EPL 657 Wireless Networks Some fundamentals: Multiplexing / Multiple Access / Duplex Infrastructure vs Infrastructureless Panayiotis Kolios
Recall: The big picture... Modulations: some basics 2
Multiplexing / multiple access / duplexing (1) Multiplexing / multiple access Signals to/from different users share a common channel using time division methods (TDM/TDMA, CSMA), frequency division methods (FDM/FDMA), code division methods (CDMA), or space division (SDMA). A combination of above is also often used
Multiplexing / multiple access / duplexing (2) Duplexing: The signals moving between two elements in opposite directions can be separated using time division duplexing (TDD) frequency division duplexing (FDD) Code Division Duplexing (CDD)
Multiplexing Goal: multiple use of a shared medium Multiplexing in 4 dimensions: space (si) time (t) frequency (f) code (c) or even a combination Important: guard spaces needed! Selective receivers/filters required to obtain/extract signal intended for user
Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high even for many users Disadvantages: Precise Synchronization necessary Can be complex Can be inefficient TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.
Example Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = packet transmision time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have packet, slots 2,5,6 idle
Frequency multiplex Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages: no dynamic coordination necessary works also for analog signals Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces Selective filters required Can be complex FDM (Frequency Division Multiplexing): frequency subdivided among N users, each user takes one; inefficient with low duty cycle users and at light load.
frequency bands Example Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle e.g. Assigned channel frequency 800 812 Mhz Needs of each user: 1Mhz f1= 800 Mhz, f2=801, Mhz... f1-f2 f11-f12
Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: better protection against tapping protection against frequency selective interference higher data rates compared to code multiplex but: precise coordination required increased complexity, and inefficiency
Code multiplex Each channel has a unique code All channels use the same spectrum at the same time (spread the spectrum- each bit is expanded to many bits-a code, e.g logical bit 1' is expanded to 010011) Advantages: bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology
Example: Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) unique code assigned to each user; i.e., code set partitioning used mostly in wireless broadcast channels (cellular, satellite, etc) all users share same frequency, but each user has own chipping sequence (i.e., code, language ) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) Note each user appears as interference to others!!!
Example: CDMA Encode/Decode
Example: CDMA two-sender interference
space division multiplex Cell structure Implements space division multiplex: base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - less for higher frequencies (e.g. UMTS)
Example: cell What is a Cell? Cell is the Basic Union in The Mobile Telecommunications System defined as the area where radio coverage is given by one base station. A cell has one or several frequencies, depending on traffic load. Fundamental idea: Frequencies are reused, but not in neighboring cells due to interference.
Example: cell planning (capacity, power, etc...) Cell splitting Decrease transmission power in base and mobile Results in more and smaller cells Reuse frequencies in noncontiguous cell groups Example: ½ cell radius leads 4 fold capacity increase (BUT higher infrastructure costs) Cell sectoring Directional antennas subdivide cell into 3 or 6 sectors Might also increase cell capacity by factor of 3 or 6
Example: Different Types of Cells
Duplex Frequency Division Duplex (FDD): Uplink and downlink transmissions use two separated radio frequencies in different frequency bands. A pair of frequency bands with specified separation is assigned for the system. Time Division Duplex (TDD): Uplink and downlink transmissions are carried over same radio frequency by using synchronized time slots that divide the physical channel into transmission and reception part. Information on uplink and downlink are transmitted reciprocally. Code Division Duplex (CDD): Uplink and downlink transmissions are carried over the same radio frequency and time using orthogonal signal sequences (different codes).
Example Radio Access FDMA/FDD (as in 1st Generation Wireless) Access is FDMA: Frequency Division Multiple Access The 1st generation mobile system uses FDMA only. Example: AMP in USA Uplink Downlink Frequency Duplex is FDD: Frequency Division Duplex The FM channels are paired with an uplink and a downlink channel for each user.
Example Radio Access TDMA (as in 2nd Generation wireless) GSM, a 2nd generation mobile system, uses 8 time slots in TDMA mode for each 200 khz carrier. Carriers are derived from frequency division over the licensed frequency band (FDMA) Frequency Time Note: Capacity in GSM is doubled by using alternate time slots to support 16 channels
Example Radio Access TDMA (as in 2nd Generation wireless) IS-136 TDMA or DAMP (Digital AMP) is the American TDMA system with 3 time slots over a 30kHz carrier TDMA6 provides 6 channels by alternating the 3 time slots
Example Radio Access TDMA/FDD (as in 2nd Generation wireless) GSM and IS-136 TDMA are TDMA/FDD Frequency Time
Example Radio Access CDMA (as in 2nd Generation wireless) IS-95, a 2nd generation mobile system, uses CDMA User E User D User C Code Sequences Frequency User B User A Time
Example Radio Access CDMA/FDD (as in 2nd Generation wireless) IS-95 is CDMA/FDD Frequency Code Sequences Time
Example Radio Access 2nd Generation Going from analog to digital and to CDMA makes more efficient use of the scarce radio resources (and expensive frequency spectrum license), and hence helps to lower the price.
Example Radio Access Wideband CDMA (3G) WCDMA allocates 10 ms (38,400 chips) frames to users. The data rate for a user may change from frame to frame (using variable length CDMA codes). Frequency 10ms, 38,400 chips per frame User E User D User C User B User A Variable data rate Time
Example Radio Access Wideband CDMA UTRA/FDD Frequency Separate carriers for Uplink downlink Time
Examples Multiplexing / multiple access / duplexing Network IEEE 802.15.1 WPAN (Bluetooth) IEEE 802.15.4 LR- WPAN (ZigBee) IEEE 802.11 WLAN (WiFi) IEEE 802.16 WMAN (WiMAX) Multiplexing / MA / duplexing TDMA / TDD CSMA/CA CSMA/CA TDM/TDMA (down/uplink) / TDD or (semi-duplex) FDD
Multiple Access protocols (MAC) Share access (time) on the common channel single shared broadcast channel two or more simultaneous transmissions by nodes cause interference only one node can send successfully at a time, therefore need multiple access protocols distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself!
Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. When more than one send at the same time, then collision 4. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 5. Simple
MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller pieces (time slots, frequency, code, space) allocate piece to node for exclusive use Random Access (e.g. Ethernet) access when data available to send (random) channel not divided, allow collisions recover from collisions Taking turns (e.g. Token ring) tightly coordinate shared access to avoid collisions
A popular wireless MAC: CSMA/CA (Collision Avoidance) Recall in wired Ethernet: CSMA/CD: carrier sensing, deferral if busy collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting in wireless CSMA/CA (Collision Avoidance) more later
Infrastructure / Infrastructureless networks
Sensor networks and VANETs are another form of infrastructureless network, with many similarities to ad-hock
infrastructure vs. ad-hoc networks (WLAN) infrastructure network AP AP wired network AP: Access Point AP ad-hoc network
Infrastructure-based networks (WLAN) Infrastructure networks provide access to other networks. Communication typically takes place only between the wireless nodes and the access point, but not directly between the wireless nodes. The access point does not just control medium access, but also acts as a bridge to other wireless or wired networks. Several wireless networks may form one logical wireless network: The access points together with the fixed network in between can connect several wireless networks to form a larger network beyond actual radio coverage.
Infrastructure-based networks (WLAN) Network functionality lies within the access point (controls network flow), whereas the wireless clients can remain quite simple. Use different access schemes with or without collision. Collisions may occur if medium access of the wireless nodes and the access point is not coordinated (e.g. DCF: CSMA/CA). If only the access point controls medium access, no collisions are possible (e.g. PCF). Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes) The access point may poll the single wireless nodes to ensure the data rate. Infrastructure-based wireless networks lose some of the flexibility wireless networks can offer in general: E.g. they cannot be used for disaster relief in cases where no infrastructure is left.
Infrastructureless No need of any infrastructure to work greatest possible flexibility Each node communicate with other nodes, so no access point controlling medium access is necessary (autonomous operation). The complexity of each node is higher implement medium access mechanisms, forwarding data Nodes within an ad-hoc network can only communicate if they can reach each other physically if they are within each other s radio range if other nodes can forward the message
Infrastructureless (Ad Hoc Networks) Some Features (typically) Lack of a centralized entity Network self-organization All the communication is carried over the wireless medium Rapid mobile host movements possible Multi-hop routing Power and computing power may be constrained
Infrastructureless (Sensor Networks) Some Features Many similarities to ad-hock networks power and computing power constrained Large number of sensors (application dependant) Limited wireless bandwidth Limited battery power Low energy use Efficient use of the small memory Data aggregation Network self-organization Collaborative signal processing Querying ability
VANET Networks: VANETS: Vehicle Ad-hock Networks Some Features Many similarities to ad-hock networks power and computing power not necessarily as constrained as in sensor or ad-hoc networks Large number of mobile nodes (cars) Network self-organization Topology dictated by road system