Review of Last Lecture MCM, Overlapping Subcarriers EE359 – Lecture 18 Outline and FFT Implementation (OFDM)

l Announcements l MCM splits high rate data stream into lower rate flat-fading substreams l Overlapping subcarriers reduces BW by factor of 2 l My OHs today are 3:45-4:45. Tom has extra OHs by appointment l Modulate symbols with IFFT at TX, Reverse structure (with FFT) in RX l HW due Fri; last HW posted later this week l Lecture next Thu 3/12 1:30-3:30 (course review+advanced topics) l Cyclic prefix makes linear convolution of channel circular, so no interference between FFT blocks in RX processing l Final info (coverage, format, extra OHs, etc) given in 3/10 lecture l Final exam 3/17, 3:30pm-6:30pm here (Thornton 102) X0 x0 TX n(t) l Final projects must be posted 3/14 at midnight (hard deadline). R bps Add cyclic Serial QAM prefix and To D/A Modulator IFFT Parallel x h(t) + l Spread Spectrum Parallel To Serial Converter XN-1 xN-1 Convert l Direct sequence (DSSS) cos(2pfct) l ISI and Interference Rejection of DSSS l RAKE Receiver RX Remove y0 Y0 cyclic R bps l Multiuser Systems prefix and Parallel QAM x LPF A/D FFT Serial to To Serial Modulator l Multiple access techniques Parallel Convert yN-1 YN-1 l Random access techniques cos(2pfct) Convert Yi=HiXi+ni 1 2

Review Continued OFDM Design Issues Intro. to Spread Spectrum

l Timing/frequency offset: l Modulation that increases signal bandwidth l Impacts subcarrier orthogonality; self-interference l Spreads modulated signal over wider BW B~1/Ts l Peak-to-Average Power Ratio (PAPR) than needed for transmission (R=log2(M)/Ts) l Adding subcarrier signals creates large signal peaks l Mitigates or coherently combines ISI l Solve with clipping or PAPR-optimized coding l Mitigates narrowband interference/jamming l Hides signal below noise (DSSS) or makes it hard l Different fading across subcarriers to track (FH) l Mitigate by precoding (fading inversion), adaptive l Also used as a multiple access technique modulation over frequency, and coding across subcarriers l MIMO-OFDM l Two types l Apply OFDM across each spatial dimension l Frequency Hopping: l Can adapt across space, time, and frequency l Narrowband signal hopped over wide bandwidth l MIMO-OFDM represented by a matrix, extends matrix l Direction Sequence: representation of OFDM alone (considered in HW) l Modulated signal multiplied by faster chip sequence 3 4

1 Direct Sequence Spread Spectrum ISI and Interference Rejection

l Bit sequence modulated by chip sequence l Narrowband Interference Rejection (1/K) S(f) S(f) S(f) I(f) s(t) sc(t) Sc(f) S(f)*Sc(f) S(f)*Sc(f) I(f)*Sc(f)

Receiver Input Despread Signal 1/Tb Info. Signal 1/Tc Tc Tb=KTc l Multipath Rejection (Autocorrelation r(t)) l Spreads bandwidth by large factor (G) aS(f) S(f) S(f)*Sc(f)[ad(t)+b(t-t)] 2 l Despread by multiplying by sc(t) again (sc(t)=1) brS’(f) l Mitigates ISI and narrowband interference Info. Signal Receiver Input Despread Signal

Can coherently combine all multipath components via a RAKE receiver 5 6

Multiuser Channels: RAKE Receiver Uplink and Downlink

l Multibranch receiver Uplink (Multiple Access x R3 l Branches synchronized to different MP components Channel or MAC): h3(t) x Demod Many Transmitters to One Receiver. y(t) sc(t) x h22(t) ^ dk Diversity Downlink (Broadcast x h21(t) x Demod x Channel or BC): h1(t) s (t-iT ) Combiner c c One Transmitter R2 to Many Receivers. x Demod R1

sc(t-NTc)

l These components can be coherently combined Uplink and Downlink typically duplexed in time or frequency l Use SC, MRC, or EGC Full-duplex radios are being considered for 5G systems 7 8

2 Bandwidth Sharing in Multiple Access Channels assigned by central controller Random vs. Multiple Access

l Frequency Division Code Space • In multiple access, channels are assigned by a centralized controller - Requires a central controller and control channel l OFDMA Time - Inefficient for short and/or infrequent data transmissions l Non-orthogonal FD • In random access, users access channel randomly when they have data to send Frequency Code Space • A simple random access scheme will be explored in homework l Time Division Time • ALOHA Schemes (not on exams/HW) l Non-orthogonal TD • Data is packetized. Frequency Code Space • Packets occupy a given time interval l Code Division Time l Code cross-correlation • Pure ALOHA • Slotted ALOHA dictates interference Frequency – send packet whenever data is available – same as ALOHA but with packet slotting l Multiuser Detection – a collision occurs for any partial overlap of – packets sent during predefined timeslots packets (nonorthogonal slots) – A collision occurs when packets overlap, l Space Division (SDMA) – Packets received in error are retransmitted after but there is no partial overlap of packets random delay interval (avoids subsequent – Packets received in error are retransmitted l Hybrid Schemes collisions). after random delay interval. 7C29822.033-Cimini-9/97 9 10

Main Points

l Spread spectrum increases signal bandwidth above that required for information transmission

l Benefits of spread spectrum: l ISI/narrowband interference rejection by spreading gain l Also used as a multiuser/multiple access technique

l Multiple access: users can share the same spectrum via time/frequency/code/space division

l Random access more efficient than multiple access for short/infrequent data transmission

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