Wireless and Telecommunication System

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This section covers the history and evolution of telecommunication, the generations of mobile/wireless systems and future trends, and the common transmission media used to carry telecommunication signals.

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History & Evolution of Telecommunication Telecommunication evolved from the electrical telegraph (1830s-40s) and telephone (1876) through analog trunk/microwave networks, to digital switching and transmission, and finally to today's packet-based, all-IP wireless/wireline convergence.

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Key milestones: telegraph → telephone → analog radio/microwave links → satellite communication → digital PCM transmission → cellular mobile systems → internet/packet-switched data → broadband wireless and fibre-optic backbones.

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Generations of Mobile/Wireless Systems Generation Key Characteristics 1G Analog cellular (e.g.

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AMPS); voice only, frequency modulation, no encryption, low capacity 2G Digital cellular (e.g.

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GSM, CDMA); voice + low-rate data (SMS), circuit switching, improved spectral efficiency and security 3G Packet-switched data added (e.g.

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UMTS/WCDMA, CDMA2000); higher data rates, mobile internet, video calling 4G All-IP, OFDMA-based (LTE/LTE-Advanced); high-speed mobile broadband, low latency 5G Enhanced mobile broadband, massive machine-type communication, ultra-reliable low-latency communication (URLLC); uses higher frequency bands (mmWave) and massive MIMO/beamforming Future Trends Trends include convergence of fixed and mobile networks, Internet of Things (IoT) connectivity, network function virtualization (NFV)/software-defined networking (SDN), and early research toward 6G (terahertz bands, AI-native networks).

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Telecommunication Transmission Media Medium Key Features Twisted pair copper cable Low cost, limited bandwidth and distance; widely used for local loop/telephone access Coaxial cable Higher bandwidth and better shielding than twisted pair; used for cable TV/broadband access Optical fibre Very high bandwidth, low attenuation, immune to electromagnetic interference; backbone of modern long-haul and access networks Wireless/radio (terrestrial & satellite) No physical medium needed; supports mobility; subject to propagation loss, fading, interference, and regulatory spectrum allocation

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This section covers radio propagation models (free space, reflection/diffraction/scattering, multipath fading), the cellular concept (frequency reuse, channel assignment, handover), and techniques for increasing coverage and capacity such as cell splitting and sectoring.

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Propagation Models Model/Mechanism Description Free space propagation model Predicts received power assuming a single, unobstructed line-of-sight path; received power decreases as 1/d² (inverse-square law) with distance d Reflection Occurs when a radio wave strikes a smooth surface much larger than its wavelength, producing a reflected wave Diffraction Occurs when a wave bends around an obstacle's edge, allowing signals to reach receivers even without a direct line of sight (shadow region) Scattering Occurs when a wave strikes an object with dimensions comparable to or smaller than the wavelength (e.g. rough surfaces, foliage), spreading energy in many directions Multipath Fading Small-scale multipath propagation: rapid fluctuations in received signal amplitude/phase over short distances or time intervals, caused by the constructive/destructive combination of multiple delayed copies of the transmitted signal.

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Rayleigh fading model: a statistical model for small-scale fading used when there is no dominant (line-of- sight) signal component — the received envelope follows a Rayleigh distribution; when a strong line-of-sight component exists, a Rician distribution is used instead.

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Cellular Concept & Frequency Reuse The cellular concept divides a service area into smaller cells, each served by a base station using a fraction of the total available channels, allowing the same frequencies to be reused in non-adjacent cells (frequency reuse) to serve a much larger number of users with limited spectrum.

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Frequency reuse strategies group cells into clusters (with a cluster size N); the same channel set is reused in the corresponding cell of every cluster, with co-channel cells separated by a reuse distance to keep interference acceptable.

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Channel assignment strategies (fixed or dynamic) determine how the available channels are allocated among cells/calls; handover (handoff) is the process of transferring an ongoing call from one cell/base station to another as a mobile user moves, without dropping the call.

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Coverage, Capacity & Cell Splitting/Sectoring Cell splitting subdivides a congested cell into smaller cells, each with its own base station at reduced transmit power, increasing capacity (more channels per unit area) as user density grows.

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Cell sectoring replaces an omnidirectional cell-site antenna with several directional antennas (e.g.

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3 sectors of 120° each), reducing co-channel interference and improving the frequency reuse factor without increasing the number of cells.

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This section covers techniques used to combat wireless channel impairments — equalization and diversity — along with spread spectrum modulation and the multiple access techniques used to share a common wireless channel among many users.

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Equalization & Diversity Techniques Technique Purpose Equalization Compensates for inter-symbol interference (ISI) caused by a frequency-selective (multipath) channel, by applying an inverse filter that approximately reverses the channel's distorting effect Diversity techniques Combat fading by providing the receiver with multiple, ideally independently-faded, copies of the same signal so that not all copies are in a deep fade simultaneously Space diversity Uses multiple physically separated antennas to obtain independently-fading signal copies Time diversity Transmits the same information at different time instants (e.g. via interleaving/coding) so fades are decorrelated in time Frequency diversity Transmits the same information over different frequency channels, since fading is frequency- dependent Spread Spectrum Modulation Spread spectrum modulation spreads a narrowband signal's energy over a much wider bandwidth than strictly necessary, using a code independent of the data, giving robustness to interference/jamming, low probability of interception, and the ability for multiple users to share the same band.

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Direct Sequence Spread Spectrum (DSSS): multiplies the data signal by a high-rate pseudo-random noise (PN) code to spread its spectrum.

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Frequency Hopping Spread Spectrum (FHSS): rapidly switches (hops) the carrier frequency among many channels according to a pseudo-random sequence known to transmitter and receiver.

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Multiple Access Techniques Technique Principle FDMA (Frequency Division Multiple Access) Each user is assigned a separate, dedicated frequency channel/sub-band TDMA (Time Division Multiple Access) Each user is assigned a separate, dedicated time slot within a shared frequency channel CDMA (Code Division Multiple Access) All users share the same frequency and time, distinguished by unique orthogonal/near-orthogonal spreading codes (based on spread spectrum) OFDMA (Orthogonal Frequency Division Multiple Access) Subdivides the channel into many orthogonal narrowband subcarriers, which are allocated among users; used in modern systems such as LTE/5G

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This section covers the fundamentals of digital and analog switching, the concept of soft switching, and the routing and signaling functions that direct calls/data through a telecommunication network.

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Digital & Analog Switching Switching Type Description Analog switching (space-division switching) Physically connects a caller's circuit to the called party's circuit through a matrix of electromechanical or crossbar switches; used in older analog exchanges Digital switching (time- division switching) Samples and encodes voice/data into digital time slots (e.g. via PCM), then switches these time slots between input and output using time-slot interchange (TSI) and/or time-space- time (TST) switching fabrics Circuit switching Establishes a dedicated end-to-end path for the entire duration of a call/session Packet switching Breaks data into packets, each routed independently through the network, sharing links among many users Soft Switching Soft switch (softswitch): a software-based call-control system that separates the call-control/signaling function from the physical media-transport (bearer) function, typically used in packet-based (VoIP/NGN) networks — contrasted with traditional hardware-based circuit switches where control and transport are tightly integrated.

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Routing & Signaling Routing: the process of selecting a path through the network for a call or data packet, based on destination address, network topology, traffic conditions, and cost.

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Signaling: the exchange of control information (not user data) needed to set up, maintain, and release a connection — e.g. call setup requests, addressing, supervision (on/off-hook), and billing information; can be in-band (over the same channel as voice) or out-of-band/common-channel (a separate dedicated signaling channel, as in SS7).

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This section covers teletraffic parameters used to size and plan telecommunication networks, traffic routing and common channel signaling in wireless networks, integrated services digital networks (ISDN), and packet vs circuit switching considerations for personal communication networks (PCN).

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Teletraffic Parameters Parameter Meaning Busy hour The continuous 60-minute period during which traffic volume/intensity is highest, used for worst- case network dimensioning Traffic intensity The average number of simultaneous calls/connections in progress, typically measured in Erlangs (one Erlang = one circuit continuously occupied for one hour) Grade of Service (GoS) The probability that a call is blocked (or delayed beyond an acceptable time) during the busy hour, due to insufficient network resources — a key network dimensioning target Service levels Defined performance targets (e.g. blocking probability, delay) that a network must meet for acceptable quality of service Traffic Routing & Common Channel Signaling Traffic routing in wireless networks directs calls/data through the network (e.g. via mobile switching centers) based on availability, load, and cost, often with alternate routing when a primary path is congested.

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Common channel signaling (CCS) uses a dedicated signaling channel, separate from the traffic-bearing channels, to exchange control information for many calls — more efficient and flexible than per-call in-band signaling (used in systems such as SS7).

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ISDN, Packet vs Circuit Switching for PCN Integrated Services Digital Network (ISDN): a digital network standard that carries voice, data, and other services over the same digital circuit-switched channels, using standardized B (bearer) and D (signaling/data) channels.

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Packet vs circuit switching for Personal Communication Networks (PCN): circuit switching suits continuous, delay-sensitive traffic (e.g. voice), while packet switching suits bursty data traffic and enables more efficient sharing of scarce wireless spectrum; modern PCN/mobile systems increasingly use packet switching (all-IP) even for voice (e.g.

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Protocol for network access: defines the rules/procedures a terminal or mobile device follows to gain access to and communicate over the network (e.g. random access procedures, contention resolution).

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This final section of the chapter covers the international and national regulatory framework governing telecommunication, including the International Telecommunication Union, Nepal Telecommunication Authority, relevant government ministry, national frequency allocation, and the Radio Act.

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International Telecommunication Union (ITU) The International Telecommunication Union (ITU) is a specialized agency of the United Nations responsible for coordinating global telecommunication standards, allocating international radio spectrum and satellite orbits, and promoting international connectivity — operating through its sectors ITU-R (radiocommunication), ITU-T (standardization), and ITU-D (development).

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Nepal's Regulatory Framework Body/Instrument Role Nepal Telecommunication Authority (NTA) The autonomous regulatory body for the telecommunication sector in Nepal — issues licenses, manages spectrum allocation domestically, sets tariff/interconnection rules, and enforces quality-of-service standards Ministry of Communication and Information (Technology) The government ministry responsible for national telecommunication/ICT policy formulation and overall sector governance National frequency allocation plan The official plan specifying which frequency bands are allocated to which services (broadcasting, mobile, satellite, defense, etc.) within Nepal, aligned with ITU international allocations Radio Act The principal national legislation governing the establishment, operation, and licensing of radio communication equipment and services in Nepal