Using Delta-Sigma Modulators in Visible Light OFDM Systems

Transcription

1 Using Modulators in Visible Light OFDM Systems Visible Light Communications (VLC) 4 The Center or Signal & Image Processing arxiv:4.7v [cs.it] 6 Jan 4 Zhenhua Yu, Arthur J. Redern and G. Tong Zhou School o Electrical and Computer Engineering, Georgia Institute o Technology, Atlanta, Georgia, USA Texas Instruments, TI Boulevard MS 8649, Dallas, TX 74, USA zhenhuayu@gatech.edu Abstract Visible light communications (VLC) are motivated by the radio-requency (RF) spectrum crunch and ast-growing Electric signal Electric signal solid-state lighting technology. VLC relies on white light emitting diodes (LEDs) to provide communication and illumination LED Light intensity Photodiode simultaneously. Simple two-level on-o eying (OOK) and pulseposition modulation (PPM) are supported in IEEE standard due Intensity modulation (IM) Direct detection (DD) On-o eying (OOK), Pulse place modulation (PPM), Pulse amplitude modulation (PAM), OFDM to their compatibility with existing constant current LED drivers, Fig.. Intensity modulation and direct detection in VLC but their low spectral eiciency have limited the achievable data rates o VLC. Orthogonal requency division multiplexing (OFDM) has been applied to VLC due to its high spectral eiciency and ability to combat inter-symbol-intererence (ISI). However, VLC-OFDM inherits the disadvantage o high peato-average power ratio (PAPR) rom RF-OFDM. Besides, the continuous magnitude o OFDM signals requires complicated mixed-signal digital-to-analog converter () and modiication o LED drivers. We propose the use o delta-sigma s in visible light OFDM systems to convert continuous magnitude OFDM symbols into LED driver signals. The proposed system has the communication theory advantages o OFDM along with the practical analog and optical advantages o simple two level driver signals. Simulation results are provided to illustrate the proposed system. I. INTRODUCTION Visible light communication (VLC) has attracted a lot o attentions or its potential to complement conventional RF communication [], [], [], [4]. VLC relies on white LEDs which already provide illumination and are quicly becoming the dominant lighting source to transmit data. VLC is motivated by a number o beneits including, but not limited to, piggybac on existing illumination inrastructure, low-cost ront-ends, more security (visible light cannot penetrate wall), no electromagnetic intererence, and being sae or human. VLC employs simple intensity modulation (IM) and direct detection (DD) schemes, which requires the modulation signal to be real-valued and positive. Single-carrier unipolar and real-valued modulations, such as on-o eying (OOK), variable pulse-position modulation (VPPM), and pulse amplitude modulation (PAM), are adopted in VLC [], []. Recently, orthogonal requency division multiplexing (OFDM) has been considered or VLC due to its ability to boost data rates and eiciently combat inter-symbol-intererence (ISI) [6], [7], [8]. OFDM can easily support multiple access (OFDMA), which is essential or multi-user broadcasting. However, transmitting OFDM symbols requires the driving circuits o a white LED to support continuous magnitude inputs, and the mixed-signal digital-to-analog converter () design is complicated. In Communication Illumination Brightness control IEEE 8..7 standard [], only two-level modulations (OOK and VPPM) are supported because their seamless compatibility min with most constant-current LED drivers [9]. Since the primary unction o the VLC is providing illumination, modiication o driving circuits o LED will be not beneicial to the application and commercialization o VLC. Moreover, the high pea to average power ratio (PAPR) maes OFDM signals sensitive to LED nonlinearities [], [], []. In this wor, we propose using a delta-sigma [], [4] to convert the OFDM signal into a two level representation and directly drive the LED. Most o the quantization noises will be pushed to the out-o-band, and the in-band subcarriers can be simply recovered at the receiver. Section II reviews modulation techniques in VLC and basic concept o delta-sigma digital to analog converters (s). Section III describes the delta-sigma based VLC-OFDM system and discusses the advantages. Numerical results are shown in Section IV and conclusions are provided in Section V. max II. BACKGROUND In this section, we review visible light communications and delta-sigma digital to analog converters (s). A. Visible light communications In visible light communication systems, white LED is utilized to simultaneously transmit inormation and illuminate. Intensity modulation (IM) is employed at the transmitter. The orward electric signal drives the LED which in turn converts the magnitude o the input electric signals into optical intensity. The human cannot perceive ast-changing variations o the light intensity, and only respond to the average light intensity. Direct detection (DD) is employed at the receiver. A photodiode (PD) transorms received optical power into the amplitude o an electrical signal. Fig. shows the concept o intensity modulation and direct detection in VLC.

2 Cloc OOK VPPM Data input 4 Interpolator... Quantization noise (OSR-) OSR (OSR-) OSR (OSR-) OSR Examples(b) o Spectrum OOK and VPPM at each waveorms stage R(z) - Q(z) 4 Low pass ilter L (a) Delta sigma input Interpolator Fig.. System diagram L Quantization V(z) (a) Delta sigma (OSR-) OSR... 4 V(z) V(z) Quantization (R(z))Q(z) noise (c) Delta sigma (OSR-) (OSR-) (OSR-) OSR OSR OSR IM/DD schemes require modulation signals in VLC to be real-valued and positive. In IEEE 8..7 standard [], on-o eying (OOK) and variable pulse-position modulation (VPPM) are supported. Fig. shows the waveorms o OOK and VPPM signals. OOK is the simplest modulation in VLC. OOK transmits the bit by turning on the light and transmits the bit by turning o the light. When transmitting the bit, the light is not necessarily turned o completely, but dimmed at a lower level relatively to the turning on when transmitting the bit. VPPM encodes the data using the position o the pulse within a time period. is represented by a positive pulse at the beginning o the period ollowed by a lower level pulse, and is represented by a lower level pulse at the beginning o the period ollowed by a positive pulse. OFDM can be applied to VLC to improve the spectral eiciency. Let {X } N/ N/ be the requency domain sequence o an OFDM symbol, where N is the number o subcarriers and is the subcarrier spacing. A Nyquist rate discrete time-domain bloc x [x, x,..., x N ] is generated by applying the inverse FFT (IFFT) operation to a requencydomain sequence as x n IFFT(X ) N/ N N/ ( X exp jπ n ), () N n,,..., N, where j. To generate real-valued and positive baseband OFDM signal, DC biased optical OFDM (DCO-OFDM) [6] was introduced or VLC. According to the property o inverse Fourier transorm, a real-valued time-domain signal x n corresponds to a requency-domain signal X that is Hermitian Fig. 4. (OSR-) OSR Signal spectrum (b) Spectrum at dierent at each stagesstage o delta sigma. symmetric; i.e., R(z) Quantization Q(z) V(z) (R(z))Q(z) Fig.. Delta sigma. V(z) X X, N/, () where denotes complex conjugate. In DCO-OFDM, the th and N/th subcarrier are null; i.e., X, X N/. Then a DC bias is added to x n to mae the signal positive. One drawbac o OFDM is the high pea-to-average power ratio (PAPR) []; i.e., B. PAPR{x n } max x n N n x n Fig. is bloc diagram o a delta-sigma and Fig. 4 shows the signal spectrum at dierent stages. A delta-sigma ()

3 Data Subcarrier assignment IFFT Add cyclic preix -bit -level biasing White LED Data Subcarrier extraction FFT Remove cyclic preix ADC Anti-alias Filtering PD Fig. 6. Visible light OFDM transmitter based on delta-sigma. [] generally comprises a interpolator, delta-sigma, a mixed signal core and an analog ilter. The input is a digital signal sampled at rate F s with a resolution o. The interpolator upsamples the signal to sampling rate L F S, where L is the over sampling ratio (OSR), and suppress the spectral replicas centered at F s, F s..., (L )F s. The role o a delta-sigma is to convert a digital signal with P levels into a digital signal with M levels, where M < P, while maintaining a high in-band signal-tonoise power ratio. Converting to a signal with only a ew levels maes the analog more tolerant to component mismatch and nonlinearities. Fig. shows the bloc diagram o a deltasigma. The operation o delta-sigma can be expressed in the z domain as V (z) ( R(z))Q(z) (4) where U(Z), V (Z), and Q(Z) denote the z transorm o the input, output and quantization error o the delta-sigma, respectively. R(z) is noise transer unction (NTF). The delta-sigma pushes quantization noise out o the signal band through the appropriate design o the NTF. A mixed signal reproduces the M level digital signal at its input and an analog lowpass ilter removes most o the out-o-band noise power and creates a P bit resolution analog signal. III. VISIBLE LIGHT OFDM TRANSMISSION BASED ON A DELTA-SIGMA MODULATOR Fig. 6 is a bloc diagram o visible light OFDM transmission that uses a delta-sigma. In a delta-sigma, interpolation is used or generating oversampled signals at the input o the delta-sigma. In our proposed ramewor, we obtain the oversampled OFDM signals by zero padding in the requency domain. Deine the in-band indices to be the set I [ N/, N/ ], and the out-o-band indices to be the set O [ NL/, N/ ] [N/, LN/ ]. The zero padded version o X can be expressed as X (L) { X, I, O. () Fig. 7. Zero padding Hermitian symmetric Zero padding Subcarrier assignment or oversampled VLC-OFDM signal. Fig. 8. requency response o emitted white light and the blue part o a typical white light (Luxeon STAR) LED []. Fig. 7 shows the subcarrier assignment or oversampled VLC- OFDM signal. An LN length IFFT is used to convert the requency domain sequence {X (L) } LN/ LN/ into a L times oversampled time-domain sequence {x n (L) } LN n. The delta-sigma is then used to convert the continuous magnitude time domain OFDM digital samples {x n (L) } LN n into one bit signals {y n (L) } LN n, where y n (L) {, }. According to Eq. (4), the data on the requencydomain subcarriers o y n (L) can be expressed as Y (L) X (L) NTF Q { (6) X NTF Q, I NTF Q, O,

4 ... OFDM signal Delta sigma output Time Fig. 9. Input and output sequence o delta-sigma (L 8). where NTF denotes the noise transer unction on the th subcarrier and Q denotes the quantization noise on the th subcarrier, respectively. Since NTF is actually a high-pass ilter, very ew distortions all on the in-band subcarriers. We use error vector magnitude (EVM) [] to quantiy the in-band distortions EVM N/ Y (L) N/ X (L) X (L) (7) N/ NTF Q N/. X The proposed has a single bit digital input and a two level analog output. To ensure that the input o the LED is positive, a bias is added to the two level analog signal, which only aects the DC component. Note that this bias could be built into the. We directly use the two level output signal (ater biasing) that contains out-o-band quantization noise to drive the LED or two reasons. First, most o the existing driving circuits o white LEDs used or the purposes o illumination are not compatible with continuous amplitude driving voltages. However, a two level input signal, lie a pulse width modulation (PWM) or OOK signal, is widely accepted []. Second, out-o-band inerence is not a concern in VLC because the LED acts as a low pass ilter (similar to a speaer in a class D ampliier audio system [6]) and no out-o-band inormation is transmitted simultaneously. As an example, the most popular white LED, which uses a blue emitter in combination with a phosphor that emits yellow light, has limited bandwidth. The requency response o emitted white light and the blue part o a typical white light (Luxeon STAR) LED is shown in Fig. 8 []. The bandwidth o the white light response is only. MHz. Any remaining out-o-band noise can be removed or ignored at the receiver, either with a low pass anti-aliasing ilter or by ignoring outo-band subcarriers ater the FFT. The delta-sigma based transmitter also has some advantages over a conventional visible light OFDM transmitter. First, the structure o a bit mixed signal is very simple and its linearity is theoretically perect. Second, the drawbac o the high PAPR o OFDM signal is avoided. Third, it is unnecessary to design a customized driving circuit or the LED to support continuous or multiple amplitude level input db signals. Delta sigma output LED output L 8, ourth order NTF Frequency (MHz) Fig.. PSD o input and output o LED. IV. NUMERICAL RESULTS To illustrate the proposed system, we choose N 6 subcarriers with a spacing o Hz, and 4-QAM modulation. The bandwidth o the OFDM signal,.87 MHz, is within the db bandwidth o the white LED rom []. The NTF o the delta-sigma was obtained rom the Matlab toolbox associated with []. Fig. 9 plots a section o the input and output o the delta-sigma and Fig. plots the power spectral density (PSD) o the input and output o the LED with a ourth order NTF and L 8. Note that most o the noise alls out o the signal band. Fig. plots the EVM o transmitted signals with various OSRs and orders o NTF. By increasing the OSR, less in-band noise is observed. However, increasing the order o NTF does not always guarantee better perormance because the stability o the loop ilter in a delta-sigma may become a problem. Dierent loop ilter designs can be created to address this issue. V. CONCLUSIONS We used a delta-sigma to convert a continuous magnitude OFDM digital signal into a two level analog signal that can directly serve as the input o a LED. This scheme

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