Frequency Management and Channel Assignment

22 

Loading.... (view fulltext now)

Loading....

Loading....

Loading....

Loading....

Full text

(1)

Frequency Management and Channel Assignment

12.1 Introduction

Achieving optimum system capacity with a limited frequency spectrum is one of the main research issues in cellular communications. In a cellular system, frequency management and channel assignment are essential in order to achieve the basic objectives of spectrum utilization as well as adaptability to traffic density. In this chapter, the various aspects leading to an efficient and effective frequency planning of cellular systems are discussed in detail.

Depending upon the system parameters, the allocated frequency spectrum is divided into a number of frequency channels. These

available frequency channels are then divided into the subsets that can be assigned to each cell. Different strategies are followed for the

assignment of these channel sets to cells. Fixed channel assignment (FCA) technique and dynamic channel allocation techniques are covered in detail. Frequency management includes operations such as

designation of set-up and voice channels, numbering the channels, and grouping voice channels into subsets.

The main objective of channel-assignment is to stabilize the

fluctuations in the probability of call blockage over the entire coverage area of a cellular network over a period of time. The channel assignment does the allocation of specific channels to cell sites and mobile units. It can be done in two ways:

o Short-term assignment, where one channel assignment per call is handled by mobile telephone switching office (MTSO).

o Long-term assignment, where a fixed channel set consisting of one or more subsets are assigned to cell site on a long-term basis.

(2)

This chapter introduces numbering of the radio channels, traffic and channel assignment, non-FCA, the simulation process followed, and the results obtained.

12.2 Numbering the radio channels

Many cellular mobile systems operate on 666 channels. Each channel consists of two frequency channel bandwidths (mobile transmit/uplink or reverse channel and cell-site transmit/downlink or forward channel) to allow duplex operation. These two channel bandwidths must be separated in frequency in order to avoid interference. The frequency separation between the uplink and downlink channels is termed as channel spacing (or) duplex spacing. In the present 800 MHz band cellular system, the separation between the mobile transmit and the cell-site transmit is specified as 45 MHz. The sub-sections that follow

describe the numbering of radio channels by discussing the frequency management chart and grouping of channels into subsets.

12.2.1 Frequency management chart

The total channels available are 832 in number. However, most mobile units and systems are still operating on 666 channels.

Figure 12.1 shows the arrangement of 666 frequency channels in block A and block B systems, each containing 333 channels. Out of these 333 available channels in each system, 312 channels are used for voice communication and 21 channels are used for controlling the system. These 21 channels are called as control channels or set-up channels. Therefore, a total of 42 channels are used for controlling the system.

(3)

Figure 12.1 Frequency management chart

In channel 1, the two frequencies available for mobile and cell-site transmit are

1. 825.030 MHz (mobile transmit) 2. 870.030 MHz (cell-site transmit)

(4)

In channel 666, the two frequencies available for mobile and cell-site transmit are

1. 844.98 MHz (mobile transmit) 2. 889.98 MHz (cell-site transmit)

Each market (i.e. each city) has two systems for a duopoly market policy with each block having 333 channels.

The 42 set-up channels also called as control channel sets are assigned as follows:

1. Channels 313–333 in block A 2. Channels 334–354 in block B

The voice channels are assigned as follows:

1. Channels 1–312 (312 voice channels) in block A 2. Channels 355–666 (312 voice channels) in block B

The 42 set-up channels (control channel sets) are assigned in the middle of all the assigned channels to facilitate scanning of those channels by frequency synthesizers. In the new additional spectrum allocation of 10 MHz, an additional 166 channels are assigned.

Since a first channel is assigned below 825 MHz (or 870 MHz), in the future, additional channels will be numbered up to 849 MHz (or 894 MHz) and will then circle back. The last channel number is 1023. There are no channels between 799 and 991 channels. New additional spectrum allocations are shown in Figure 12.2.

(5)

Figure 12.2 New additional spectrum allocations 12.2.2 Grouping into subsets

Since there are 21 set-up channels for each block, it is logical to group 312 voice channels into 21 subsets of 15 voice channels each (the last subgroup has 12 voice channels only). Each subset then consists of 16 channels – – 15 voice channels and one control channel. In each set, the closest adjacent channel is 21 channels away as shown in Figure 12.1. The channel separation is provided in such a way that it is sufficient to meet the adjacent channel isolation requirement.

The 16 channels in each subset can be mounted on a frame and connected to a channel combiner. Wide separation between adjacent channels is required for meeting the requirement of minimum isolation.

12.3 Set-up channels

The set-up channels are also called as control channels. They are designated to set-up calls in the system. But even without set-up

channels, a system could work where all the channels are in either block A or block B which will be used as voice channels. If a frequency reuse technique is applied to a cellular system, the set-up channels act as control channels.

(6)

The set-up channels are classified with respect to their application. They are

o access channels o paging channels

12.3.1 Access channels

Access channels are used for calls originating from mobile. When a mobile set scans all the 21 set-up channels (in block A), two conditions are considered:

o If no set-up channels are operational in block A, then the mobile unit switches automatically to block B.

o If there is a strong set-up channel with no message detected then within the second setup, it will be selected by the scanner.

12.3.2 Paging channels

Paging channels are used for calls originating from land. Every cell site is assigned its own control or set-up channels. For example, FOCC is the forward set-up channel in which every cell site are mainly used to page the mobile unit with control message of same mobile station.

o The same message is transmitted by different set-up channels and there is no simulcast interference.

o A better algorithm is used to page from all the cell sites.

12.4 Traffic and channel assignment

The vehicular traffic density of a coverage area is a critical element and must be determined before a system is designed. This traffic pattern in busy hours can be confined to different zones within the service area. If the traffic pattern predominates over the simple signal coverage pattern, then the cell-site selection will be based on the traffic pattern. Choice of the initial cell sites should be based on the signal covered in zones of heavy vehicular traffic. This means that the cell site would most likely be located at the centre of those zones.

(7)

If call traffic data are collected while the system is operating, then we can update the call traffic data at each cell site to correlate with the vehicular traffic data. This information will be useful for determining whether new cell splitting is needed. If it is, then we must determine how many radios should be installed at the new site and where it is to be located. These decisions are all related to frequency channel assignment.

12.4.1 Fixed channel assignment

In FCA, each cell assigns its own frequency channel to the mobile subscribers within its cell. Channel assignment is primarily based on causing least co-channel and adjacent channel interference in the

cellular system. The channel assignment for each voice call is determined by MTSO on a short-term basis. In a FCA, the set-up and voice channels are usually assigned to the cell site for relatively long periods. Channels in a channel set are usually 21 channels apart and must meet minimum frequency spacing requirements of a multi-channel transmitter

combiner. Channels are usually numbered in order of increasing frequency. Regardless of the number of channels in a channel set, the highest channel set is frequency adjacent to the lowest channel set.

The following are the advantages of FCA:

o Fixed parameters (power, frequency) for transceivers.

o Good performance under uniform- and/or high-traffic loads as cells independently decide their channel allocation decisions.

o If each cell is allocated to a pre-determined set of voice channels then the call is blocked and all the channels are occupied.

o Borrowing strategy: A cell is allowed to borrow channels from a neighbouring cell if all of its own channels are occupied.

o Mobile switching centre (MSC) supervises the borrowing procedure to ensure no disrupting calls or interference with any of the calls in progress in the donor cell.

(8)

12.4.2 Dynamic channel assignment

In dynamic channel assignment (DCA), the central common pool

maintains all the available channels. Channels are assigned dynamically as new requests for radio resource (for a fresh originating call or handoff of existing call) arrive in the system. This also implies that when the use of assigned channel is completed, the channel currently in use is

returned to the central pool.

In order to achieve optimum system capacity with limited frequency spectrum, many DCA schemes have been proposed to allocate the channels more efficiently. In a cellular system, a mobile subscriber

moves from one cell to another and continuation of communication link is ensured with suitable handoff mechanism. This demands for

additional and flexible radio resources utilization. One way to ascertain minimum blocking probability is to increase the number of channels per cell. Then every cell would expect to have a large number of channels. However, because a limited frequency band is allocated for cellular communication, there is an upper limit to the maximum number of channels, thereby restricting the number of available channels that can be assigned to each cell. Another way is non-uniform FCA based on the amount of traffic expected to be served in different cells as per the statistical traffic data. Another alternative arrangement could be dynamic assignment of channels to different cells, as per the current demand. This may be done from a central pool of channels, or a combination of both FCA and DCA.

The following are the advantages of DCA:

o No fixed channels are assigned to each cell.

o Out of the available channels, any channel can be assigned to any cell on need basis.

o The serving base station (BS) requests a channel from the MSC whenever a call request is made.

(9)

o Consideration of the likelihood of future blocking in the cell, the frequency use of the candidate cell, the reuse distance of the channel, and other cost

functions.

o MSC needs to collect real-time data on channel occupancy, traffic distribution, and received signal strength indicator (RSSI) of all channels on a continuous basis, which increases storage and computational load on the system.

12.4.3 Channel sharing scheme

When a particular cell needs more channels in order to meet the

increased traffic demand, the channels of another sector at the same cell site can be shared to meet the short-term overload traffic. Channel

sharing can be done from one of the two adjacent sectors of the

neighbouring cells in a sectored cellular system configuration. Shared channels can be returned back when the channels become available in the shared sector. This scheme is called the ordered channel assignment scheme with rearrangement.

An alternate scheme is channel assignment with sharing and reassignment. This scheme makes sure that channel-sharing

arrangement causes minimum impact on call-blocking probability in neighbouring cells. Reassignment of shared channels is done in order to provide maximum assistance to the neighbouring cells to meet the

temporary increased traffic demand. The channel can also be ordered based on which channels provide better performance. Channel sharing always increases the trunking efficiency of the channels.

12.4.4 Channel-borrowing scheme

The channel-borrowing scheme is used for slow growing systems on a long-term basis as an alternate to the costly cell-splitting technique to handle increased traffic. One approach to address increased traffic of either mobile originating calls or handoff calls in a cell is to borrow free available channels from neighbouring cells. A simple channel-borrowing scheme implies that if all channels assigned to a cell have already been

(10)

used, then additional channels as per the current need can be borrowed from any other cell (preferably adjacent cells) that has some free unused channels. In addition, the central cell site can also borrow channels from neighbouring cells. The extent of borrowing channels depends on the traffic density in the area.

12.5 Channel assignment algorithms

There are several algorithms available for non-FCA. They are listed below:

o Fixed channel algorithm (FCA): This algorithm is the most commonly adopted in many cellular systems. Here, each cell assigns its own radio channels to the vehicles within its cell.

o Dynamic channel algorithm (DCA): Here no fixed channels are assigned to each cell. Therefore, any channel in a composite of 312 radio channels can be assigned to the mobile unit. This means that a channel is assigned directly to a mobile unit. On the basis of overall system performance, the DCA can also be used during a call.

o Hybrid channel algorithm (HCA): This is a combination of FCA and DCA. A portion of the total frequency channels will use FCA and the rest will use DCA. o Borrowing channel algorithm (BCA): It uses FCA as a normal assignment

condition. When all fixed channels are occupied, then the cell borrows channels from the neighbouring cells.

o Forcible-borrowing channel algorithm (FBCA): In this case, if a channel is in operation and the situation warrants it, then channels must be borrowed from the neighbouring cells and at the same time another voice channel will be assigned to continue the call in the neighbouring cell.

Channel cannot be borrowed frequently from adjacent cells.

12.6 Simulation process and results

On the basis of FBCA, FCA, and BCA algorithms, a seven-cell reuse pattern with an average blocking of 3 per cent is assumed.

The simulation model is described as follows:

(11)

o Determine the state of the vehicle in the cell (idle, off-hook, on-hook, and handoff).

Method of implementation

There are many different ways of implementing FBCA. In a general sense, FBCA can also be applied while accounting for the forcible borrowing of the channels within a fixed-channel set to reduce the chance of co-channel assignment in a reuse cell pattern.

Reuse distance

The FBCA algorithm is based on assigning a channel dynamically but obeying the rule of reuse distance. The distance between the two cells is reuse distance, which is the minimum distance at which no co-channel interference would occur. If all the channels in the neighbouring cells cannot be borrowed because of interference problems, the FBCA stops.

12.6.1 Blocking

Two types of blocking are possible in FBCA algorithm:

1. Average blocking: It happens mostly in non-uniform traffic. 2. Handoff blocking: It happens mostly in uniform traffic.

Average blocking

Two average blocking cases illustrating this simulation are shown

in Figure 12.3. In a uniform traffic condition (Fig. 12.3(a)), the 3 per cent blocking of both BCA and FBCA will result in a load increase of 28 per cent, compared to 3 per cent blocking of FCA. There is no difference between BCA and FBCA when a uniform traffic condition exists. In a non-uniform traffic distribution (Fig. 12.3(b)), the load increase in BCA drops to 23 per cent and that of FBCA increases to 33 per cent, as at an average blocking of 3 per cent. The load increase can be utilized in another way by reducing the number of channels. The percent increase in load is same as the percent reduction in the number of channels.

(12)

Figure 12.3 Averaging blocking in spatially (a) uniform and (b)

(13)

Handoff blocking

Handoff blocking is not considered as the regular cell blocking which can only occur at the call set-up stage. In both BCA and FBCA, load is

increased almost equally to 30 per cent, as compared to FCA at 3 per cent handoff blocking in uniform traffic (shown in Fig. 12.4(a)).

For a non-uniform traffic distribution, the load increase of both BCA and FBCA at 4 per cent blocking is about 50 per cent (Fig. 12.4(b)), which is a big improvement, considering the reduction in interference and blocking. Otherwise, there would be multiple effects from

(14)

Figure 12.4 Handoff blocking in spatially (a) uniform and (b)

non-uniform traffic distribution 12.7 Summary

In a cellular system, frequency management and channel assignment are essential in order to achieve the basic objectives of spectrum utilization as well as adaptability to traffic density.

o Frequency management includes operations such as

o designating the set-up and voice channels

o numbering the channels

o grouping voice channels into subsets, and so on

o Channel assignment does the allocation of specific channel to the cell sites and mobile units:

o The channel assignment can be done in two ways. They are as follows:

o Short-term assignment: During a call, a particular

channel is assigned to a mobile unit on a short-term basis, that is the channel is assigned only during the call duration. After the call, the channel will be altered.

o Long-term assignment: A fixed channel set

consisting of one or more subsets is assigned to a cell site on a long-term basis.

o The set-up channels are classified as access channels and paging channels.

o Fixed channel assignment/(algorithm) (FCA): This algorithm is the most commonly adopted in many cellular systems. Here, each cell assigns its own radio channels to the vehicles within its cell.

o Dynamic channel assignment/(algorithm) (DCA): Here, no fixed channels are assigned to each cell. Therefore, any channel in a composite of 312 radio channels can be assigned to the mobile unit. This means that a channel is assigned

(15)

directly to a mobile unit. On the basis of overall system performance, DCA can also be used during a call.

o Hybrid channel assignment/(algorithm) (HCA): This is a combination of FCA and DCA. A portion of the total frequency channels will use FCA and the rest will use DCA.

o Borrowing channel assignment/ (algorithm) (BCA): It uses FCA as a normal assignment condition. When all fixed channels are occupied, then the cell borrows channels from the neighbouring cells.

o Forcible-borrowing channel assignment/(algorithm) (FBCA): Here, if a channel is in operation and the situation warrants it, channels must be borrowed from the neighbouring cells and at the same time, another voice channel will be assigned to continue the call in the neighbouring cell.

o Channel cannot be borrowed frequently from adjacent cells. o There are two types of blocking possible in FBCA algorithm.

o Average blocking: It happens mostly in non-uniform

traffic.

o Handoff blocking: It happens mostly in uniform traffic.

o Queuing of handoff calls can increase traffic capacity. Example problem 12.1

A full-duplex wireless cellular system is allocated a total spectrum of 20 MHz and each simplex channel has 25 kHz RF bandwidth. Determine the following:

1. Total number of full-duplex channels available.

2. Number of channels per cell site if K = 4 cell reuse pattern is employed. Solution

Given data:

Total allocated RF spectrum bandwidth = 20 MHz Channel bandwidth per simplex channel = 25 KHz

1. To determine number of full-duplex channel

Channel bandwidth per simplex channel = 25 kHz Number of channels in a duplex link = 2

(16)

Number of full-duplex channels = total bandwidth/duplex channel bandwidth

Number of full-duplex channels = 20 MHz/50 kHz

Hence, total number of duplex channels = 400 channels.

2. To determine number of channels per cell site

Number of cells in one cluster, K = 4 (given)

Number of channels per cell site = total number of channels/K = 400/4 =100.

Hence, number of channels per cell-site = 100 channels.

Example problem 12.2

A full-duplex wireless cellular system is allocated a total spectrum of 25 MHz and each simplex channel has 15 kHz RF bandwidth. Determine the following:

1. Total number of full-duplex channels available.

2. Number of channels per cell site if K = 7 cell reuse pattern is employed. Solution

Given data:

Total allocated RF spectrum bandwidth = 25 MHz Channel bandwidth per simplex channel = 15 KHz

1. To determine number of full-duplex channel

Channel bandwidth per simplex channel = 15 kHz Number of channels in a duplex link = 2

Therefore, duplex channel bandwidth = 15 × 2 = 30 kHz

Number of full-duplex channels = total bandwidth/duplex channel bandwidth

Number of full-duplex channels = 25 MHz/30 kHz

Hence, total number of duplex channels = 833 channels.

2. To determine number of channels per cell site

(17)

Number of channels per cell site = total number of channels/K = 833/7 =119.

Hence, number of channels per cell site = 119 channels.

Example problem 12.3

Calculate the number of set-up and voice channels per cell for a cellular system having a total spectrum allocation of 60 MHz which uses two 25 kHz simplex channels to provide full duplex set-up and voice channels. Assume that the system is designed with nine-cell frequency-reuse pattern and 1 MHz of the total spectrum is exclusively allocated for set-up channels.

Solution

Given data:

Total allocated RF spectrum bandwidth = 60 MHz Channel bandwidth per simplex channel = 25 kHz Number of cells in one cluster = 9

Allocated RF bandwidth for set-up channels = 1 MHz

o To determine duplex channel bandwidth

Channel bandwidth per simplex channel = 25 kHz Number of channels in a duplex link = 2

Therefore, duplex channel bandwidth = 25 × 2 = 50 kHz

o To determine number of full-duplex channels

Number of full-duplex channels = Total bandwidth/duplex channel bandwidth

Number of full-duplex channels = 60 MHz/50 kHz

Hence, total number of duplex channels = 1,200 channels

o To determine total number of set-up channels

Duplex channel bandwidth = 50 kHz (As calculated in Step 1) Total number of available set-up channels = 1 MHz/50 kHz = 20

(18)

Total available 20 number of set-up channels can be distributed among nine cells in a cluster as 7 cells can have 2 set-up channels each, and remaining 2 cells can then have 3 set-up channels each which means a total (7 × 2 + 2 × 3 ) of 20 set-up channels in a system.

o To determine total number of voice channels

Available RF bandwidth for voice channels = 60 MHz – 1 MHz= 59 MHz

Total number of available voice channels = 59 MHz/50 kHz = 1180

o To distribute number of voice channels per cell

Total 1180 number of available voice channels can be distributed among nine cells in a cluster as8 cells can have 131 voice channels

each, and remaining 1 cell can then have 132 voice channelswhich

means a total (8 × 131 + 1 × 132) of 1180 voice channels in a system. Review questions

1. What is meant by frequency management and channel assignment? 2. What is known as FOCC?

3. Define blocking? What are the types of blocking? What is their significance? 4. Explain briefly about FCA.

5. What is the importance of frequency management chart? 6. What are the methods for reducing interference?

7. Write the procedure to allot the channels for the travelling mobile units. (Refer Section 12.4)

8. Explain the channel assignment to the cell sites based on the adjacent channels. (Refer Section 12.4)

9. Give the structure of the channels in 800 MHz system with frequency ranges. (Refer Section 12.2.1)

10. Explain how set-up channels act as control channels in a cellular system? (Refer Section 12.3)

11. What are the advantages of reuse-partition scheme? (Refer Section 12.2) 12. Explain the following,

i. Channel sharing (Refer Section 12.4.3) ii. Channel borrowing (Refer Section 12.4.4) iii. Underlay and overlay (Refer Section 12.3)

iv. Set-up channel (Refer Section 12.3.2) v. Paging channel (Refer Section 12.2.1)

(19)

vi. Voice channel (Refer Section 12.2.1)

2. How a reuse-partition scheme reduces the number of cell sites? Explain it with suitable examples. (Refer Section 12.2.1)

3. Differentiate between FCA and non-FCA in detail. (Refer Section 12.4.1) 4. Discuss the concept of frequency management concern to the numbering the

channels and grouping into the subset. (Refer Sections 12.2 and 12.3) 5. Explain in brief the grouping of voice channels into subsets. (Refer Section

12.2.2)

6. Explain how paging channels are used for the land originating calls? (Refer Section 12.3.2)

7. What do you understand by non-FCA? Describe the corresponding algorithms. (Refer Section 12.5)

8. Describe the grouping of the voice, set-up, and paging channels. (Refer Sections 12.2.2, 12.3, and 12.3.2)

9. Explain how the 666 channels are divided into groups? (Refer Section 12.2.1) 10. What are the different techniques to utilize the frequency spectrum? Explain

with brief explanation. (Refer Section 12.2.4)

11. Explain the forcible-borrowing channel assignment algorithm and its implementation? (Refer Section 12.5.5)

Objective type questions and answers

1. The main function of the frequency management is

a. increasing gain

b. increasing power

c. dividing total number of channels into subsets d. adding the given number of channels

2. Numbering the channel is done by the following channel

a. RVC

b. RCC

c. FVC

d. FCC

2. What basis in a fixed channel set that consists of one or more subsets is assigned to a cell site?

(20)

b. in dynamic basis

c. long-term basis

d. temporary basis

2. What is the method of channel assignment to a mobile unit during call in progress?

a. short-term basis b. in dynamic basis

c. long-term basis

d. temporary basis

2. Allocation of specific channels to a cell site is known as

a. frequency management

b. frequency allotment

c. channel assignment

d. channel modelling

2. What is the other name of set-up channels?

a. reverse channels

b. forward channels

c. control channels d. traffic channel

2. In set-up channels every two-way channel contains a ——— bandwidth.

3. In normal case, ——— set-up channels can be used for paging and accessing.

4. All the set-up channels carry only ———.

5. The access channel can be designated by ——— as a channel apart from set-up

channels in the cell site.

Answers: 1. (c), 2. (d), 3. (c), 4. (a), 5. (c), 6. (c), 7. 30 MHz, 8. One, 9.

Data information, 10. MTSO.

Open book questions

1. Explain the different channel assignment algorithms in detail. 2. Explain the significance of FBCA algorithm.

(21)

4. Why is it necessary to form frequency channel groups?

5. How is voice channels assigned for establishment of voice calls?

6. Which channel-assignment approach can be effectively deployed to handle increased traffic situation?

7. On what basis channels are assigned in an overlapped cell-based system? 8. Explain in detail access channels and operational techniques.

9. The U.S. AMPS system is allocated 50 MHz of spectrum in the 800 MHz range and provides 832 channels. Forty-two of those channels are control channels. The forward channel frequency is exactly 45 MHz greater than the reverse channel frequency.

i. Assume a base station transmits control information on channel 352, operating at 880.560 MHz. What is the

transmission frequency of the subscriber unit on transmitting on channel 352?

ii. The A-side and B-side cellular carriers evenly split AMPS channels. Find the number of voice channels and the number of control channels for each carrier.

iii. Suppose that you are chief engineer of a cellular system using seven-cell reuse. Purpose a channel assignment strategy for a uniform distribution of user throughout your cellular system specifically, assume that each cell has three control channels (1200 sector is employed) and specify the number of voice channels you would assign to each control in your system.

iv. For an ideal hexagonal cellular layout which has identical cell coverage, what is the distance between the centers of two nearest co-channel cells for seven-cell reuse and for four-cell reuse?

2. What are the common principles of channel allocation schemes?

Further reading

Beach M., Propagation and System Aspects, University of Bristol, Future Communication Systems course, April 1994.

Classen, F., Meyr, H., and Sehier, P. “Maximum likelihood open loop

carrier synchronizer for digital radio,” Proceedings ICC’93, pp. 493–497, 1993.

(22)

Lee, W. C. Y. “Elements of Cellular Radio System,” IEEE Transactions on Vehicular Technology, 35 (May 1986): pp. 48–56.

Lee, W. C. Y. Mobile Cellular Telecommunications System. New York: McGraw-Hill, 1989.

Van Nee, R., and Prasad R. OFDM for Wireless Multimedia

Figure

Updating...