Broadband OFDM-FDMA System for the Uplink of a Wireless LAN

Broadband OFDM-FDMA System for the Uplink of a Wireless LAN Dirk Galda and Hermann Rohling Department of Telecommunications,TU of Hamburg-Harburg Eißendorfer Straße 40, 21073 Hamburg, Germany Elena Costa, Harald Haas, and Egon Schulz ICM N MR ST 8, SIEMENS AG Grassbrunn, Germany Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 1

Overview Motivation OFDM Transmission Technique Multiple Access Schemes for OFDM OFDM-FDMA Uplink Peak-to-average ratio in OFDM communications DFT-spreading Types of spreading matrices Mapping scheme Single user case OFDM-FDMA User data rate adaptation Summary Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 2

Properties of the OFDM Transmission Technique Typical parameters of future WLANs System bandwidth B 20 MHz Sampling time T 50 ns Max. multi-path delay τ max < 2 µs Single Carrier: Symbol duration mainly depends on data rate Symbol duration << Max. multi-path delay Inter symbol interferences (ISI) occur High complexity equalizer required Multi Carrier: Transmission on parallel, orthogonal subcarriers Symbol duration >> Max. multi-path delay No ISI and no ICI if guard interval is used Simple one tap equalizer for each subcarrier Time GI Data 1 GI Data 2 GI GI Data 1 GI Data 2 GI GI Data 1 GI Data 2 GI Path 1 Path 2 Path 3 GI Data 1 GI Data 2 GI Path P τ 1 τ P Data window Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 4

Advantages of OFDM Multiple Access Schemes OFDM-FDMA OFDM-CDMA Frequency Frequency Code Time No multiple access interferences Flexible resource allocation Channel adaptive Time Exploit frequency diversity Robust against interferences Combine OFDM-FDMA with individual spreading of user data to profit from advantages of both concepts! Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 5

OFDM-FDMA Uplink System Structure Encoder + Interleaver System Properties: Base Station Decoder + Deinterleaver Modulation User separation by orthogonal sets of subcarriers (OFDM-FDMA) Demodulation Spreading Individual spreading of user data by orthogonal matrix Equalization/ Detection FDMA- Mapping Single- or multi-code detection of each user s data FDMA- Demapping IFFT FFT Add Guard Interval LO Mobile Terminal MT LO Remove Guard Interval PA DUC + DAC ADC + DDC MT Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 6

Fundamental Questions in OFDM Systems Two major questions in OFDM system design OFDM Transmitter H OFDM Receiver Frequency Problem: High power amplifier Non-linearity of the PA causes inand out-of-band distortions Frequency selective channel Problem: Large attenuation of a number of subcarriers causes flat fading BER Possible solutions: Amplifier input backoff (IBO) Correction functions Adaptive mapping Optimised Coding Possible solutions: Interleaving + Coding Spreading + Coding can be solved jointly by spreading with DFT matrices. Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 7

Overview Motivation OFDM Transmission Technique Multiple Access Schemes for OFDM OFDM-FDMA Uplink Peak-to-average ratio in OFDM communications DFT-spreading Types of spreading matrices Mapping scheme Single user case OFDM-FDMA User data rate adaptation Summary and future work Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 8

The Peak-to-Average Problem Limited linear range of the power amplifier causes clipping Input backoff: OFDM Transmitter D/A Effects of clipping: In-band distortion of the signal Increased BER is a quality of service aspect and a selling argument Nobody will buy such a system! S(jω) [db] 10 0-10 -20-30 -40 A 0 Maximum amplitude of amplifier P S Input signal power Out-of-band emissions Other services will be disturbed by the system Nobody will allow such a system! -50-60 -70-4 -3-2 -1 0 1 2 3 IBO = 5 db Ω[2π] Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 9

Selection of Spreading Matrices Requirements on spreading matrices: Orthogonal: Separate data symbols of different users without interference Orthonormal: Spread symbol energy equally over the frequency band since the channel transfer function is unknown Unitary: Preserve distance of code symbols after spreading Possible spreading matrices with these properties: Walsh-Hadamard matrix (WH) Discrete Fourier transform matrix (DFT) Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 11

DFT-Spreading Single User Case OFDM system with DFT spreading over all subcarriers N C N C N C Coding + Interleaver Modulation S/P DFT IFFT P/S Add Guard Equivalent system Coding + Interleaver Modulation Add Guard An OFDM system with DFT spreading is equivalent to a single-carrier system with guard interval Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 12

OFDM-FDMA System with DFT-Spreading DFT spreading can be used in the uplink of an OFDM-FDMA system Coding + Interleaver Modulation S/P Insertion of unmodulated subcarriers result in a periodic time signal DFT Subcarrier Mapping IFFT P/S Add Guard Each period of the time signal is the IDFT of the used subcarriers OFDM time signal: The time signal is a repetition of the DFT spreading matrix input Equivalent system: GI A A A GI Time Coding + Interleaver Modulation Symbol Repition Frequency Shift Add Guard Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 13

OFDM Spectrum Imag Imag Signal envelope: 2.5 2 1.5 1 0.5 0-0.5-1 -1.5-2 -2.5-2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 2.5 Real 2.5 2 1.5 1 0.5 0-0.5-1 -1.5-2 Without DFT-Spreading With DFT Spreading -2.5-2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 2.5 Real 3 db level PA S(jω) [db] 10 0-10 -20-30 -40-50 -60 OFDM (IBO = 5 db) OFDM (IBO = 13 db) OFDM with DFT spreading (IBO = 5 db) OFDM Spectrum after clipping -70-4 -3-2 -1 0 1 2 3 Ω[2π] Result: IBO can be reduced when DFT spreading is employed! 4 Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 14

Use of π/4-qpsk for Subcarrier Modulation Single carrier technique for PAR reduction: Imag Real OFDM with DFT spreading and π/4-qpsk subcarrier modulation: Imag 2.5 2 1.5 1 0.5 0-0.5-1 -1.5-2 -2.5-2.5-2 -1.5-1 -0.5 0 0.5 1 1.5 2 2.5 Real 3 db level PA S(jω) [db] 10 0-10 -20-30 -40-50 -60-70 -4-3 -2-1 0 1 2 3 IBO = 5 db OFDM OFDM (DFT) OFDM (DFT,π/4-QPSK) Ω[2π] 4 Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 15

User Rate Adaptation DFT-Spreading can be used with different user data rates Magnitude User 1 User 2 User 3 Not used Frequency Subcarrier spacing Number of periods per OFDM symbol MT1 MT2 MT3 GI A A A A GI + GI B B GI + GI C C C C C C C C GI BS One OFDM symbol Time Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 16

Summary Spreading can be used as a technique to modify the transmit signal A DFT spreading matrix can reduce the peak-to-average ratio of and OFDM-FDMA uplink system without a performance loss compared to spreading with Walsh-Hadamard matrices An OFDM-FDMA system with DFT spreading can be implemented as a single-carrier transmitter and a conventional OFDM-FDMA receiver Single-carrier techniques can be employed to further reduce the peak-to-average ratio Different user data rates can be realized by modifying the spacing of equidistant subcarriers (number of periods) Broadband OFDM-FDMA System for the Uplink of a Wireless LAN 18