Mobile Cellular Systems SLIDE #2.1 MOBILE COMPUTING NIT Agartala, Dept of CSE Jan-May,2012 ALAK ROY. Assistant Professor Dept. of CSE NIT Agartala Email-alakroy.nerist@gmail.com
What we will learn in this topic: Cellular Concept Cellular Architecture Frequency Reuse Multiple Access Methods FDMA, TDMA, and CDMA In particular, we focus on CDMA. 2
Cellular Implementations (Gs) 1G: Analog cellular systems (450-900 MHz) Frequency shift keying for signaling FDMA for spectrum sharing NMT- Nordic Mobile Telephone (Europe), AMPS Advanced Mobile Phone System (US) 2G: Digital cellular systems (900, 1800 MHz), < 14.4 kbps TDMA/CDMA for spectrum sharing Circuit Switching GSM-Global System for Mobile communications (Europe), IS-136 Interim Standard 136 (US), PDC Pacific Digital Cellular (Japan), CDMAOne US, Korea, Asia. 3
Cellular Implementations (Gs) 2.5G: Packet switching extensions, 14.4 144 kbps Digital: GSM to GPRS (General Packet Radio Service) Analog: AMPS to CDPD (Cellular Digital Packet Data) CDMA2000 1X 3G: High speed (384 kbps to 2 Mbps), data and Internet services IMT-2000 (International Mobile Telecommunications) CDMA2000 1X EV-DO, EV-DV, 3X, W-CDMA etc. 4
Cellular Implementations(Gs) 4G: High speed (20 100 Mbps in mobile mode), Conversed data and voice over IP, Seamless and smooth handoff. OFDM (Orthogonal FDM) and MC-CDMA (Multi Carrier CDMA) Hybrid Architecture: integration of Wireless LAN (WiFi, Bluetooth) and wide area 5
1G Cellular Systems Many Different Standards: AMPS (US) NMT (Northern Europe) TACS (Europe) NTT (Japan) many others... Spectrum around 800 and 900 MHz 6
Frequency Division Duplex (FDD) Forward Link mobile Reverse Link base station Two separate frequency bands are used for forward and reverse links. Typically, 25 MHz in each direction. AMPS: 824-849 MHz (forward or downlink) 869-894 MHz (reverse or uplink) 7
Frequency Division Multiple Access (FDMA) The spectrum of each link (forward or reverse) is further divided into frequency bands Each station assigned fixed frequency band frequency bands idle idle idle 8
Number of User Channels in AMPS Bandwidth allocated to each user in each link (forward or reverse) is 30 KHz. No. of user channels = Total bandwidth / user bandwidth = 25 MHz / 30 khz = 833 Is it enough? 9
Frequency Reuse Radio coverage, called a cell. f The same frequency can be reused in different cells, if they are far away from each other f 10
Cellular Architecture MS MS Mobile Station BSC Base Station Controller MSC Mobile Switching Center PSTN Public Switched Telephone Network BSC MSC PSTN segmentation of the area into cells 11
Geometric Representation Cells are commonly represented by hexagons. Why hexagon? How about circle? How about square, or triangle? 12
Hexagonal Cells 13
Channel Reuse The total number of channels are divided into K groups. K is called reuse factor or cluster size. Each cell is assigned one of the groups. The same group can be reused by two different cells provided that they are sufficiently far apart. 14
Example: K = 7 15
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Reuse Distance How far apart can two users share the same channel? It depends on whether signal quality is acceptable or not. The larger the distance between the two users, the better the signal quality. How to measure signal quality? 17
Nyquist Bandwidth (for noiseless channel) For binary signals (two voltage levels) C = 2B With multilevel signaling C = 2B log 2 M M = number of discrete signal or voltage levels 18
Signal Quality The signal quality depends on the ratio between signal power and interference (noise) power. S I i S I i Interference from the i-th interfering BS. This is called signal-to- noise (interference) ratio (SNR or SIR). 19
Signal-to-Noise Ratio Ratio of the power in a signal to the power contained in the noise that s present at a particular point in the transmission Typically measured at a receiver Signal-to-noise ratio (SNR, or S/N) ( SNR) db 10log10 signal power noise power A high SNR means a high-quality signal, low number of required intermediate repeaters SNR sets upper bound on achievable data rate 20
Shannon Capacity Formula Equation: C B log 2 1 SNR Represents the theoretical maximum data rate that can be achieved for noisy channel In practice, only much lower rates are achieved Formula assumes white noise (thermal noise) Impulse noise is not accounted for Attenuation distortion or delay distortion not accounted for 21
Example of Nyquist and Shannon Formulations Spectrum of a channel between 3 MHz and 4 MHz ; SNR db = 24 db B SNR SNR 4 MHz 3 MHz db 24 db 10log 251 Using Shannon s formula 1MHz 10 SNR C 10 6 log 2 6 1 251 10 8 8Mbps 22
Example of Nyquist and Shannon Formulations How many signaling levels are required? C 2B 8 10 4 M 6 log 16 log 2 2 M 2 M 10 6 log 2 M 23
HANDOFF IN CELLULAR SYSTEMS 24
Handoff in Cellular Systems Handoffs occur when a mobile host starts communicating with a new base station. Continuation of an active call when the mobile is crossing cell boundaries Parameters: Minimize Forced Termination Prob Minimize Call Blocking Prob. More handoffs means more FTP 25
Handoff Decisions Usually based on received signal strengths (RSS) from Current BS and neighboring BSs. Handoff occurs when the signal from the serving BS is weak while the signal from a neighboring BS is strong. The received signal strength is averaged over time using an averaging window to remove momentary fading due to geographical and environmental factors Ping Pong Effect Signal strength fluctuates randomly Several handoffs occur back and forth between two BSs. 26
Sample RSS 27
Traditional Handoff Algorithms Choose B new (i.e. handoff from B old to B new ) if 1. P new > P old, Relative Signal Strength: The RSSs are measured over time and the BS with strongest signal is chosen to handoff. Due to signal fluctuations, several handoffs can be requested while B old s RSS is still sufficient to serve the Mobile Station (MS). So, more Ping-Pong effect and more FTP. 2. P new > P old and P old < T, Relative Signal Strength with Threshold: Introduces a threshold value to overcome the ping-pong effect. Handoff process is initiated if B new s RSS is stronger than B old s RSS and B old s RSS is lower than the threshold (T). ** Here, P new : B new s RSS by MS and P old : B old s RSS by MS 28
Traditional Handoff Algorithms 3. P new > P old + H, Relative signal strength with Hysteresis: Handoff process is initiated when the B new s RSS exceeds the B old s RSS by the hysteresis value H. 4. P new > P old + H and P old < T, Relative Signal Strength with Hysteresis and Threshold: Combines both the threshold and hysteresis values concepts to come with a technique with minimum number of handoffs. The handoff is requested when the B old s RSS is below the threshold (T) and B new s RSS is stronger than B old s RSS by the hysteresis value H. All the above techniques should ensure initiation of handoff before the B old s RSS falls below Receiver Threshold (minimum acceptable RSS for call continuation). Otherwise ongoing call is dropped. A multi-level thresholds based algo which assigns diff thresholds to the users according to their speed. 29
Example Receiver Threshold 30
Handoff What happens when a user is mobile? - Especially when crossing a cell boundary while continuing the call. Handoff strategy is invoked. Find a new base station, allocating new voice and control channel of the new BS Process handoff higher priority over new call invocation, when allocating unused channels in a cell Optimal received signal level to initiate a handoff. Delta =P handoff P usable, should not be too small or large Large -> unnecessary handoff, small->insufficient time to complete handoff before a call is lost due to weak signal Dropped call also due to excessive delay by MSC (loads, no channels free in nearby cell) 31
Who and When Who initiates handoff Network Controlled Handoff (NCHO): The network determines RSSI based on the locator receiver signal strength info from all BSs. MSC decides whether handoff is required or not. Used in AMPS. Load on network is high. Typically needs 5-10 secs. Mobile Assisted Handoff (MAHO) : Mobile helps the tower, mobile periodically measures the received power from surrounding BSs and report to serving BS. Handoff initiated when the recv power from a neighboring BS exceeds the power recv from the current BS (by a certain amt or period), MSC decides when to handoff. Used in GSM. Typically needs ~ 1sec. Mobile Controlled Handoff (MCHO): MS determines handoff. MS make necessary measurements and if the RSS of a surrounding BS exceeds a threshold, it initiates handoff. Used in DECT. Typically needs ~ 100ms. 32
Who and When When to initiate handoff (the reasons for handoff) When the mean signal (over some predetermined time) from the current BS is below some threshold (or by using any one of the previous 4 methods). That is to avoid termination of calls. To release some channels in the current cell, so that new calls can be accommodated channel used by a mobile affected by another phone using the same channel in a different cell, then the call is transferred to a different channel of the same cell or on a different channel in another cell to avoid interference. To reduce potential interference to other cells/users in Micro cell/ Macro cell based system 33
Types of Handoff Hard handoff Mobile user is passed between disjoint towers that assign different frequency or adapt different air-interface technology. Communication channel is released first and the new channel is acquired later from the neighboring cell. Service disruption, reduction in QoS Used by systems which use TDMA and FDMA such as GSM and GPRS Soft handoff Mobile user communicates to two or more towers simultaneously and the signal is treated as a multipath signal Can establish multiple connections with neighboring cells. Used in CDMA systems, where the cells use same frequency band using diff code words. Each MS maintains an active set of BSs, where BSs are added and removed based on 2 RSS thresholds. So addition or removal of a BS to the active set is called Soft handoff. Used by IS-95 and WCDMA Without handoff 34
Horizontal handoff Types of Handoff Mobile user is passed between towers of homogeneous networks Example: handoff involving two GSM cells. Vertical handoff Handoff between towers of different types of networks. Handoff in heterogeneous networks. More complex compared to Horizontal handoff. Example: handoff from one GSM based cell to one WCDMA-based cell. 35
High priority for Handoff In non-prioritization schemes, new calls and handoff calls are treated the same way. Idle channels of BS are assigned due to FCFS basis. Does not provide lower forced termination prob In order to provide lower forced termination prob, prioritization schemes assigns more channels to the handoff calls. Two prioritization schemes are: Guard Channels (GC) and Queuing Handoff calls (QHC). 36
Guard Channels(GC) Reserves some fixed or adaptively changing number of channels for handoff calls only. The rest of the channels are used by new and handoff calls. Forced termination prob is decreased. Cost is an increase in call blocking prob, decrease in total carried traffic. BSs can get number of MSs in pre handover zone (PHZ) from neighboring BSs and accordingly reserve that many number of guard channels. Assign number of channels adaptively. When FTP exceeds a predefined limit, the guard channel no is increased. The no of guard channels is decreased in case BS does not use reserved guard channels significantly. 37
Queuing Handoff Calls (QHC) Queues the handoff calls when all of the channels are occupied in a target BS. (Queuing does not guarantee a zero FTP, since large delays will cause RSS to drop below recvr thres) When a channel is released, it is assigned to one of the handoff calls in the queue. A new call request is assigned a channel if the queue is empty and if there is at least one free channel in the BS. Time interval between handoff initiation and receiver threshold makes it possible to use queuing handoff calls. QHC can be used with or without guard channel scheme. QHC can be timer based: when a channel is released at BS, a timer is started. If a handoff request is done before the timer expires, channel is assigned to it. Otherwise, channel can be assigned to new or handoff calls depending on their arrival order. 38
Queuing Handoff Calls QHC can be Measurement based (MBPS): Handoff calls are assigned priority dynamically based on the power level they have. Calls with power level close to receiver threshold have the highest priority. Provided better results from FCFS basis. QHC can be Most Critical First based (MCF): determines the first handoff call that will be cut off and assigns the first released channel to that call. Use simple radio measurements to predict the first cut off call. 39
Queuing Handoff Calls QHC can be with guard channels: Both new calls and Handoff calls are queued. A number of guard channels are reserved for handoff calls. When new calls are congested, a channel from guard channels is used if it is available. 40
Other practical problems with handoff High speed vehicles can cross many small cells in a short time (due to wide range of mobile velocities). Umbrella cell which is co-located with some smaller microcells. Large cell with a powerful tower to handle high speed vehicles (Macrocell/Microcell concepts). No of handoff is minimized for high speed devices and provide additional microcell channels for pedestrian users. Ping-Pong Effect 41
Other practical problems with handoff Another problem is called cell dragging. Happens when the user (in LOS) moves slowly away from the cell and the tower didn t recognize it due to strong average signal. (RSS at B old may be above handoff threshold, so no HO) Creates interference and traffic management problem because users has meanwhile traveled deep within the neighboring cell. 42
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