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Wireless Communications: System Design. Dr. Mustafa Shakir. Issues in cell to cell moving. What are different levels of handoff. (1) Intra Cell (2) Inter cell (3) Inter system Importance of handoff.
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Wireless Communications:System Design Dr. Mustafa Shakir
Issues in cell to cell moving • What are different levels of handoff. • (1) Intra Cell (2) Inter cell (3) Inter system • Importance of handoff. • When no priority to handoff call blocking would be equal for call initiation and call handoff. • There are two strategies to give a priority to handoff. • (1) Guard Channel: 100% guaranty for successful handoff but It will cause low trunking efficiency. • (2) Queuing Of Handoff Request: There can be unsuccessful handoffs due to long delay in queue. --“Probability of forced termination” decreases at the cost of reduced Total Carried Traffic. -- Queuing is possible because of the time available between the Threshold power level and the Hand off power level.
Solution For More Handoffs • Umbrella Cell Approach: • Micro cells inside A macro cell. ---- Macro cell is defined by high power and lengthy tower. ---- Micro cells are defined inside the macro cell with less power and less height towers. ---- High speed MS are handled by macro cell and low speed subscribers are handled by micro cells. ---- This strategy increases the no of capacity channels per unit area and decreases the no of handoffs.
Interference • It is a major limiting factor in the performance of cellular radio systems. (In comparison with wired comm. Systems, the amount and sources of interferences in Wireless Systems are greater.) • Creates bottleneck in increasing capacity • Sources of interference are: 1. Mobile Stations 2. Neighboring Cells 3. The same frequency cells 4. Non-cellular signals in the same spectrum • Interference in Voice Channels: Cross-Talk • Urban areas usually have more interference, because of: a)Greater RF Noise Floor, b) More Number of Mobiles
Major Types Of Interference • Co-Channel Interference (CCI) • Adjacent Channel Interference (ACI) • Other services: like a competitor cellular service in the same area • The cells that use the same set of frequencies are called co-channel cells. • The interference between signals from these cells is called Co-Channel Interference (CCI). • Cannot be controlled by increasing RF power. Rather, this will increase CCI. • Depends on minimum distance between co-channel cells. 1) Co-Channel Interference and System Capacity
The yellow cells use the same set of frequency channels, and hence, interfere with each other. In case of N=7, there are 6 first-layer co-channels. • In constant cell size and RF power, CCI is a function of Distance between the co-channel cells(D), and the size of each cell (R). • Increasing ratio D/R, CCI decreases. • Define Channel Reuse Ratio = Q = D/R
Signal-to-interference ratio • S is the power of the signal of interest and Ik is the power of kth interference. • The signal strength at distance d from a source is • That is, received signal power is inversely related to nth power of the distance. • where n = path loss exponent
R D • For hexagonal geometry, D/R can be calculated: • Smaller Q provides larger capacity, since that would mean smaller N. (Capacity 1/N). • Larger Q improves quality, owing to less CCI. • for N=3, Q=3, N=7, Q=4.58, N=12, Q=6, N=13, Q=6.24
Then we can express the SIR in terms of distance • where the denominator represents the users in neighboring clusters using the same channel. • Let D k=D be the distance between cell centers. Then • Note how S/I improves with the frequency reuse N. • Analog systems: U.S. AMPS required S/I ~= 18dB For n = 4, the reuse factor for AMPS is N 6.49, so N =7. • Now, let us consider the worst case for a cluster size of N= 7. The mobile is at the edge of the cell. Express C/I as a function of actual distances.
Worst Case Design Worst case carrier-to-interference ratio Let n = 4 and D/R = q, Let reuse N = 7, then Compute C/I and get C/I = 17.3 dB
If S/I min = 15 dB, what is the capacity for n = 4, n = 3 (a) n = 4, N = 7 N =7 can be used (b) n = 3, N = 7 E
(2) Adjacent Channel Interference • Interference from channels that are adjacent in frequency, • The primary reason for that is Imperfect Receiver Filters which cause the adjacent channel energy to leak into your spectrum. • Problem is severer if the user of adjacent channel is in close proximity. Near-Far Effect • Near-Far Effect: The other transmitter(who may or may not be of the same type) captures the receiver of the subscriber. • Also, when a Mobile Station close to the Base Station transmits on a channel close to the one being used by a weaker mobile: The BS faces difficulty in discriminating the desired mobile user from the “bleed over” of the adjacent channel mobile.
Near-Far Effect: Case 1 Unintended Tx Strong “bleed over” BS as Tx Mobile User Rx Weaker signal The Mobile receiver is captured by the unintended, unknown transmitter, instead of the desired base station
Near-Far Effect: Case 2 BS as Rx Weaker signal Strong “bleed over” Desired Mobile Tx Adjacent Channel Mobile Tx The Base Station faces difficulty in recognizing the actual mobile user, when the adjacent channel bleed over is too high.
Minimization of ACI • Careful Filtering ---- min. leakage or sharp transition • Better Channel Assignment Strategy • Channels in a cell need not be adjacent: For channels within a cell, Keep frequency separation as large as possible. • Sequentially assigning cells the successive frequency channels. • Also, secondary level of interference can be reduced by not assigning adjacent channels to neighboring cells. • For tolerable ACI, we either need to increase the frequency separation or reduce the pass band BW.
What is power control ? • Both the BS and MS transmitter powers are adjusted dynamically over a wide range. • Typical cellular systems adjust their transmitter powers based on received signal strength. TYPES OF POWER CONTROL • Open Loop Power Control It depends solely on mobile unit, not as accurate as closed loop, but can react quicker to fluctuation in signal strength. In this there is no feed back from BS. • Closed Loop Power Control In this BS makes power adjustment decisions and communicates to mobile on control channels
Why power control ? • Near-far effect • Mechanism to compensate for “channel fading” • Interference reduction, • prolong battery life
Improving Capacity in Cellular Systems Cost of a cellular network is proportional to the number of Base Stations. The income is proportional to the number of users. Ways to increase capacity: New spectrum – expensive. PCS bands were sold for $20B. Architectural approaches: cell splitting, cell sectoring, microcell zones. Dynamic allocation of channels according to load in the cell (non-uniform distribution of channels). Improve access technologies.
Cell Splitting Cell Splitting is the process of subdividing the congested cell into smaller cells (microcells), Each with its own base station and a corresponding reduction in antenna height and transmitter power. Cell Splitting increases the capacity since number of clusters over coverage region would be increased thus increasing the number of channels. New cells added having smaller radius than original cells and by installing these smaller cells (called microcells ) between existing cells , capacity increases due to additional number of channels per unit area.
An Example The area covered by a circle with radius R is four times the area covered by the circle with radius R/2 The number of cells is increased four times The number of clusters the number of channels and the capacity in the coverage area are increased Cell Splitting does not change the co-channel re-use ratio Q =D/R
Transmit Power New cells are smaller, so the transmit power of the new cells must be reduced How to determine the transmit power? The transmit power of the new cells can be found by examining the received power at the new and old cell boundaries and setting them equal Pr(at the old cell boundary) is proportional to Pt1 * R-n Pr(at the new cell boundary) is proportional to Pt2 * (R/2)-n
Transmit Power Take n=4, we get Pt2 = Pt1/16 We find that the transmit power must be reduced by 16 times or 12 dB in order to use the microcells to cover the original area. While maintaining the same S/I.
Application of cell splitting When there are two cell sizes one cant simply use the same transmit power for all cells. If larger transmit power used for all cells some smaller cells would not be sufficiently separated from co channel cells. Using smaller Pt the larger cells might be left unserved. So old channel broken to two channel groups corresponding to smaller and larger cell reuse. Larger cell for less frequent hand off. Antenna down tilting focusing radiated energy from base station to the ground to limit radio coverage of newly formed cells.
Cell Sectoring Co channel interference may be reduced by replacing omni directional antenna by several directional antennas. Given cell will receive interference and would transmit with fraction of available co channel cells. Each sector uses directional antenna at the B.S and assigned a set of channels. Partitioning into three 120 deg. sectors or six 60 deg. sectors. Amount of CCI reduced by number of sectors. Reduced Tx Power… 32
Effects of Sectoring Reduction in interference offered by sectoring would enable to reduce the cluster size N and additional degree of freedom in channel assignment. Increased number of antennas with shrinking cluster size and decrease in trunking efficiency due to channel sectoring at base station. Since sectoring reduces the coverage area of a particular group of channels the number of handoffs increases Available channels subdivided and dedicated to a specific antenna thus making up of several smaller pools contributing to decrease in trunking efficiency.
Repeaters To provide dedicated coverage for hard to reach areas Radio retransmitters for range extension. Upon receiving signals from base station forward link the repeater amplifies and reradiates the base station signals to specific coverage region. In building wireless coverage by installing Distributed Antenna Systems. Repeaters must be provisioned to match the available capacity from the serving base station.
Microcell Zone The increased number of handoff as a result of sectoring would result in an increased load on switching and control link elements of the mobile system. Division into microcell zones and each of the three are connected to a single base station and share the same radio equipment. Zones connected by a coaxial cable, fiber optic cable or microwave link to the base station. Handoff not required while mobile travels between zones within cell. Channel switching and a channel active only within zone of travelling.
Scenario In Micro cell zone scenario each hexagon represents a zone while the group of three hexagons represent a cell. Zone Radius Rz is one hexagon radius. Capacity of Microcell is directly related to distance betw. Cochannel cells and not zones. No handoffs is required at the MSC. The base station radiation is localized and interference is reduced
Trunking and Grade of Service (GOS) • Trunking • A means for providing access to users on demand from available pool of channels. • With trunking, a small number of channels can accommodate large number of random users. • Telephone companies use trunking theory to determine number of circuits required. • Trunking theory is about how a population can be handled by a limited number of servers.
Terminologies • Erlang: • One Erlang: When a circuit is busy for one hour it handled a traffic of one erlang. • Grade of Service (GOS): probability that a call is blocked (or delayed). • Set-Up Time: Traffic intensity is measured in Erlangs: • time to allocate a channel. • Blocked Call: Call that cannot be completed at time of request due to congestion. Also referred to as Lost Call.
Terminologies Contd. • Holding Time: (H) Average duration of typical call. • Load: Traffic intensity across the whole system. • Request Rate: () Average number of call requests per unit time.
Traffic Measurement (Erlangs) • Traffic per user Au = H where is the request rate and H is the holding time. • For U users the load is A= U Au • If traffic is trunked in C channels, then the traffic intensity per channel is Ac= UAu /C • Erlang B:
Example • Example : An urban area has 2 million residents. Three competing cellular systems provide service: • System A 394 cells x 19 channels/cell. • System B 98 cells x 57 channels/cell. • System C 49 cells x 100 channels/cell. • For each user = 2 calls/hr, H = 3min, GOS = 2% blocking. Find the number of users that can be supported by each system. Note that these are not simultaneous users. • System A: • Au= H = 2 x 3/60 = 0.1 Erlangs. • From the curve for GOS = 0.02 and C = 19 => A = 12 Er. • Users per cell (U) = A/Au = 12/0.1 = 120 users • 120 users/cell x 394 cells = 47,280 users can be served. • Market penetration = 2.36%.
No. of subscribers • System C: • Prob Blocking = 2% = 0.02 • C =100 • Au = H = 2 x 3/60 = 0.1 Erlangs. • From table, A = 88 Erlangs. • Users per cell U = A/Au = 88/0.1 =880 users • 880 users/cell x 49 cells = 43,120. • Market penetration = 2.156%. • System B: • Prob Blocking = 2% = 0.02 • C =57 • Au = H = 2 x 3/60 = 0.1 Erlangs. • From table, A = 45 Erlangs • Users per cell U = A/Au = 45/0.1 = 450 users • 450 users/cell x 98 cells = 44,100. • Market penetration = 2.21%.
