Mobile Communications Lecture Notes PDF

Lecture 7: Wireless Wide Area Networks (WWANs) – Mobile Communications Dr. Abdulmalik Alwarafy Some content adapted from the book “5G Mobile and Wireless Communications Technology” by Osseiran et al. Lecture 7 Learning Objectives • Understand Wireless WANs, in particular mobile/cellular nets • U...

Lecture 7: Wireless Wide Area Networks (WWANs) – Mobile Communications Dr. Abdulmalik Alwarafy Some content adapted from the book “5G Mobile and Wireless Communications Technology” by Osseiran et al. Lecture 7 Learning Objectives • Understand Wireless WANs, in particular mobile/cellular nets • Understand the fundamental aspects underpinning cellular communications • Provide overview & rationale for each generation (1/2/3/4/5 G) • Explain architecture & operational aspects of each & discuss associated technical, economic & regulatory issues • Maps onto CLO 3 o “Explain protocols underlying the design of modern communication systems” Most fundamental requirements of cellular communications • Spectrum re-use • Multiple access schemes • Duplex operation • Service continuity (hand-off) • Performance & Scalability 3 Frequency/Spectrum Re-use • Early schemes (1G) used single transmitter to cover wide area: o Limited number of channels o Large waiting lists (more people waiting than those being served) • Need 25kHz for sufficient audio quality + guard band o Can support only 40 users using 1MHz band o With 100MHz, only 4000 users supported ▪ Operators today only have tens of MHz allocated ▪ Typically acquired at exorbitant prices (Billions of $) • Millions of subscribers today o Spectrum efficiency is vital! ▪ Frequency re-use comes to the rescue! 4 Frequency/Spectrum Re-use Available spectrum • In practice cell-edge boundary varies due to: oRadio propagation, terrain, receiver sensitivity • Signal strength gradually reduces towards edge oOverlap at some edges, coverage hole at others • Adjacent cells allocated different frequencies to overcome this overlapping problem Cell Size & Capacity Available spectrum • Shrink cell size to enable more frequency re-use • Use small low-powered base stations to cater to areas with more users • Need to deploy more infrastructure: oHigher Capital Expenditure (CAPEX) ▪ The more BS, the higher CAPEX & vice versa Cell Size & Capacity • Types of Cells: oMacro Cells (~10km radius, sparse areas) oMicro Cells (~1km radius, dense areas) oPico Cells (areas of buildings / tunnels etc.) oFemto (Small) Cells (inside the home) Multiple Access • Allow multiple users to access system • Resource sharing principles: oFDMA oTDMA oCDMA oOFDMA oNon-orthogonal Multiple Access (NOMA) Duplex Operation • Half Duplex: Push-to-talk system (e.g. a walky-talky) o Cannot transmit & receive at the same time o Delay between talking & listening • Full Duplex: Talk in both directions at same time o Uplink (UL) & Downlink (DL) o Using Frequency Division Duplex (FDD) ▪ Tx on one frequency (f1) & Rx on another frequency (f2) o Using Time Division Duplex (TDD) ▪ Tx & Rx on same frequency in different timeslots ▪ Short bursts so no noticeable delay for user Service Continuity • Mobile moves from one cell to the other • Handover to maintain service continuity https://www.youtube.com/watch?v=lSwRfjgUYq Q&t=4s&ab_channel=EzEdChannel o When to handover? Whom (which cell) to handover to? o Smooth re-route without interruption to service quality! • Typical handover: o User assisted: mobile best placed to monitor strength of signals from different Base Stations o Network-initiated: only network knows channel availability & when to initiate handover • Handovers for load balancing too! Evolution 1st Generation (1G) System NMT • Established a market that continues to grow • Expensive: Use limited to business customers • No standardisation, no roaming • Nordic Mobile Telephone (NMT) o Used in Scandinavia AMPS • Advanced Mobile Phone System (AMPS) o Used in North America • Total Access Communications System (TACS) o Used in UK & other countries TACS 1st Generation (1G) System • Used Frequency Modulation (FM) for voice • Typically used frequencies in 900MHz band o NMT used frequencies around 450MHz • Channel spacing: o 25kHz for NMT & TACS o 30kHz for AMPS 1G Mobile Equipment • Big bulky phones with poor battery life, capacity, & coverage • Mobile Identifier Number: o3 digit area code, 3 digit exchange number, 4 digit subscriber id oUsed for routing calls • Mobiles also had electronic serial number (ESN): o32-bit long: 8-bit manufacturer code, 18bit serial no., 6-bits reserved oUnique for each device, tampering rendered phone inoperable 1G Systems - Summary • Analog systems: major achievement of their time • Grew rapidly leading to capacity crunch • Did not have security features oEasy to eavesdrop into calls • New systems required to mitigate these drawbacks • Led to migration from 1G to 2G Motivation behind 2G Capacity Security Roaming • Improve capacity over 1G systems • Design a secure system with call privacy • Support roaming across country borders 16 2G Frequency Bands 900MHz 1800MHz 1900MHz 800MHz • Original aim was to use 900MHz band o DCS 1800, PCS 1900, 800 MHz deployments emerged • Most successful system with 1+ Billion subscribers in 2004 • Global standard: o Enabled international roaming o Global market with economies of scale realised o Helped to drive down costs, increase widespread adoption 2G System Architecture • Base Transceiver Station (BTS): transmits & receives signals from mobile station (MS) • Base Station Controller (BSC): manages one or more BTS o Handles channel set-up, security, authentication, paging, handovers • Mobile Switching Centre (MSC): interfaces with other MSCs o Interfaces with Authentication Centre (AuC) to authenticate users o Interfaces with HLR, VLR registers to resolve location information for call routing o Coordinates handovers o Interfaces with PSTN 2G Equipment Identity • International Mobile Equipment Identity (IMEI): o15 digit number used to identify equipment oHardcoded into a mobile during manufacture time oChecked with Equipment Identity Register (EIR) when trying to access the system ▪ White Listed: Access allowed ▪ Black Listed: Access blocked (e.g., stolen/unapproved device) 2G Subscriber Identity • International Mobile Subscriber Identity (IMSI): o15 digit number contained in SIM card oEnables operator to link phone number with subscriber oAuthentication key: ▪ Stored on SIM card, used to generate cipher key (used as a session key) GSM Specification • Uses digital technology: TDMA + FDMA Image Source: Google Image Search • Frequencies: UL 890 – 915, DL 935 – 960 MHz • 25MHz band divided into 125 carriers (200kHz each) o Each carrier divided into 8 TDMA timeslots (aka Burst Period) which lasts 0.577ms o 8 burst periods comprise a TDMA frame which lasts 4.615ms Signalling Methods • Control channels: o Used to page/call mobile (registration/call reception) o Take access request (call initiation) o Control signals transmitted using FSK with Manchester coding 1. Forward control channel (FCC): o For mobiles joining network o For mobiles looking for alternatives if handoff required 2. Reverse control channel (RCC): o Carries registration message from mobile o Call setup messages when a number is dialled o Responding to paging requests from BTS during incoming call GSM operation – Joining Network 1. Upon startup, MS scans all available frequencies from surrounding BTSs 2. Records signals received from all BTSs & the broadcast information 3. Identifies strongest beacon frequency o Sends registration message using Random Access Channel (RACH) to this BTS GSM operation - Joining Network 4. BTS forwards this to MSC via BSC 5. MSC queries the Equipment Identity Register (EIR) & Authentication Centre (AuC) & based on the reply, it authenticates MS 6. MS is now ready to make or receive calls GSM Operation – Call Initiation • MS sends an access request over the RACH to BTS • BTS receives the access message: oIt directs MS to specific radio channel where call setup can be progressed oSubsequently the call can commence GSM Operation – Call Reception • Caller places the call • MSC checks location register for last known location of Callee • MSC sends page message on paging channel to group of BTSs in region • When the callee receives paging message, it responds on reverse control channel • Serving BTS allocates radio channel & timeslot to callee so that call can be completed • Call continues until either end hangs up GSM Operation – Handovers (HO) • HO triggers: 1. Drop in signal quality ▪ E.g. user records moving average of signal strength ▪ Declining trend could trigger a HO request 2. Load balancing ▪ Move users from heavily loaded to lightly loaded BTS • The TX transmits in 1 out of 8 slots • The RX receives in 1 out of 8 slots • In the remaining 6 slots: o MS scans other channels looking for stronger beacons o Reports back measurements to serving BTS GSM Operation – Handovers (HO) • Network knows: oLink quality between MS & serving BTS oStrength of signals from other BTS at MS oAvailability of channels in neighbouring BTS • Network initiates HO (if required): oNotifies new BTS to reserve channels oNotifies MS to establish link with new BTS oNew BTS confirms HO completion oOld BTS clears state & frees up channels GSM Summary • Multi-stakeholder, cross-border joint initiative which has crossed billion subscribers • Transition from 1G Analog to 2G Digital system • Brings the best of both worlds FDMA + TDMA to improve efficiency • Delivered the promise of capacity, security & global roaming • Improvements still required on data rate! 2.5G - General Packet Radio Service (GPRS) • GSM is circuit switched • GPRS introduces packet switching: o Support for IP-based connectivity on top of GSM o Higher data rate compared to GSM • Packet switching improves utilization through statistical multiplexing o More suitable for bursty nature of data traffic • IP-based operation eases migration to newer generations 30 GPRS Architecture • Builds on top of GSM architecture • The network between BTS & BSC unchanged • Introduced two new elements to provide data services: 1. Serving GPRS Support Node (SGSN): o Provides data services within network o Authentication, location tracking, QoS monitoring o Link between MS & GGSN GPRS Architecture 2. Gateway GPRS Support Node (GGSN): o gateway to the packet network o MS must attach to SGSN to receive data services GPRS Operation • • • • Upon startup, MS registers with network as follows: Transmits a burst on RACH Identifies itself, indicates it wants to provide location update Network authenticates MS, SGSN notes its location (in case of incoming data for this MS) GPRS Operation • MS then enters standby mode (periodically updates location) • MS monitors Packet Paging Channel (PPCH) for any incoming alerts • MS enters ready mode when it has data to send/receive • MS sends packet Tx request using Packet Random Access Channel (PRACH) in the UL • BTS sends a grant on Packet Access Grant Channel (PAGCH) o Indicates resources & parameters to use in UL data transfer 2.5G GPRS - Summary • Introduced IP-based packet switching service on top of circuit switched GSM • Improved resource utilization through statistical multiplexing • Offered on-demand data services: oDon’t have to be always-on thereby reducing bill for customer Motivation for 3G • 2G introduced voice & text messaging • 2.5G (GPRS) introduced packet switching & higher rates • Advances in other technology areas: o Popularity of over-the-top VoIP services o Increasing capability of end user devices o Improvement in human-computer interfaces • Competition: WiFi offering higher speeds at little/no cost • Need for more speed & ability to support diverse services was essential to remain in the game! o 3G was designed to meet these needs 38 3GPP Standardisation Process • 3GPP standardisation process addresses four aspects: 1. Requirements: o Agreement on what is to be achieved 2. Architecture: o Deciding the main building blocks & interfaces 3. Detailed Specifications: o Specify every interface in detail 4. Testing & Verification: o Verify that the design works in practice & to test inter-operability 3GPP Facts & Figures ~400 Companies from 39 Countries 50 delegate days per year 40 documents per year 1.2 specs per Release New Release every ~18 months Approved CRs per year per Release North America 22% 8000 Europe 36% 7000 6000 5000 R14 4000 3000 2000 1000 Asia 42% R5 R99 R4 R6 R7 R8 R9 R10R11 R12 R13 0 Participation by Region (by TSG#77) Source: 3GPP Presentation Template 3GPP Releases • Key Milestones: o Rel 97. GPRS 1998 o Rel 99. UMTS 2000 o Rel 4. All IP Core 2001 o Rel 8. LTE 2008 o Rel 9. LTE HeNB 2009 o Rel 12. MTC LTE Cat0, D2D 2015 o Rel 13. LTE-LAA, LTEM Cat 1 2016 o Rel 15. 5G Phase 1 (end 2018) o Rel 16. 5G Phase 2 (2020) Source: https://www.radio-electronics.com/info/cellulartelecomms/3gpp/standards-releases.php Ways to support higher data rates 1. Increase spectrum 2. Increase transmit power of transmissions o Need to consider interference, battery life implications 3. Reduce distance between TX & RX (shrinking cell size) o Better propagation of radio waves (less loss) o But might increase cost (more no. of cells required) 4. Increase no. of antennas at RX (receive-antenna diversity), beamforming at TX (narrower directed beams) o Could have implications on cost & shape of devices W-CDMA Technology • 5MHz bandwidth, unlike 1.5MHz in CDMA-2000, & therefore the “W” prefix for CDMA • Better spectral efficiency over 2G technology • Neighbouring cells can use same frequency • W-CDMA uses spread spectrum, so: ohas anti-jamming properties ohard to eavesdrop therefore, better security 3G UMTS Architecture Source: Google Images • Core network: Switching/Routing & provide transit to in/out traffic o Combines circuit & packet switched elements • UMTS Radio Access Network (UTRAN) comprises: o Radio Network Controller (RNC): handles power control, channel allocation, RRM, handover etc. & Node B serves UEs • UE Side: Mobile Equipment & a Universal SIM (USIM) Power Control • BSs should receive all UEs with similar power o If not, farther users will be at a disadvantage ▪ Stronger signals from near UEs, weaker from farther UEs • Once connected, BS measures received power for each UE during each time slot o Sends a bit to UE indicating power to be stepped up/down • Change in propagation implies constant change may be inevitable 3G-3G & 3G-2G Handovers • UE moves out of range of 3G cell & has to be handed over to another frequency (hard handover) o Handover to another frequency channel • UE moves from one 3G cell to another 3G cell o Adjacent cells can be on same frequency (unlike in 2G systems) o UE may hear from other BS simultaneously (soft-handover) • RNC makes the handover decision • Inter-System Handover: o Handover from 3G cell to GSM cell (no 3G coverage available in area where UE has moved to) ▪ Many common network elements between 2G & 3G ▪ Fallback to GSM supported Evolution after 3G: HSDPA • Increase in data consumed wirelessly • Disparity in usage in UL & DL o Bulk of the data traffic in DL o Immediate Need: Improve DL data rate • High Speed Downlink Packet Access (HSDPA) was developed (Rel. 5): o HSDPA uses higher order modulation (16-QAM) unlike QPSK used in WCDMA ▪ Higher order modulation requires higher SNR (less resilient to noise) o Dynamic adaptation of modulation & power to use based on channel state information (CSI) reports