Team 1 LU3 – Protocols

1. Team 1LU3 – Protocols

2. Q1 • What are the various modulation protocols that can be used in WiMAX?

3. Q2 • Research on the concept of QPSK Standard showing its constellation diagram. How many types of QPSK are there? • Quadrature Phase Shift Keying (QPSK) can be interpreted as two independent BPSK systems (one on the I-channel and one on Q), and thus the same performance but twice the bandwidth efficiency • Large envelope variations occur due to abrupt phase transitions, thus requiring linear amplification

4. QPSK Constellation Diagram Q Q • Quadrature Phase Shift Keying has twice the bandwidth efficiency of BPSK since 2 bits are transmitted in a single modulation symbol I I Carrier phases {0, /2, , 3/2} Carrier phases {/4, 3/4, 5/4, 7/4}

5. Modulating sin Bit1 Bit2 bRbI 0 0 A A 0 1 A -A 1 0 -A A 1 1 -A -A modulating cos 1,0 0,0 1,1 0,1 Constellation for QPSK

6. Q Q I I Types of QPSK Q • Conventional QPSK has transitions through zero (i.e. 1800 phase transition). Highly linear amplifiers required. • In Offset QPSK, the phase transitions are limited to 900, the transitions on the I and Q channels are staggered. • In /4 QPSK the set of constellation points are toggled each symbol, so transitions through zero cannot occur. This scheme produces the lowest envelope variations. • All QPSK schemes require linear power amplifiers I Conventional QPSK Offset QPSK /4 QPSK

7. Q3: QAM • Research on the concept of QAM Standard showing its constellation diagram. • Quadrature amplitude modulation (QAM) is a modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers—hence the name of the scheme.

8. Q3 Diagram of a 16-QAM “constellation”.

9. Q3 Diagram of a 64-QAM OFDM (Quadrature Amplitude Modulation Orthogonal Frequency Division Multiplexing) “constellation”.

10. Q4: Trade off in M-QAM • The disadvantages of this technique are that again (as it was with amplitude modulation) the rate of frequency changes is limited by the bandwidth of the line, and that distortion caused by the lines makes the detection even harder than amplitude modulation • Less tolerance to noise / error than QPSK

11. Q5: What is OFDM? • OFDM- Orthogonal Frequency Division Multiplexing, is an FDM modulation technique for transmitting large amounts of digital data over a radio wave. • OFDM works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. • OFDM reduces the amount of crosstalk in signal transmissions. 802.11a, WLAN, 802.16 and WiMAX technologies uses OFDM. • Why OFDM? • -Very easy and efficient in dealing with multi-path • -Robust again narrow-band interference • -Ideal for slow changing multipath channels with high time spread • -No need for time equalization (complex FIR Filters). • -Simple frequency equalization (phase correction) • -Best Bps/Hz performance • -Large number of carriers makes things easier: • -Synchronization is more robust and simple • -Easier to adapt to notches and ingress • -Better immunity to impulsive noise • -Increase the robustness against frequency selective fading channels

12. Q6: How OFDM Works? • Parallel Data Streams • Data Encoding is based on Amplitude Modulation • Multiple Carriers are combined through the Fourier Series • Computed by Inverse Fast Fourier transform

13. Q6: Difference between OFDM and FDM • Orthogonal FDM deals with this multipath problem by splitting carriers into smaller subcarriers, and then broadcasting those simultaneously. This reduces multipath distortion and reduces RF interference (a mathematical formula is used to ensure the subcarriers' specific frequencies are "orthogonal," or non-interfering, to each other), allowing for greater throughput. • In frequency-division multiplexing, multiple signals, or carriers, are sent simultaneously over different frequencies between two points. However, FDM has an inherent problem: Wireless signals can travel multiple paths from transmitter to receiver (by bouncing off buildings, mountains and even passing airplanes); receivers can have trouble sorting all the resulting data out.

14. Random data generation Channel Encoding Mapping IFFT Cyclic Prefix insertion Output Data Channel decoding De-mapping FFT Cyclic Prefix removal Q7: OFDM Block Diagram Transmitter S  P S  P FEC PHY Layer Setup Receiver

15. Q7: Wimax Transmitter IFFT S  P FEC Pulse Shaper & DAC

16. Q7: Channel Coding • Two types of errors have to be catered for • 􀂉 Errors due to Guassian white noise which are • distributed independently of time • 􀂉 Burst errors due to various phenomena • 􀂄 Implementation • 􀂉 Convolutional code and Viterbi decoding algorithm • 􀂉 RS codes

17. Data Randomization Reed-Solomon Encoding Convolutional Encoding Interleaving Q7: FEC • Scrambling • Implemented with PRBS generator • 15-stage shift register • XOR gates in feedback • RS-encoding • Derived from RS(N=255, K=239, T=8) • Shortend and punctured • CC Encoder • Native code rate ½ • Supports punctureing to acheive variable code rate • Interleaver • Two step permutation • First step:adjacent coded bits are mapped onto non-adjacent subcarriers • Second step: adjacent coded bits are mapped alternately onto less or more significant bits of the constellation FEC BLOCK

18. Q7: Convolution Codes • Convolutional Coding 􀂄 The basic idea is to send ‘sequences on the channel’ corresponding to a sequence of input bits such that 􀂉 The closest output sequences possible differ in maximum positions 􀂄 The decoder tries to find the sequence which was most probable for a given received sequence at the output of the channel • Viterbi algorithm is an efficient way to find that sequence 􀂄 Code Rate = ½ 􀂄 Constraint Length = 5

19. Q7: Reed Solomon Code • RS coding is a block coding technique • In our case 10 extra ‘check bits’ are attached to each OFDM symbol. • These 10 bits can correct 5 bit errors in one OFDM symbol • We use (n=48, k=38) RS codes

20. Q7: Interleaving • Burst errors are often contiguous • To randomize the location of errors, interleaving is used • This involves writing data in a certain order and reading it in a different order prior to transmission

21. Q7: Constellation Mapping • Following mappings are supported • 16-QAM, 64 QAM, BPSK and QPSK • The mappings can be selected on the data rate required • For same power the higher constellation mappings normally suffer from worse Prob. Of error • Output of the mapper is complex number $specifying the ‘ weightage (amplitude and phase)’ of respective complex carriers

22. IFFT • IFFT is an efficient operation to do the following operation 􀂉 Multiply the symbol for each carrier with the carrier, generate the waveform and sum all the weighted carrier waveforms • FFT is an efficient operation to extract the coefficients or weightage (amplitude and phase) of the all carriers from the waveform • Frequency-domain spreading: • Spreading is achieved by choosing conjugate symmetric inputs for the input to the IFFT. • Exploits frequency diversity and helps reduce the transmitter complexity/power consumption. • Time-domain spreading: • Spreading is achieved in the time-domain by repeating the same information in an OFDM symbol on two different sub-bands. • Exploits frequency diversity as well as enhances performance.

23. Q7: Cyclic Extension • It is required to make DFT (FFT for implementation) to be valid ! • DFT is valid only for periodic signals in time. • The cyclic prefix should be longer than channel maximum length

24. Q7: Pulse Shaper & DAC • To have less spectral leakage (side lobes) • RC filtering is often used • Additional time is required which reduces efficiency. Therefore OFDM overheads per symbol • Windowing function (Hamming etc) • Guard in some cases • DAC to convert digital to analogue to prepare for transmission

25. Q8: Adv & Disadv of OFDM • OFDM is spectrally efficient. • IFFT/FFT operation ensures that sub-carriers do not interfere with each other. • OFDM has an inherent robustness against narrowband interference. • Narrowband interference will affect at most a couple of tones. • Information from the affected tones can be erased and recovered via the forward error correction (FEC) codes. • OFDM has excellent robustness in multi-path environments. • Cyclic prefix preserves orthogonality between sub-carriers. • Cyclic prefix allows the receiver to capture multi-path energy more efficiently. • The latency of symbol increases with no. of carriers thus causing delay – greater the tones greater the latency

26. channel carrier magnitude subchannel Subchannels are 312 kHz wide in 802.11a and HyperLAN II frequency Q9: Orthogonal Frequency Division Multiplexing • Adapted by current wireless standards • IEEE 802.11a/g, Satellite radio, etc… • Broadband channel is divided into many narrowband subchannels • Multipath resistant • Equalization simpler than single-carrier systems • Uses time or frequency division multiple access

27. User 1 User 2 magnitude frequency . . . Base Station - has knowledge of each user’s channel state information thru ideal feedback from the users User K Orthogonal Frequency Division Multiple Access (OFDMA) • Adapted by IEEE 802.16a/d/e BWA standards • Allows multiple users to transmit simultaneously on different subchannels • Inherits advantages of OFDM • Exploits multi-user diversity

28. Q9: OFDMA-TDMA Principles Using OFDMA/TDMA, Sub Channels are allocated in the Frequency Domain, and OFDM Symbols allocated in the Time Domain.

29. DownLink OFDMA Symbol

30. DownLink Specification • Burst Structure is defined from one Sub-channel in the Frequency domain and n OFDMA time symbols in the time domain, each burst consists of N data modulated carriers. • Adaptive Modulation and Coding per Sub-Channel in the Down-Link • Forward APC controlling (+6dB) – (-6dB) digital gain on the transmitted Sub-Channel • Supporting optional Space Time Coding employing Alamouti STC. • Supporting optional Adaptive Array.

31. Q9: OFDM • Basic idea: divide spectrum into several 528 MHz bands. • Information is transmitted using OFDM modulation on each band. • OFDM carriers are efficiently generated using an 128-point IFFT/FFT. • Internal precision is reduced by limiting the constellation size to QPSK. • Information bits are interleaved across all bands to exploit frequency diversity and provide robustness against multi-path and interference. • 60.6 ns prefix provides robustness against multi-path even in the worst channel environments. • 9.5 ns guard interval provides sufficient time for switching between bands.

33. Multi-band OFDM System Parameters • System parameters for mandatory and optional data rates: * Mandatory information data rate, ** Optional information data rate

34. More Details on the OFDM Parameters • 128 total tones: • 100 data tones used to transmit information (constellation: QPSK) • 12 pilots tones used for carrier and phase tracking • 10 guard tones (used to be known as dummy tones) • 6 NULL tones • Exact use of guard tones is left to implementer – adds a level of flexibility in the standard. • Advantages of using guard tones: • Can relax the analog transmit and receive filters. • Helps to relax filter specifications for adjacent channel rejection. • Can be used to help reduce peak-to-average power ratio (PARP). • Could be used to transmit proprietary information (data/signaling). • If data is mapped onto the guard tones, then this scheme is analogous to the concept of Excess Bandwidth as used in Single-carrier Systems. • Received signal in guard tones can be combined appropriately with the remaining information data tones to improve performance.