Outline : Wireless Networks Lecture 7: Physical Layer Coding and Modulation. From Signals to Packets. Code Division Multiple Access.

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1 Outline : Wireless Networks Lecture 7: Physical Layer Coding and Modulation Peter Steenkiste Departments of Computer Science and Electrical and Computer Engineering Spring Semester Peter A. Steenkiste 1 RF introduction Modulation and multiplexing Channel capacity Antennas and signal propagation Equalization and diversity Modulation and coding» Coding and modulation» Amplitude, frequency, phase» Code division multiple access» OFDM Some newer technologies Spectrum access Peter A. Steenkiste 2 From Signals to Packets Code Division Multiple Access Packet Transmission Packets Bit Stream Digital Signal Analog Signal Sender Receiver Header/Body Header/Body Header/Body Peter A. Steenkiste 3 Users share spectrum and time, but use different codes to spread their data over frequencies» DSSS where users use different spreading sequences» Use spreading sequences that are orthogonal, i.e. they have minimal overlap» hopping with different hop sequences The idea is that users will only rarely overlap and the inherent robustness of DSSS will allow users to recover if there is a conflict» Overlap = use the same the frequency at the same time» The signal of other users will appear as noise Peter A. Steenkiste 4 Page 1

2 CDMA Principle CDMA Example Basic Principles of CDMA» D = rate of data signal» Break each bit into k chips - user-specific fixed pattern» Chip data rate of new channel = kd If k=6 and code is a sequence of 1s and -1s» For a 1 bit, A sends code as chip pattern <c1, c2, c3, c4, c5, c6>» For a 0 bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6> Receiver knows sender s code and performs electronic decode function S u d d1 c1 d2 c2 d3 c3 d4 c4 d5 c5 d6 c6 <d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender s code Peter A. Steenkiste 5 User A code = <1, 1, 1, 1, 1, 1>» To send a 1 bit = <1, 1, 1, 1, 1, 1>» To send a 0 bit = < 1, 1, 1, 1, 1, 1> User B code = <1, 1, 1, 1, 1, 1>» To send a 1 bit = <1, 1, 1, 1, 1, 1> Receiver receiving with A s code» (A s code) x (received chip pattern) User A 1 bit: 6 -> 1 User A 0 bit: -6 -> 0 User B 1 bit: 0 -> unwanted signal ignored Peter A. Steenkiste 6 CDMA for Direct Sequence Spread Spectrum Categories of Spreading Sequences Spreading Sequence Categories» Pseudo-noise (PN) sequences» Orthogonal codes For FHSS systems» PN sequences most common For DSSS systems not employing CDMA» PN sequences most common For DSSS CDMA systems» PN sequences» Orthogonal codes Peter A. Steenkiste 7 Peter A. Steenkiste 8 Page 2

3 CDMA Discussion CDMA Example CDMA does not assign a fixed bandwidth but a user s bandwidth depends on the load» More users results more noise and less throughput for each user, e.g. more information lost due to errors» How graceful the degradation is depends on how orthogonal the codes are» TDMA and FDMA have a fixed channel capacity» Contention based access is more flexible TDMA Tradeoffs between CSDM and contention-based TDMA? Weaker signals may be lost in the clutter» This will systematically put the same node pairs at a disadvantage not acceptable» The solution is to add power control, i.e. nearby nodes use a lower transmission power than remote nodes CDMA cellular standard.» Used in the US, e.g. Sprint Allocates MHz for base station to mobile communication.» Shared by 64 code channels» Used for voice (55), paging service (8), and control (1) Provides a lot error coding to recover from errors.» Voice data is 8550 bps» Coding and FEC increase this to 19.2 kbps» Then spread out over MHz using DSSS; uses QPSK Peter A. Steenkiste 9 Peter A. Steenkiste 10 Outline How Do We Increase Rates? RF introduction Modulation and multiplexing Channel capacity Antennas and signal propagation Equalization and diversity Modulation and coding» Coding and modulation» Amplitude, frequency, phase» Code division multiple access» OFDM Some newer technologies Spectrum access Peter A. Steenkiste 11 Two challenges related to multipath: As rates increase, symbol times shrink and the effects of inter-symbol interference becomes more pronounced» See earlier examples selective fading starts to have a bigger impact because there is less redundancy in the signal We would like an encoding and modulation solution that has longer symbol times and allows us to fight frequency selective fading more effectively Peter A. Steenkiste 12 Page 3

4 Inter-Symbol-Interference OFDM - Orthogonal Division Multiplexing Transmitted signal: Received Signals: Line-of-sight: Reflected: The symbols add up on the channel Distortion! Delays Peter A. Steenkiste 13 Distribute bits over N subcarriers that use different frequencies in the band B» Multi-carrier modulation» Each signal uses ~B/N bandwidth Since each subcarrier only encodes 1/N of the bit stream, each symbol takes N times longer in time Challenge is efficiently packing many subcarriers in a band - later Peter A. Steenkiste 14 OFDM Transmission Distributing Bits over Subcarriers selective fading distorts wide-band signals Multipath causes ISI Time Channel impulse response Single Channel 2 Channels Time Channels are transmitted at different frequencies (sub-carriers) Narrow band signals Longer symbols Peter A. Steenkiste 15 Time 8 Channels Resistance improves with number of channels Peter A. Steenkiste 16 Page 4

5 -Selective Radio Channel Benefits of Narrow Band Channels Power response [db] Channel impulse response 1 Channel (serial) Time Channel transfer function Signal is broadband 0 2 Channels Interference of reflected and LOS radio waves results in frequency dependent fading Impact is reduced for narrow channels Peter A. Steenkiste 17 8 Channels Channels are narrowband Peter A. Steenkiste 18 Fighting ISI Adjacent Symbol Interference (ASI) Symbol Smearing Due to Channel selective fading will only affects some subcarriers» May be able to simply amplify affected subcarriers No need for complex dynamic equalizer» Use redundancy to deal with data loss on bad subcarriers Further reduce ISI effects by sending a cyclic prefix before every burst of symbols» Can be used to absorb delayed copies of real symbols, without affecting the symbols in the next burst» Prefix is a copy of the tail of the symbol burst to maintain a smooth symbol» E.g. a cyclic prefix of 64 symbols and data bursts of 256 symbols using QPSK modulation Peter A. Steenkiste 19 Slide Prof Harris, SDSU Peter A. Steenkiste 20 Page 5

6 Guard Interval Inserted Between Adjacent Symbols to Suppress ASI Cyclic Prefix Inserted in Guard Interval to Suppress Adjacent Channel Interference (ACI) Peter A. Steenkiste 21 Slide Prof Harris, SDSU Peter A. Steenkiste Slide Prof Harris, SDSU 22 Subcarriers are Orthogonal Densely Packing OFDM Channels Peaks of spectral density of each carrier coincide with the zeros of the other carriers» Carriers can be packed very densely with minimal interference» Requires very good control over frequencies Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10 Conventional multicarrier techniques frequency Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 Ch.3 Ch.5 Ch.7 Ch.9 Saving of bandwidth 50% bandwidth saving Peter A. Steenkiste 23 Orthogonal multicarrier techniques frequency Peter A. Steenkiste 24 Page 6

7 OFDM Spectrum Use OFDM Discussion OFDM provides both frequency and time diversity» Data is spread out over multiple sub carriers and also over time through interleaving» Combining OFDM with MIMO adds space diversity Can increase throughput by increasing the number of subcarriers» Does not affect symbol time! Degree of redundancy can be adjusted per subcarrier -> different data rates Peter A. Steenkiste 25 Peter A. Steenkiste 26 Example: a OFDM Transmitter Uses OFDM with up to 48 subcarriers» Used for data,pilots for control, and guard bands Subcarrier spacing is MHz Subcarriers are modulated using BPSK, QPSK, 16-QAM, and 64-QAM Uses a convolutional code at a rate of ½, 2/3, ¾, or 5/6 to provide forward error correction Results in data rates of 6, 9, 12, 18, 24, 36, 48, and 54 MBps Cyclic prefix is 25% of a symbol burst (16 vs 64) OFDM is also used for higher g rates } (0) Convolutional Encoder (3) Cyclic Prefix Serial to Parallel DAC..... (4) (5).. Modulation (6) ifft Parallel To Serial Peter A. Steenkiste 27 Peter A. Steenkiste 28 f c Page 7

8 OFDM in Putting the Pieces Together Uses punctured code: add redundancy and then drop some bits to reach a certain level of redundancy Coding encodes the data bits into a digital signal» Scrambling, interleaving» FEC» DSSS Modulation super-imposes x-bit symbols onto the carrier» QAM, etc. Use of frequency and space diversity maximizes SNR at receiver» FH, MIMO, receiver MRC, User data bits Digital Signal Modulated Carrier Peter A. Steenkiste 29 Peter A. Steenkiste 30 Page 8