1 / 60

EE 6331, Spring, 2009 Advanced Telecommunication

EE 6331, Spring, 2009 Advanced Telecommunication. Zhu Han Department of Electrical and Computer Engineering Class 23 Apr. 20 th , 2009. Outline. Review CDMA OFDM 2G-3G-4G Project 2 due on the exam Exam Coding is important: Linear and CRC CDMA Nyguist Three Criteria

alyn
Download Presentation

EE 6331, Spring, 2009 Advanced Telecommunication

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. EE 6331, Spring, 2009Advanced Telecommunication Zhu Han Department of Electrical and Computer Engineering Class 23 Apr. 20th, 2009

  2. Outline Review CDMA OFDM 2G-3G-4G Project 2 due on the exam Exam Coding is important: Linear and CRC CDMA Nyguist Three Criteria Term Presentation: 15 min., 13 slides, 2 questions, evaluation Multiple Access: Chapter 5 of my book Chapter 3 and 4 next week ECE6331

  3. Spread-spectrum transmission • Three advantages over fixed spectrum • Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background. • Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter. • Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently. ECE6331

  4. Direct Sequence Spread Spectrum Unique code to differentiate all users Sequence used for spreading have low cross-correlations Allow many users to occupy all the frequency/bandwidth allocations at that same time Processing gain is the system capacity How many users the system can support ECE6331

  5. Spreading & Despreading • Spreading • Source signal is multiplied by a PN signal: 6.134, 6.135 • Processing Gain: • Despreading • Spread signal is multiplied by the spreading code • Polar {±1} signal representation ECE6331

  6. Spreading & Despreading ECE6331

  7. CDMA Example R Receiver (a base station) Data=1011… Data=0010… A B Transmitter Transmitter (a mobile) Codeword=101010 Codeword=010011 Data transmitted from A and B is multiplexed using CDMA and codewords. The Receiver de-multiplexes the data using dispreading. ECE6331

  8. CDMA Example – transmission from two sources 1 0 1 1 A Data 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 A Codeword 1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0 1 0 1 1 0 0 A Signal 0 0 1 0 B Data 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 B Codeword 1 0 1 0 1 0 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 B Signal Transmitted A+B Signal ECE6331

  9. CDMA Example – recovering signal A at the receiver A+B Signal received A Codeword at receiver Integrator Output Comparator Output 0 1 0 0 Take the inverse of this to obtain A ECE6331

  10. CDMA Example – recovering signal B at the receiver A+B Signal received B Codeword at receiver Integrator Output Comparator Output 1 1 0 1 Take the inverse of this to obtain B ECE6331

  11. CDMA Example – using wrong codeword at the receiver A+B Signal received Wrong Codeword Used at receiver Integrator Output Comparator Output X 0 1 1 Noise Wrong codeword will not be able to decode the original data! ECE6331

  12. Near Far Problem and Power Control • At a receiver, the signals may come from various (multiple sources. • The strongest signal usually captures the modulator. The other signals are considered as noise • Each source may have different distances to the base station • In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. • Therefore the receiver power at the base station for all mobile users should be close to eacother. • This requires power control at the mobiles. • Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power. B pr(M) M M M M ECE6331

  13. Frequency Hopping Spread Spectrum • Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. • Military, bluetooth ECE6331

  14. Orthogonal frequency-division multiplexing • Special form of Multi-Carrier Transmission. • Multi-Carrier Modulation. • Divide a high bit-rate digital stream into several low bit-rate schemes and transmit in parallel (using Sub-Carriers) ECE6331

  15. OFDM bit loading • Map the rate with the sub-channel condition • Water-filling ECE6331

  16. CDMA (IS 95 A) IS 95 B GSM GPRS W-CDMA 3X UWC-136 TDMA EDGE cdmaOne IS-95A 1X No 3X Road Map 1XRTT/3XRTT cdma2000 4G 1999 2000 2001 2002 IS-95B 3G Phase 1 3G Phase 2 2G 2.5G ECE6331

  17. Multiple Access How can we share a wireless channel: Results in Wireless Media Access Control Protocols How we can change base stations: Results in Handoff algorithms and protocols How can we seamlessly support mobile applications over wireless links: Results in mobility protocols like Mobile IP, Cellular IP, etc. How can we design efficient transport protocols over wireless links: Results in solutions like SNOOP, I-TCP, M-TCP, etc. How different wireless networks/systems are designed? Bluetooth, IEEE 802.11, GSM, etc. ECE6331

  18. Wireless System Architecture and Functions Applications TCP/IP Neighbor Discovery and Registration, Multicasting, Power Saving Modes, Address Translation (IP-MAC), Routing, Quality of Services, Subnet Security Wireless Subnet Controller Medium Access Control, MAC level Scheduling, Link Layer Queueing, Link Layer Reliability – ACKs, NACKs, …. Wireless Link Layer (Layers 1 and 2 in ISO/OSI Network Reference Model) Link Controller Transceiver Frame Controller Framing and frame synchronization, error control, CRC, bit scrambling, widening, …. Carrier frequency, channel bandwidth, carrier detect, Captude detect, channel data rate, modulation, Received signal strength (RSSI), transmit power, Power control, … Physical Radio ECE6331

  19. Medium Access Control Wireless spectrum (frequency band) is a very precious and limited resource. We need to use this resource very efficiently We also want our wireless system to have high user capacity A lot of (multiple) users should be able to use the system at the same time. For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system. The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…) ECE6331

  20. Wireless Media Access Control Random Schemes (Less-Coordinated) Examples: MACA, MACAW, Aloha, 802.11 MAC,… More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs Coordinated Schemes Examples: TDMA, FDMA, CDMA More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,… Polling based Schemes Examples: Bluetooth, BlueSky,… Access is coordinated by a central node Suitable for Systems that wants low-power, aims to carry voice and data at the same time. ECE6331

  21. Duplexing It is sharing the media between two parties. If the communication between two parties is one way, the it is called simplex communication. If the communication between two parties is two- way, then it is called duplex communication. Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver. Duplex communication achieved by: Time Division (TDD) Frequency Division (FDD) Some other method like a random access method ECE6331

  22. Duplexing Usually the two parties that want to communication in a duplex manner (both send and receive) are: A mobile station A base station Two famous methods for duplexing in cellular systems are: TDD: Time Division Duplex FDD: Frequency Division Duplex ECE6331

  23. Duplexing - FDD • A duplex channel consists of two simplex channel with different carrier frequencies • Forward band: carries traffic from base to mobile • Reverse band: carries traffic from mobile to base F M B R Base Station Mobile Station Reverse Channel Forward Channel frequency fc,,F fc,R Frequency separation Frequency separation should be carefully decided Frequency separation is constant ECE6331

  24. Duplexing - TDD • A single radio channel (carrier frequency) is shared in time in a deterministic manner. • The time is slotted with fixed slot length (sec) • Some slots are used for forward channel (traffic from base to mobile) • Some slots are used for reverse channel (traffic from mobile to base) M B Mobile Station Base Station Slot number 0 1 2 3 4 5 6 7 … F R F R F R F R …. channel Reverse Channel Forward Channel time Ti+1 Ti Time separation ECE6331

  25. Duplexing – TDD versus FDD FDD FDD is used in radio systems that can allocate individual radio frequencies for each user. For example analog systems: AMPS In FDD channels are allocated by a base station. A channel for a mobile is allocated dynamically All channels that a base station will use are allocated usually statically. More suitable for wide-area cellular networks: GSM, AMPS all use FDD TDD Can only be used in digital wireless systems (digital modulation). Requires rigid timing and synchronization Mostly used in short-range and fixed wireless systems so that propagation delay between base and mobile do not change much with respect to location of the mobile. Such as cordless phones… ECE6331

  26. Multiple Access - Coordinated We will look now sharing the media by more than two users. Three major multiple access schemes Time Division Multiple Access (TDMA) Could be used in narrowband or wideband systems Frequency Division Multiple Access (FDMA) Usually used narrowband systems Code Division Multiple Access Used in wideband systems. ECE6331

  27. Narrow- and Wideband Systems Narrowband System The channel bandwidth (frequency band allocated for the channel is small) More precisely, the channel bandwidth is large compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel) AMPS is a narrowband system (channel bandwidth is 30kHz in one-way) Wideband Systems The channel bandwidth is large More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel. A large number of users can access the same channel (frequency band) at the same time. ECE6331

  28. Narrow- and Wideband Systems Narrowband Systems Could be employing one of the following multiple access and duplexing schems FDMA/FDD TDMA/FDD TDMA/TDD Wideband systems Could be employing of the following multiple access and duplexing schemes TDMA/FDD TDMA/TDD CDMA/FDDCDMA/TDD ECE6331

  29. Cellular Systems and MAC ECE6331

  30. Frequency Division Multiple Access • Individual radio channels are assigned to individual users • Each user is allocated a frequency band (channel) • During this time, no other user can share the channel • Base station allocates channels to the users B fN,F f1,F f2,F f2,R f1,R fN,R … M M M ECE6331

  31. Features of FDMA An FDMA channel carriesone phone circuit at a time If channel allocated to a user is idle, then it is not used by someone else: waste of resource. Mobile and base can transmit and receive simultaneously Bandwidth of FDMA channels are relatively low. Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low) Lower complexity systems that TDMA systems. ECE6331

  32. Capacity of FDMA Systems Frequency spectrum allocated for FDMA system … Guard Band channel Guard Band Bt : Total spectrum allocation Bguard: Guard band allocated at the edge of the spectrum band Bc : Bandwidth of a channel AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz channel bandwidth (simplex): Number of channels is 416. ECE6331

  33. Time Division Multiple Access The allocated radio spectrum for the system is divided into time slots In each slot a user can transmit or receive A user occupiess a cyclically repeating slots. A channel is logically defined as a particular time slot that repeats with some period. TDMA systems buffer the data, until its turn (time slot) comes to transmit. This is called buffer-and-burst method. Leaky bucket Requires digital modulation ECE6331

  34. TDMA Concept Downstream Traffic: Forward Channels: (from base to mobiles) 1 2 3 … N 1 2 3 …. N … Logical forward channel to a mobile Base station broadcasts to mobiles on each slot Upstream Traffic: Reverse Channels: (from mobile to base) 1 2 3 … N 1 2 3 …. N … Logical reverse channel from a mobile A mobile transmits to the base station in its allocated slot Upstream and downstream traffic uses of the two different carrier frequencies. ECE6331

  35. TDMA Frames Multiple, fixed number of slots are put together into a frame. A frame repeats. In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels. In TDMA/FDD: a different carrier frequency is used for a reverse or forward Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa). Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame. ECE6331

  36. General Frame and Time Slot Structure in TDMA Systems One TDMA Frame Preamble Information Trail Bits Slot 1 Slot 2 Slot 3 … Slot N Guard Bits Sync Bits Control Bits Information CRC One TDMA Slot A Frame repeats in time ECE6331

  37. A TDMA Frame Preamble contains address and synchronization info to identify base station and mobiles to each other Guard times are used to allow synchronization of the receivers between different slots and frames Different mobiles may have different propagation delays to a base station because of different distances. ECE6331

  38. Efficiency of a Frame/TDMA-System • Each frame contains overhead bits and data bits. • Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits. bT: total number of bits in a frame Tf: frame duration (seconds) bOH: number of overhead bits Number of channels in a TDMA cell: m: maximum number of TDMA users supported in a radio channel ECE6331

  39. TDMA TDMA Efficiency GSM: 30% overhead DECT: 30% overhead IS-54: 20% overhead. TDMA is usually combined with FDMA Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other. There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels) For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA. ECE6331

  40. Contemporary TDMA Systems ECE6331

  41. Spread Spectrum Access SSMA uses signals that have transmission bandwidth that is several orders of magnitued larger than minimum required RF bandwidth. Provides Immunity to multipath interference Robust multiple access. Two techniques Frequency Hopped Multiple Access (FHMA) Direct Sequence Multiple Access (DSMA) Also called Code Division Multiple Access – CDMA ECE6331

  42. Capacity of CDMA Systems • Uplink Single-cell System Model • Assumptions • Total active users Ku • The intra-cell MAI can be • modeled as AWGN • Perfect power control is assumed • Random sequences User 2 ... User 1 User k . . . . . . ... User n User Ku ECE6331

  43. Capacity of CDMA Systems Coarse estimate of the reverse link (uplink) capacity Assumptions: Single Cell. The interference caused by other users in the cell can be modeled as AWGN. Perfect power control is used, i.e. the received power of each user at the base station is the same. If the received power of each user is Ps watts, and the background noise can be ignored (ex: microcells), then the total interference power (MAI) at the output of the desired user’s detector is where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as: ECE6331

  44. Scheduling and Spectrum Allocation For TDMA system: whose information to transmit, Scheduling Problem Round Robin Opportunistic scheduling: channel good? transmits Fairness: Max-min fair and proportional fair Cross-layer design: delay issue For FDMA system: where to load the bits Bit loading problem For OFDMA system Time and frequency slots are assigned Complicated assignment problem Single Cell without interference case, or multicell interference case Channel Allocation problem Cognitive radio Wireless ad hoc/sensor networks Admission control Reject the users if there is no more resources Handoff has higher priority ECE6331

  45. Random Access Packet Radio Protocols Multihop radio network that carries packets Not circuit oriented like GSM, CDMA, etc. Example Protocols Pure Aloha Slotted Aloha CSMA Protocols 1-persistent CSMA non-persistent CSMA p-persistent CSMA CSMA/CD Reservation Protocols Reservation Aloha PRMA Others MACA, MACAW IEEE 802.11 MAC ECE6331

  46. Pure Aloha Algorithm: A mobile station transmits immediately whenever is has data. It then waits for ACK or NACK. If ACK is not received, it waits a random amount of time and retransmits. Ignoring the propagation delay between mobiles and base station: B The time difference between the time a mobile send the first bit of packet and the time the base station receives the last bit of the packet is given by 2T. T = C/P T: packet time. C: channel data rate (bps) P: packet length (bits) Ack/Nack Data M3 M1 M2 During this 2T period of time, the packet may collide with someone else packet. ECE6331

  47. Contention for Aloha ECE6331

  48. Throughput of Aloha Normalized Throughput ~0.185 0.5 Normalized Channel Occupancy ECE6331

  49. Slotted Aloha ECE6331

  50. Reservation Protocols Reservation Aloha Packet Reservation Multiple Access ECE6331

More Related