Chapter 8 Traffic Channel Allocation

Transcription

1 Chapter 8 Traffic Channel Allocation Prof. Chih-Cheng Tseng tsengcc@niu.edu.tw EE of NIU Chih-Cheng Tseng 1

2 Introduction What is channel allocation? It covers how a BS should assign traffic channels to the MSs. A given radio spectrum is to be divided into a set of disjointed channels that can be used simultaneously while minimizing interference in adjacent channel by allocating channels appropriately (especially for traffic channels) S total channels equally partitioned among N cells with each cell with S channels as S=S total /N, e.g., 140/7=20 Channel allocation schemes can be divided in general into Static versus Dynamic Fixed Channel Allocation (FCA) Dynamic Channel Allocation (DCA) Hybrid Channel Allocation (HCA) EE of NIU Chih-Cheng Tseng 2

3 Fixed Channel Allocation (FCA) In FCA, a set of channels is permanently allocated to each cell. Number of available channels S is divided into sets, the minimum number of channel sets N required is related to the frequency reuse distance D as N = D 2 /3R 2 If a cell of cluster A 1 borrows channel, there should not be interference with cells A 2, A 3, A 4, A 5, A 6, and A 7 A 1,1 : Channels 1-20, A 1,2 : Channels A 1,3 : Channels 41-60, A 1,4 : Channels A 1,5 : Channels , A 1,6 : Channels A 1,7 : Channels A 6 A 5 A 7 A 2 A 1,5 A 1,4 A 1,3 A 1,6 A 1 A 1,7 A 1,2 A 4 A 3 EE of NIU Chih-Cheng Tseng 3

4 Simple Borrowing Schemes In SB schemes, cell (acceptor cell) that has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls. Borrowing from the richest: Borrowing can be done from an adjacent cell which has largest number of free channels. Borrow-first-available scheme: Select the first free channel found for borrowing using a search algorithm. To be available for borrowing, the channel must not interfere with existing calls. EE of NIU Chih-Cheng Tseng 4

5 Complex Channel Borrowing using Sectored Cell-based Wireless System A 6 c b a A 7 c a A 2 b c a b A 1 c a A b x 3 c b a Although x borrows channels from a would violate the reuse distance, the direction of sector x are different from the sector a s in A 2, A 3, and, A 4 A 5 c b a A 4 c b a EE of NIU Chih-Cheng Tseng 5

6 Simple Channel Borrowing Schemes Scheme Simple Borrowing (SB) Borrow from the Richest (SBR) Basic Algorithm (BA) Basic Algorithm with Reassignment Borrow First Available Description A nominal channel set is assigned to a cell, as in the FCA case. After all nominal channels are used, an available channel from a neighboring cell is borrowed. Channels that are candidates for borrowing are available channels nominally assigned to one of the adjacent cells of the acceptor cell. If more than one adjacent cell has channels available for borrowing, a channel is borrowed from the cell with the greatest number of channels available for borrowing. This is an improved version of the SBR strategy which takes channel blocking into account when selecting a candidate channel for borrowing. This scheme tried to minimize the future call blocking probability in the cell that is most affected by the channel borrowing This scheme provides for the transfer of a call from a borrowed channel to a nominal channel whenever a nominal channel becomes available Instead of trying to optimize when borrowing, this algorithm selects the first candidate channel it finds EE of NIU Chih-Cheng Tseng 6

7 Dynamic Channel Allocation (DCA) In DCA schemes, all channels are kept in a central pool and are assigned dynamically to new calls. After each call is completed, the channel is returned to the central pool. Select the most appropriate channel for any call based on current allocation and current traffic, with the aim of minimizing the interference. EE of NIU Chih-Cheng Tseng 7

8 Classification of DCA Schemes DCA schemes can be centralized or distributed The centralized DCA scheme involves a single controller selecting a channel for each cell Centralized DCA schemes can theoretically provide the best performance. Provide a useful benchmark to compare practical decentralized DCA schemes The enormous amount of computation and communication among BSs leads to excessive system latencies and renders centralized DCA schemes impractical. The distributed DCA scheme involves a number of controllers scattered across the network (MSCs) EE of NIU Chih-Cheng Tseng 8

9 Centralized DCA Schemes (1/2) First Available (FA) The first available channel within the reuse distance encountered during a channel search is assigned to the call. Locally Optimized Dynamic Assignment (LODA) Based on the future blocking probability in the neighborhood of the cell where a call is initiated. Selection with Maximum Usage on the Reuse Ring (RING) A channel is selected which is in use in the most cells in the cochannel set. If none is available, the selection is made based on the FA scheme. EE of NIU Chih-Cheng Tseng 9

10 Centralized DCA Schemes (2/2) Mean Square (MSQ) The MSQ scheme selects the available channel that minimizes the mean square of the distance among the cells using the same channel. EE of NIU Chih-Cheng Tseng 10

11 Distributed DCA Schemes Assigns traffic channels based on one of the three parameters: Co-channel distance select channels that neighboring co-channel cells are not using sometimes adjacent channel interference also taken into account Signal strength measurement select channel that the anticipated Co-Channel Interference Ratio (CCIR) is above a threshold Signal to noise ratio (SNR) satisfy desired SNR ratio EE of NIU Chih-Cheng Tseng 11

12 Comparison between FCA and DCA (1/2) FCA Performs better under heavy traffic Low flexibility in channel assignment Maximum channel reusability Sensitive to time and spatial changes Unstable grade of service per cell in an interference cell group High forced call termination probability Suitable for large cell environment Low flexibility DCA Performs better under light/moderate traffic Flexible channel allocation Not always maximum channel reusability Insensitive to time and time spatial changes Stable grade of service per cell in an interference cell group Low to moderate forced call termination probability Suitable in microcellular environment High flexibility EE of NIU Chih-Cheng Tseng 12

13 Comparison between FCA and DCA (2/2) FCA Radio equipment covers all channels assigned to the cell Independent channel control Low computational effort Low call set up delay Low implementation complexity Complex, labor intensive frequency planning Low signaling load Centralized control DCA Radio equipment covers the temporary channel assigned to the cell Fully centralized to fully distributed control dependent on the scheme High computational effort Moderate to high call set up delay Moderate to high implementation complexity No frequency planning Moderate to high signaling load Centralized, distributed control depending on the scheme EE of NIU Chih-Cheng Tseng 13

14 Other Channel Allocation Schemes Based on different criterion being used as a potential way of optimizing the performance. Many other channel allocation schemes have been suggested Hybrid Channel Allocation (HCA) Flexible Channel Allocation Handoff Channel Allocation EE of NIU Chih-Cheng Tseng 14

15 Hybrid Channel Allocation (HCA) (1/2) HCA schemes are the combination of both FCA and DCA techniques. The total number of channels available for service is divided into fixed and dynamic sets The fixed set contains a number of nominal channels that are assigned to cells as in the FCA schemes and, in all cases, are to be preferred for use in their respective cells. The dynamic set is shared by all users in the system to increase flexibility. EE of NIU Chih-Cheng Tseng 15

16 Hybrid Channel Allocation (HCA) (2/2) Request for a channel from the dynamic set is initiated only when the cell has exhausted using all its channels from the fixed set. Optimal ratio: ratio of number of fixed and dynamic channels 3:1 (fixed to dynamic), provides better service than fixed scheme when traffic load less than 50% (105 versus 35 channels). Beyond 50%, fixed scheme perform better. EE of NIU Chih-Cheng Tseng 16

17 Flexible Channel Allocation (FCA) Similar to hybrid scheme with channels divided into fixed and flexible (emergency) sets Fixed sets used to handle lighter loads Variations in traffic (peaks in time and space) are needed to schedule emergency channels Two types: Scheduled and Predictive Scheduled: Prior estimate is done about traffic change Predictive: Traffic intensity and blocking probability is monitored in each cell all the time EE of NIU Chih-Cheng Tseng 17

18 Channel Allocation in One-Dimensional Systems Call initiated a b c d e Reuse distance D If a new call is initiated in cell 1, with the current allocation of channels a, b, c, d, e as shown. It is better to assign channel e to mobile in cell 1. Assuming that as MS in cell 1 moves to cell 2, MS in cell 7 moves to cell 8. EE of NIU Chih-Cheng Tseng 18

19 Reuse Partitioning-Based Channel Allocation Each cell is divided into concentric and equal-size zones. Inner zone being closer to BS would require lesser power to attain a desired channel MS with the best SIR are assigned a group of channels that have the smallest reuse distance. EE of NIU Chih-Cheng Tseng 19

20 Overlapped Cells-Based Channel Allocation Splitting cell into smaller cells (pico, micro cells), to handle increased traffic. Highly moving MSs are assigned channels from the cell Cell 2 3 MS with low mobility are assigned to microor pico-cells 4 Microcell EE of NIU Chih-Cheng Tseng 20

21 Use of Overlapped Cell Areas If MS in shaded area does not find a free channel in cell A, it can take the free channel from cells B or C. If a new call in cell A can not find a free channel, some of the existing connections in the shaded area of cell A will be forced to handoff to cells B or C. C A B EE of NIU Chih-Cheng Tseng 21

22 System Modeling The following assumptions are made to obtain an approximate model of system All MSs are assumed to be uniformly distributed through the cell. Each MS moves at a random speed and to an arbitrary random direction. The arrival rate of originating call is given by O. The arrival rate of handoff call is given by H. The call service rate is given by. EE of NIU Chih-Cheng Tseng 22

23 A Generic System Model for A Cell with S Channels H O O : originating call rate H : handoff call rate S Channels : service rate EE of NIU Chih-Cheng Tseng 23

24 Analysis Model The following parameters are defined in the analysis model P(i): the probability of i channels to be busy O : the arrival rate of an originating call in the cell H : the arrival rate of a handoff call from neighboring cells B O : the blocking probability of originating calls S: the total number of channels allocated to a cell : the call service rate c : the average call duration EE of NIU Chih-Cheng Tseng 24

25 Basic Modeling (1/3) The states of a cell can be represented by (S+1) states Markov model. A transition diagram of M/M/S/S model as shown below. O + H O + H O + H O + H 0 i-1 i i (i+1) S S State transition diagram EE of NIU Chih-Cheng Tseng 25

26 Basic Modeling (2/3) The state equilibrium equation for state i can be given as ( ) O H P i P( i 1), 0 i S i The sum of all states must to be equal to one: The steady-state probability P(i) is easily found as follows: i ( O H) P( i) P(0), 0 i S where i i! P(0) S i0 ( O H) i i! Pi ( ) 1 i 1 EE of NIU Chih-Cheng Tseng 26

27 Basic Modeling (3/3) The blocking probability for an originating call when all S channels are busy, can be expressed by: B O P( S) S i0 ( O H) S S! The blocking probability of a handoff request at this state is also the forced termination probability of a handoff call is B H =B O This is Erlang B formula covered in Chapter 5 O i i! H S i EE of NIU Chih-Cheng Tseng 27

28 Channel Partitioning under Integrated Traffic Originating calls Blocked Forced Termination of Hand off calls X Transmission delay of originating calls Transmission Delay of Hand off calls A channel allocation scheme that provides different level of priority to real-time and handoff traffic Analytical model to evaluate system performance A novel recursive method leads to optimal system design EE of NIU Chih-Cheng Tseng 28

29 Modeling for Channel Reservation Why should we provide a higher priority to handoff calls? From users view, the dropping of handoff calls is more serious and irritating than the blocking of originating calls How to provide a higher priority to handoff calls? One approach is reserve S R channels exclusively for handoff calls among the S channels in a cell EE of NIU Chih-Cheng Tseng 29

30 System Model with Reserved Channels System model with reserved channels for handoff No blocking of originating calls till less than S C channels are busy S H. S R Reserved only for handoff calls S C.. O 2 1 Channels EE of NIU Chih-Cheng Tseng 30

31 Analytical Model (1/3) The state balance equations can be obtained as and S i0 Pi ( ) 1. ip( i) ( O H ) P( i 1), 0 i SC ip( i) HP( i 1), SC i S O + H 0 O + H S C H S C (S C +1) H S S State transition diagram EE of NIU Chih-Cheng Tseng 31

32 Analytical Model (2/3) The steady-state probability P(i) can be obtained as Pi () i ( O H) P(0), 0 i S i C i! SC isc ( O H ) H P(0), S i C i S i! where P(0) ( ) ( ) SC i S SC is O H O H H i i i0 i! is 1 i! C C 1 EE of NIU Chih-Cheng Tseng 32

33 Analytical Model (3/3) The blocking probability B O for an originating call is given by (at least S C channels busy): B O S P() i is The blocking probability B H for a handoff call is (all S channels busy) or forced termination probability of handoff call is: C SSC O H H BH P( S) P(0) S S! C S EE of NIU Chih-Cheng Tseng 33

34 Homework P8.2 P8.11 P8.18 P8.20 EE of NIU Chih-Cheng Tseng 34