, non-extended VCH B, non-extended VCH A, extended VCH B, extended VCH A, Control Channels B, Control Channels 799 717 667 666 716 355 312 354 334 001 1023 991 333 313 Legend: System A System B VOICE CHANNELS CONTROL CHANNELS CHANNELS VOICE CHANNELS non-extended exten- ded non-extended extended A’ B VOICE A”
20. Transmit and Receive Channel Separation
21. Cell Clusters – Frequency Reuse R 1 2 3 4 5 6 7 D N=7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 11 12 10 9 8 1 2 3 4 5 6 7 11 12 10 9 8 D R N=12 3 2 1 2 4 3 4 R D N=4 1
27. <ul><li>C/I is Carrier-to-Interference Ratio </li></ul><ul><li>AMPS modulation characteristics require 18 dB co-channel C/I over single interferer ( 17dB over </li></ul><ul><li>multiple interferers ) </li></ul><ul><li>Between a pair of sites using same channel, three C/I regions exist: </li></ul><ul><ul><li>Site A C/I better than 18 dB </li></ul></ul><ul><ul><li>Neither site gives usable C/I </li></ul></ul><ul><ul><li>Site B C/I better than 18 dB </li></ul></ul><ul><li>Other sites needed to serve region where neither A nor B has good C/I </li></ul><ul><li>Rate of signal decay determines how close next co-channel site can be and how many additional sites on other channels are needed in between </li></ul><ul><li>With proper planning, it is possible to determine required separation between co-channel sites to avoid interference </li></ul>Co-Channel Interference Model -120 -110 -100 -90 -80 -70 -60 Distance, km 1 3 5 7 9 11 13 15 17 19 21 23 25 0 Site A Site B -50 C/I = 18 dB C/I = 18 dB Good Service Good Service Interference RSSI, dBm Frequency Reuse Scenario
28. Co-Channel Interference Uplink/Reverse Path <ul><li>Co-channel interference can occur on uplink, downlink, or both </li></ul><ul><li>On the uplink, interference occurs at cell site receiver, from mobiles in surrounding co-channel cells </li></ul>1 1 1 2 3 4 5 6 7 4 6 4 7 2 7 2 5 3 5 3 6 1 1 1 1
29. Co-Channel Interference Downlink/Forward Path <ul><li>On the downlink, interference occurs at mobile user’s receiver due to signals from surrounding co-channel cells </li></ul>1 1 1 1 2 3 4 5 6 7 4 6 4 7 2 7 2 5 3 5 3 6 1 1 1 1
30. Tower Distance to Cell Radius (D/R) Ratio
31. <ul><li>Use D/R to determine how few cells ( N ) can be tolerated in frequency assignment repetition pattern </li></ul><ul><ul><li>Co-channel cells must be separated by at least D/R </li></ul></ul>D/R Determines Required Minimum N Example of N=7 D f1 f7 f2 f1 f3 f6 f4 f2 f5 f4 f7 f1 R R D = Distance between two co-channel transmitters R = Coverage radius where a cell is the best server N = ( ) 2 3 D R /
32. Reuse Plan Design Criteria <ul><li>Two design constraints with cellular reuse plans </li></ul><ul><ul><li>Need to maintain C/I ratio of 18 dB to avoid co-channel interference, but... </li></ul></ul><ul><ul><li>Want to divide channel sets into as few groups as possible to maximize number of channels per cell </li></ul></ul><ul><li>Closely related to development is the D/R ratio </li></ul><ul><ul><li>Distance between cells using same channel sets versus radius of each cell </li></ul></ul><ul><li>We can show that </li></ul>
33. Frequency Reuse Implications of N <ul><li>N is number of cells in frequency reuse pattern and is critically important since it determines: </li></ul><ul><ul><li>Capacity of an individual cell </li></ul></ul><ul><ul><ul><li>Channels per cell = </li></ul></ul></ul><ul><ul><ul><li>(total channels) / N </li></ul></ul></ul><ul><ul><ul><li>As N goes up, capacity progressively decreases </li></ul></ul></ul><ul><ul><li>Interference </li></ul></ul><ul><ul><ul><li>As N goes up, interference becomes progressively less troublesome </li></ul></ul></ul>Channels per Cell* D/R 395 198 132 99 79 66 56 49 44 1.732 2.449 3.000 3.464 3.873 4.243 4.583 4.899 5.196 40 5.477 36 5.745 N 1 2 3 4 5 6 7 8 9 10 11 12 33 6.000 * Assumes use of 395 voice channels including expanded spectrum
34. Channel Assignment <ul><li>Channel set - collection of channels which could be assigned at one cell </li></ul><ul><li>Channels in a channel set </li></ul><ul><ul><li>Normally are N channels apart, where N is reuse factor </li></ul></ul><ul><ul><li>Must meet combiner minimum frequency spacing requirements </li></ul></ul><ul><li>Sets 1 and 3 (i.e.,1 and N) are adjacent frequencies </li></ul>If N=3, for example: 1 2 3 4 5 6 7 8 9 10 12 Channels Freq. Channel Set 1 1, 4, 7, 10, . . . Channel Set 2 2, 5, 8, 11, . . . Channel Set 3 3, 6, 9, 12, . . . 1 2 3 2 3 1 1 1 2 3 3 2 N=3
35. Cell Arrangement as a Function of N <ul><li>N=1 Lethal, works well in CDMA </li></ul><ul><ul><li>Awful C/I: Every neighbor is co-channel </li></ul></ul><ul><ul><li>Every neighbor cell is adjacent channel too! </li></ul></ul><ul><ul><li>Center 1/3 of each cell OK, rest is lost in horrible interference </li></ul></ul><ul><li>N=2 Better, but still lethal </li></ul><ul><ul><li>Each cell still has 2 co-channel neighbors </li></ul></ul><ul><ul><li>Each cell has 4 adjacent channel neighbors </li></ul></ul>1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 2 2 2 2 2 2
36. <ul><li>N = 3 Better, but still lethal </li></ul><ul><ul><li>Co-channel neighbors are now spaced at D/R of 3.0 - better, but not 18 dB.... </li></ul></ul><ul><ul><li>Each cell has 6 adjacent channel neighbors - all neighbors are adjacent!! </li></ul></ul><ul><li>N = 4 Better, but still lethal </li></ul><ul><ul><li>Co-channel neighbors are now spaced at D/R of 3.464 </li></ul></ul><ul><ul><li>Each cell has 4 adjacent channel neighbors </li></ul></ul>Cell Arrangement as a Function of N 2 3 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 1 2 3 4 1 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 3 4 4 4 2 4 4 4
37. <ul><li>N = 5 Better, but not good enough </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>2 at D/R of 3.0 </li></ul></ul></ul><ul><ul><ul><li>4 at D/R of 4.58 </li></ul></ul></ul><ul><ul><li>Some cells have 2 adjacent channel neighbors, some have 3 </li></ul></ul><ul><li>N = 6 Better, but not by much </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>2 at D/R of 3.464 </li></ul></ul></ul><ul><ul><ul><li>2 at D/R of 4.58 </li></ul></ul></ul><ul><ul><ul><li>2 at D/R of 6.0 </li></ul></ul></ul><ul><ul><li>Some cells have 2 adjacent channel neighbors, some have 3 </li></ul></ul>Cell Arrangement as a Function of N 1 5 2 3 4 1 5 2 1 2 1 2 3 1 5 2 3 4 1 5 4 1 5 1 5 6 2 3 4 1 5 6 2 3 4 1 5 6 4 1 6 2 1 2 3 6 1 2 3 4 5 1
38. <ul><li>N = 7 First arrangement that works in most propagation environments, giving 18+ dB C/I </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>6 at D/R of 4.58 </li></ul></ul></ul><ul><ul><li>Each cell always has 2 adjacent channel neighbors </li></ul></ul><ul><li>N = 8 Better, but not worthwhile </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>4 at D/R of 4.58 </li></ul></ul></ul><ul><ul><ul><li>2 at D/R of 6.0 </li></ul></ul></ul><ul><ul><ul><li>2 at D/R of 6.93 </li></ul></ul></ul><ul><ul><li>Of the eight cells in the cluster, 2 have 2 adjacent-channel neighbors and 4 have 1 adjacent channel neighbor </li></ul></ul>Cell Arrangement as a Function of N 1 5 3 8 2 6 4 7 1 5 3 7 1 7 1 5 3 8 2 6 4 1 8 2 6 1 8 2 6 4 1 5 4 7 1 4 7 1 5 3 8 1 2 3 4 5 6 7 4 6 1 1 1 1 1 1 4 7 2 7 2 5 3 5 3 6
39. <ul><li>N = 9 Significant improvement </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>6 at D/R of 5.20 </li></ul></ul></ul><ul><ul><li>Out of 9 cells in cluster, 4 have 1 adjacent channel neighbor and 3 have 2 such neighbors </li></ul></ul><ul><li>N = 10 Not impressively better </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>2 at D/R of 4.58 </li></ul></ul></ul><ul><ul><ul><li>2 at D/R of 6.0 </li></ul></ul></ul><ul><ul><ul><li>2 at D/R of 6.06 </li></ul></ul></ul><ul><ul><li>Out of 10 cells in cluster, </li></ul></ul><ul><ul><ul><li>6 have 1 adjacent channel neighbor </li></ul></ul></ul><ul><ul><ul><li>3 have 2 adjacent-channel neighbors </li></ul></ul></ul>Cell Arrangement as a Function of N 1 2 3 4 5 6 7 8 9 1 2 3 6 7 1 2 6 7 4 4 5 8 9 1 3 5 8 9 1 1 4 6 8 1 3 5 7 1 2 3 4 5 6 7 8 9 10 1 2 4 6 1 3 5 9 1 2 6 7 9 10 1 3 7 9 10 1 4 8 1 5 8
40. <ul><li>N = 11 Slightly better </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>2 at D/R of 4.58 </li></ul></ul></ul><ul><ul><ul><li>2 at D/R of 6.06 </li></ul></ul></ul><ul><ul><ul><li>4 at D/R of 7.14 </li></ul></ul></ul><ul><ul><li>Out of 11 cells in cluster, eight each have one adjacent channel neighbor </li></ul></ul><ul><li>N = 12 Excellent but inefficient </li></ul><ul><ul><li>Co-channel neighbors farther away </li></ul></ul><ul><ul><ul><li>6 at D/R of 6.0 </li></ul></ul></ul><ul><ul><li>No adjacent-channel neighbors </li></ul></ul>Cell Arrangement as a Function of N 3 7 11 6 10 10 9 12 4 12 8 1 5 1 8 1 9 8 1 9 5 5 2 3 2 3 6 10 7 11 8 5 1 9 4 12 2 3 2 11 1 1 10 7 7 10 7 6 2 3 4 5 6 7 10 11 1 9 8 2 3 6 7 10 1 2 6 10 1 2 5 6 10 1 9 3 4 7 10 11 1 8 3 7 1 4 5 11 1 9 8 5 11 1 9 4 11 1 8
41. Cellular Network Architecture
42. Diagramming a Network <ul><li>Build a basic cluster of cells using the N factor </li></ul><ul><ul><li>Try to minimize number of adjacent channels </li></ul></ul><ul><ul><li>Build a cluster that is repeatable </li></ul></ul>G B D E A F C
43. Building a Network <ul><li>Duplicate the cluster as many times as needed to cover designated area </li></ul><ul><li>Only one thing is left </li></ul><ul><ul><li>Supervisory audio tone (SAT) code </li></ul></ul>D G B E A F C G B D E A F C G B D E A F C G B D E A F C G B D E A F C G B D E A F C 1 G B E A F C
44. SAT Assignment Plan for N=7 Omni Systems <ul><li>SAT frequencies are transmitted on analog voice channels to: </li></ul><ul><ul><li>Provide circuit supervision </li></ul></ul><ul><ul><li>Validate identity of measured signals during locate process preceding handoff </li></ul></ul><ul><li>SATs are assigned so that : </li></ul><ul><ul><li>No cell has near co-channel neighbors with same SAT </li></ul></ul><ul><ul><li>No adjacent cell has same SAT </li></ul></ul>SAT Frequencies SAT 0 5970 Hz SAT 1 6000 Hz SAT 2 6030 Hz 2 0 1 1 0 1 0 2 0 1 0 2 0 2 1 2 0 2 1 2 1 0 1 2 1 0 1 0 2 0 1 0 2 0 2 0 1 2 1 0 1 0 1 2 0 0 2 0 2
46. Summary <ul><li>In this module, you were introduced to: </li></ul><ul><li>History of wireless communication </li></ul><ul><li>Cell splitting </li></ul><ul><li>Channel grouping </li></ul><ul><li>C/I ratio </li></ul><ul><li>D/R ratio </li></ul><ul><li>Network architecture </li></ul>
47. Summary <ul><li>In this module, you were introduced to: </li></ul><ul><li>The history of wireless communication </li></ul><ul><li>Cell splitting </li></ul><ul><li>Channel grouping </li></ul><ul><li>C/I ratio </li></ul><ul><li>D/R ratio </li></ul><ul><li>Network architecture </li></ul>
48. Industry Contributors <ul><li>AT&T Wireless ( http://www.attwireless.com ) </li></ul><ul><li>Lucent Technologies, Inc. ( http://www.lucent.com ) </li></ul><ul><li>Motorola ( http://www.motorola.com ) </li></ul><ul><li>RF Globalnet ( http://www.rfglobalnet.com ) </li></ul><ul><li>Telcordia Technologies, Inc ( http://www.telcordia.com ) </li></ul>The following companies provided materials and resource support for this module:
49. Individual Contributors <ul><li>The following individuals and their industry or educational institutions provided materials, resources, and development input for this module: </li></ul><ul><li>Dr. Cheng Sun </li></ul><ul><ul><li>California Polytechnic State University </li></ul></ul><ul><ul><li>http://www.calpoly.edu/ </li></ul></ul><ul><li>Dave Voltmer </li></ul><ul><ul><li>Rose-Hulman Institute of Technology </li></ul></ul><ul><ul><li>http:// www.rose-hulman.edu / </li></ul></ul>