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NOC 2011                                                                                                                  IT - 1




                         Optical Spatial Modulation with
                         Transmitter-Receiver Alignments
                                                           (Invited Paper)
           Raed Mesleh                 Hany Elgala, Marwan Hammouda, and Irina Stefan                         Harald Haas
      University of Tabuk                     Jacobs University Bremen                The University of Edinburgh
47315/4031 Tabuk, Saudi Arabia                28759 Bremen, Germany                    Edinburgh, EH9 3JL, UK
  Email:raed.mesleh@ieee.org Email:h.elgala,m.hammouda,&i.stefan}@jacobs-university.de Email:h.haas@ed.ac.uk



   Abstract—Optical spatial modulation (OSM) is a pulsed modu-        (LOS) paths. Imaging receivers can not collect light over
lation technique for indoor optical wireless (OW) communication       a field of view (FOV) as wide as a non-imaging angle-
systems, proposed recently in [1]. In this paper, the performance     diversity receivers, since the elements of the latter can be
of OSM is significantly enhanced by proper geometrical align-
ment of transmit and receive units. In OSM, multiple transmit         oriented in any direction. Also, the size of the imaging lens
units are considered where only a single transmit unit is activated   and the detector array depend on the location and is not
at a particular time instance and all others are turned off. The      suitable for all applications. Another OW MIMO system with
incoming bits are grouped in blocks with length equivalent to         subcarrier multiplexing (SCM) is proposed in [7] where a
the base-two logarithm of the number of transmit units. A block       spatial multiplexing technique with zero forcing (ZF) detection
of bits forms a spatial symbol, and the actual transmit units are
considered as spatial constellation points. A 4x4 multiple-input      is considered. The performance of the system depends on the
multiple-output (MIMO) model inside a room is considered to           achievable signal to interference plus noise ratio (SINR) values
evaluate the performance of the proposed OSM system. It is            for different configurations. It is shown that with transmitter
shown that the proposed scheme is very efficient in terms of           semiangles (φ 1 ≥ 20◦ ), the separation between transmit and
                                                                                      2
power and bandwidth as compared to on-off keying (OOK), pulse         receive units should be larger than 1.5 m to achieve a bit-
position modulation (PPM), and pulse amplitude modulation
(PAM).                                                                error-ratio (BER) lower than 10−4 . An alternative OW MIMO
                                                                      system is the OSM system proposed in [1] to increase the data
                      I. I NTRODUCTION                                rate by adopting the SM concept [8]. OSM enhances the data
   The key design challenge for OW is to increase the data            rate by a factor of log2 Nt , where Nt is the number of transmit
rate while maintaining high power efficiency. For instance,            units considered. However, the error performance depends on
PPM is a power efficient modulation technique, at the expense          the locations and the directions of the transmit and receive
of an increased bandwidth to achieve higher data rate. The            units. It is shown that when the transmit units are directed
data rate can be enhanced by considering PAM at the expense           toward the floor and the receive units are directed toward the
of a significant power penalty. A compromise between power             ceiling in a medium-sized room, the performance of the OSM
and bandwidth efficiencies can be achieved by considering              system deteriorates due to the high optical channel correlation
OOK [2, 3].                                                           and a BER of 10−3 requires signal to noise ratio (SNR) above
   MIMO techniques for radio frequency (RF) systems have              20 dB. The same BER can be achieved at 5 dB and 8 dB SNR
been studied intensively over the last decade. As a result, the       for PPM and OOK, respectively [2].
IEEE 802.11n amendment is proposed to significantly improve               The performance of OSM can be significantly enhanced
network throughput over previous wireless local area network          by de-correlating the MIMO channel matrix. In this paper,
(WLANs) standards by using MIMO [4]. The expected maxi-               the optical MIMO channel matrix is de-correlated by properly
mum raw physical net bit rate is boosted from 54 Mbps to 130          aligning the transmit and the receive units. The aligned setup
Mbps for two parallel streams and 600 Mbps for four parallel          creates a full rank MIMO channel matrix which significantly
streams.                                                              enhances the power efficiency of OSM. A gain of about 14 dB
   A spatial multiplexing indoor MIMO technique for visible           in SNR is reported. The SNR distribution for a practical
light communication (VLC) technology using orthogonal fre-            aligned scenario supports 10−3 BER on a large area inside
quency division multiplexing (OFDM) is considered in [5].             a moderate size room. In addition, OSM performance is com-
It is shown that the channel is highly correlated which               pared to OOK, PPM, and PAM in terms of power efficiency
significantly degrades the bit error performance at different          as well as bandwidth efficiency.
locations inside the room. The performance can be enhanced               The remainder of this paper is organized as follows: The
remarkably by considering the imaging diversity receivers             OSM system model is presented in Section II. Performance
technique to de-correlate the MIMO channel matrix [6]. This           analysis along with comparison to OOK, PPM, and PAM are
is achieved at the expense of placing a lens at proper locations      presented in Section III. Finally, Section IV concludes the
between the transmitters and the receivers for line-of-sight          paper.


 978-1-86135-373-3/11/$25.00 ©2011 IEEE                          1
NOC 2011                                                                                                                                IT - 1




                                                                        path gain between LED and PD i for the k th path given
                                                                                   ∞
                                                                           ¯ (k)
                                                                        by hi =
                                                                                       (k)
                                                                                      hi (t) dt. The notation n(t) indicates an Nr
                                                                                   −∞
                                                                        dimensional noise vector. The noise is the sum of the receiver
                                                                        thermal noise and shot noise due to ambient light which
                                                                        can be modeled as independent and identically distributed
                                                                        additive white Gaussian noise (AWGN) with double sided
                                                                        power spectral density σ 2 [2, eqn.(21)]. For the analysis in
                                                                        this paper, H(t) is an Nr × Nt × (K + 1) normalized indoor
                                                                        optical MIMO channel tensor, defined as
Fig. 1: OSM communication system model. The LED mapper                               ⎡                                        ⎤
maps input bits to LED indices. Each sequence of log2 (Nt )                             h11 (t)    h12 (t) · · · h1Nt (t)
                                                                                     ⎢ h21 (t)     h22 (t) · · · h2Nt (t) ⎥
input bits correspond to a certain LED index.                                        ⎢                                        ⎥
                                                                            H (t) = ⎢       .         .      ..        .      ⎥ (3)
                                                                                     ⎣      .
                                                                                            .         .
                                                                                                      .         .      .
                                                                                                                       .      ⎦
                         II. OSM S YSTEM M ODEL                                        hNr 1 (t) hNr 2 (t) · · · hNr Nt (t)
   The system model of the OSM approach is depicted in
                                                                        The channel paths are obtained numerically via ray tracing
Fig. 1. A MIMO system consisting of Nt transmit units (in
                                                                        technique as discussed in details in [9, eqn.(13)].
particular, light emitting diodes (LEDs)) and Nr receive units
(in particular, photo diodes (PDs)) is illustrated. The bits               At the receiver, the PD converts the optical signal to
                                                                        electrical signal and applies the optimal SM detector [10] to
to be transmitted at each time instance, q(t), are grouped
                                                                        detect the active transmit unit as follows,
as the row vectors of the matrix x(t). For illustration pur-
poses, the bits to be transmitted in three time instances are                    ˜ =                   s ¯
                                                                                          arg max py y|¯, H
q (t) = 1 0 0 1 1 1 . Assuming Nt = 4, each                                                      √
log2 (Nt ) bits are transmitted at one time instance and grouped                     =    arg min ρ h s 2 − 2 yT h s                     ,          (4)
                                                  T
as follows, x (t) =         10      01      11      , where (·)T                                                                             T
                                {t=1}   {t=2}   {t=3}                                     K            K                   K
denotes the transpose. The bits in this matrix are mapped to            where h =              ¯ (k)
                                                                                               h1            ¯ (k)
                                                                                                             h2      ···         ¯ (k)
                                                                                                                                 hN r             is an
one of the transmitting LEDs. The selected LED ( ) transmits                             k=0           k=0                 k=0
                                                                        Nr dimensional vector containing the sum of the channel
a non-return to zero (NRZ) pulse with an optical power                  path gains from transmit unit to each receive unit, H =  ¯
(intensity) s = Pt at a particular time instance while all other          h1 h2 · · · hNt is the channel matrix after the optical
LEDs are off. The intensity level carries no information and            to electrical conversion at the receiver, ¯ is the transmitted
                                                                                                                  s
can be utilized to optimize power consumption and range. In             column vector from the matrix s(t) at this time instance, and
the considered example, assuming the mapping table in Fig. 1,
                                                                                                                    √ ¯ 2
the resultant matrix is given by,                                                      s ¯
                                                                                py y|¯, H = π −Nt exp − y − ρH¯ F        s         (5)
                      ⎡                        ⎤
                           0       0      0
                                                                        is the probability density function (pdf) of y conditioned on
                      ⎢ 0         s       0 ⎥                                                                     ¯
                      ⎢                        ⎥                        the transmitted vector ¯ and the channel H. The notation · F
                                                                                               s
              s(t) = ⎢ 0           0      s    ⎥.            (1)
                      ⎣                        ⎦                        stands for the Frobenius norm of a vector or a matrix.
                          s        0      0                                The estimated transmit unit index ˜ is then used to decode
                                {t=1}   {t=2}   {t=3}
                                                                        the original information bits by inverse mapping process using
   Each column of the matrix s(t) is transmitted at a single            the same mapping table as used at the transmitter.
time instance where each element in the column vector corre-
sponds to the respective transmit unit. As can be seen, at any                           III. P ERFORMANCE A NALYSIS
give time only one transmit unit is active, all others transmit           For Monte Carlo simulations, the considered 4x4 MIMO
zero power. The transmission is made over the optical MIMO              system inside a room is depicted in Fig. 2(a). The height of
channel H(t).                                                           the transmit units from the ground is given by z in meters and
   The received signal can be written as,                               the transmitters half power angle is given by φ 1 in degrees.
                       √                                                                                                   2
                                                                        The transmit units are placed on the ceilings with the exact
                y(t) = ρH(t) ⊗ s(t) + n(t)                  (2)
                                                                        locations shown in Fig. 2(b) and Fig. 2(c) for the unaligned
                                                       ¯
                                                    r2 P 2
where ⊗ denotes time convolution, ρ = σ2r is the aver-                  and the aligned scenarios, respectively. The receive units are
age electrical SNR at each PD, r is the PD responsivity,                placed on a desktop with height 1 m from the floor. The exact
            Nr
¯      1           (i)                                                  locations are shown in Fig. 2(b) and Fig. 2(c) for the unaligned
Pr =   Nr         Pr is the average received optical power at each
            i=1                                                         and the aligned scenarios, respectively.
                         K                                                For the unaligned scenario, the transmit units are directed
PD with Pr
             (i)
                   =         ¯ (k)
                             hi Pt being the average received optical
                       k=0                                              toward the floor and the receive units are directed toward the
power at PD i when LED is active, K is the number of the                ceiling. The receivers FOV is 90◦ . In Fig. 3, the average
                                        ¯ (k)
considered channel reflection paths, and hi is the channel               BER is plotted versus the average electrical SNR at each


                                                                   2
NOC 2011                                                                                                                                                                                                                                                                                              IT - 1




                                                                                                                                   0
                                                                                                                                  10                                                                                                                            0
                                                                                                                                                                                                                                                               10



                                                                                                                                   −1                                                                                                                           −1
                                                                                                                                  10                                                                                                                           10



                                                                                                                                   −2                                                                                                                           −2
                                                                                                                                  10                                                                                                                           10




                                                                                                                            BER




                                                                                                                                                                                                                                                         BER
                                                                                                                                   −3                                                                                                                           −3
                                                                                                                                  10                                                                                                                           10




                                                                                                                                   −4                                                                                                                           −4
                                                                                                                                  10                                                                                                                           10
                                                                                                                                                                        φ1/2 = 35◦ simulation
                                                                                                                                                                                                                                                                                z = 5m simulation
                                                                                                                                                                        φ1/2 = 35◦ analytical                                                                                   z = 5m analytical
Fig. 2: (a) A 4x4 optical MIMO model inside a room. (b) Tx-
                                                                                                                                   −5                                                                                                                           −5
                                                                                                                                  10                                                                                                                           10
                                                                                                                                                                                    ◦
                                                                                                                                                                        φ1/2 = 40 analytical                                                                                    z = 6.5m analytical
                                                                                                                                                                        φ1/2 = 60◦ analytical                                                                                   z = 8m analytical
Rx locations for the unaligned scenario. (c) Tx-Rx locations                                                                       −6                                                                                                                           −6
                                                                                                                                  10                                                                                                                           10
for the aligned scenario. Each Tx-Rx pair are directed toward                                                                                      0                    2       4          6
                                                                                                                                                                                        SNR (dB)
                                                                                                                                                                                                       8   10                                  12                    0         2       4       6
                                                                                                                                                                                                                                                                                            SNR (dB)
                                                                                                                                                                                                                                                                                                        8       10   12



each other.
             0
            10                                                     0
                                                                  10
                                                                                                                                   Fig. 4: Aligned OSM performance analysis.

             −1
            10                                                     −1
                                                                  10                                                                                                            1.00 at Rx1                                                         1




                                                                                                                                       Channel Impulse Response
                                                                                                                                                                                0.55 at Rx2
                                                                                                                                                                   1            0.53 at Rx3




                                                                                                                                                                                                                    Channel Impulse Response
                                                                                                                                                                                0.55 at Rx4                                                    0.8
             −2
            10                                                     −2
                                                                  10                                                                                              0.8                                                                                                    LOS
                                                                                                                                                                                                                                                                         First reflection
                                                                                                                                                                  0.6
                                                                                                                                                                                                                                               0.6
                                                                                                                                                                  0.4
      BER




                                                            BER




             −3                                                    −3
            10                                                    10

                                                                                                                                                                  0.2
                                                                                                                                                                                                                                               0.4
                          φ1/2 = 35◦ simulation                             z = 5m simulation
                                                                                                                                                                   0
             −4
            10            φ1/2 = 35◦ analytical                    −4
                                                                  10        z = 5m analytical                                                                      4
                          φ1/2 = 40◦ simulation                             z = 6.5m simulation                                                                                                                                                0.2
                                                                            z = 6.5m analytical                                                                             2           Rx1                     4
                          φ1/2 = 40◦ analytical
                                                                            z = 8m simulation
             −5                                                    −5
            10                                                    10                                                                                                                               2
                          φ1/2 = 60◦ simulation                                                                                                                    Y[m]                                                                             0
                                                                            z = 8m analytical                                                                                   0 0                X[m]                                              0              10        20      30        40
                          φ1/2 = 60◦ analytical                                                                                                                                                                                                                           Time (nsec)
             −6                                                    −6
            10                                                    10
                  0   5      10     15       20   25   30               0   5     10     15       20   25   30
                                  SNR (dB)                                             SNR (dB)
                                                                                                                     Fig. 5: Channel impulse response for the aligned scenario
                                                                                                                     between Tx2 and Rx1.
Fig. 3: Unaligned OSM performance analysis. In the left
subfigure, φ 1 values of 35◦ , 40◦ , and 60◦ are considered at
             2                                                                                                          Compared to the results in Fig. 3, the aligned OSM sys-
z = 6 m. In the right subfigure, transmit units are considered                                                        tem performance enhances by at least 14 dB in SNR. This
at heights z =5, 6.5, and 8 m for φ 1 = 30◦ .
                                    2                                                                                enhancement is not due to lower path loss for the aligned
                                                                                                                     scenario since the SNR is the same for all compared systems.
receiver. The results for at least 106 channel realizations are                                                      The improvement is rather due to de-correlating the MIMO
considered. Analytical and simulation results are presented for                                                      channel matrix by direct alignment of the transmit and receive
Nt = Nr = 4, z = 6 m for different values of φ 1 , and for
                                                     2
                                                                                                                     units. Further reduction in the FOV creates a diagonal channel
φ 1 = 30◦ for different values of z. The analytical results are
  2
                                                                                                                     matrix and further enhances the performance.
upper bounds and calculated as discussed in [1, Section III,                                                            For the aligned scenario as in Fig. 2(c), the channel impulse
eqns. (5) and (6)]. The analytical BER equation is also shown                                                        response (LOS) when a single transmitter Tx2 is on and is
in Table I, where N (κ, ν) is the number of bit errors when                                                          aligned to the location of Rx1 is shown in the left-subplot of
choosing κ instead of ν as transmit unit index.                                                                      Fig. 5 (φ 1 is 60◦ , FOV is 45◦ , z = 6 m). Rx1 is aligned to
                                                                                                                               2
   The performance of the unaligned system is poor and for                                                           the location of Tx2 and the other receive units are directed
all considered φ 1 angles, see the left sub-plot in Fig. 3,
                  2
                                                                                                                     toward the ceiling. The de-correlation is clearly noticed from
SNR values above 20 dB are required to achieve 10−2 BER.                                                             the obtained amplitudes at the different locations of the four
According to the right sub-plot in Fig. 3, at 20 dB SNR, a                                                           transmit units. For the same aligned scenario, the channel
BER ≤ 10−3 can not be achieved regardless of the considered                                                          impulse response (LOS and first reflection path) between Tx2
height, z.                                                                                                           and Rx1 is shown in the right-subplot of Fig. 5. It can be
   The performance can be significantly enhanced by aligning                                                          found that the 3 dB bandwidth for this path is 30 MHz and
the transmit and receive units as depicted in Fig. 2(c) and                                                          that the channel delay spread is approximately 33 ns. At this
reducing the receivers FOV to 45◦ . Performance results for                                                          delay spread, 96% from the overall power is captured.
this scenario are shown in Fig. 4. It can be observed that                                                              The SNR distributions on a horizontal plan at 1 m from
neither changing the angles, nor changing the heights affect                                                         the floor for the unaligned and the aligned scenarios (a single
the simulated BER performance. However, slight differences                                                           transmit unit, Tx2, is turned on and an AWGN model intro-
are noticeable between analytical and simulation results which                                                       duced in [2, eqn.(21)] is considered) are shown in Fig. 6(a) and
can be attributed to numerical errors caused by the small                                                            Fig. 6(b), respectively. For the unaligned scenario, it can be
variation between the channel path gains with different heights                                                      seen that the maximum achieved SNR is 11 dB. The achieved
and angles.                                                                                                          maximum SNR would not allow a useful BER as can be found


                                                                                                                 3
NOC 2011                                                                                                                            IT - 1




                                                                          TABLE I: OSM, OOK, PPM, and PAM performance
                                                                       Scheme                             BER                        Data rate
                                                                                                           √
                                                                        OOK                              Q   ρ                          B
                                                                                                   L       Llog2 L                   log2 L
                                                                       L-PPM                       2
                                                                                                     Q        2
                                                                                                                   ρ                   L
                                                                                                                                            B
                                                                                              2(L−1)            log 2 L
                                                                       L-PAM                  Llog2 L
                                                                                                      Q        (L−1)2
                                                                                                                        ρ            Blog2 L
                                                                                        Nt   Nt
                                                                                   1                               ρ             2
                                                                        OSM        Nt
                                                                                                   N (κ, ν)Q       4
                                                                                                                       hk − hν       Blog2 Nt
                                                                                        κ=1 ν=1
                                                                                             ν=κ



                   (a)                                (b)
                                                                                                  IV. C ONCLUSIONS
Fig. 6: SNR distribution. A single transmitter Tx2 is on (15 W          OSM performance can significantly be enhanced by a proper
optical power), φ 1 is 60◦ , and the receiver is directed toward
                  2                                                  alignment of transmit and receive units. The resultant scheme
the ceiling. (a) unaligned scenario. Tx2 directed toward the         is shown to be very efficient in terms of power and band-
floor and FOV is 90◦ . (b) aligned scenario. Tx2 is aligned to        width efficiency as compared to the existing OW modulation
the location of Rx1 and FOV is 45◦ .                                 techniques. The simplicity of the scheme makes it a strong
                                                                     candidate for practical implementations. The considered OSM
                    −1
                   10
                                           4−PPM, 10 Mbps
                                           OSM, 40 Mbps
                                                                     system is shown to achieve a BER slightly better than OOK
                                           OOK, 20 Mbps

                    −2
                                           4−PAM, 40 Mbps            and a data rate that is twice that of OOK. The power efficiency
                   10
                                                                     of OSM is expected to be significantly enhanced by increasing
                    −3
                                                                     the number of receive units and/or considering channel coding
             BER




                   10


                    −4
                   10                                                techniques. In addition, the bandwidth efficiency of OSM
                    −5
                                                                     can be boosted by increasing the number of transmit units.
                   10
                                                                     These will be addressed in detail in future publications and in
                    −6
                   10
                        0   5     10
                                SNR (dB)
                                           15               20       experimental demonstrations.

Fig. 7: Performance comparison between aligned OSM, OOK,                                             R EFERENCES
4-PPM, and 4-PAM and assuming a bandwidth of 30 MHz.                  [1] R. Mesleh, R. Mehmood, H. Elgala, and H. Haas, “Indoor MIMO
                                                                          Optical Wireless Communication Using Spatial Modulation,” in IEEE
                                                                          International Conference on Communications (ICC’10), Cape Town,
                                                                          South Africa, 22–27 May 2010, pp. 1–5.
from results in Fig. 3. However, for the aligned case, SNR            [2] J. M. Kahn and J. R. Barry, “Wireless Infrared Communications,”
values between 7.5 dB and 8.5 dB are achieved in several                  Proceedings of the IEEE, vol. 85, no. 2, pp. 265–298, Feb. 1997.
locations. These SNRs are sufficient to achieve at least 10−3          [3] B. Wilson and Z. Ghassemlooy, “Pulse Time Modulation Techniques for
                                                                          Optical Communications: A Review,” in In the Proceeding of the IEE
BER according to the obtained BER results in Fig. 4.                      on Optoelectronics, vol. 140, no. 6, Dec. 1993, pp. 347–357.
   Finally, the performance of the aligned OSM (φ 1 is 60◦ ,
                                                       2
                                                                      [4] BROADCOM             Corporation,       “802.11n:      Next-Generation
                                                                          Wireless      LAN     Technology,”     White    paper,   BROADCOM
FOV is 45◦ , z = 6 m) system is compared to the performances              Corporation, Tech. Rep., Apr. 2006, retrieved Aug. 4, 2006
of OOK, 4-PPM, and 4-PAM [2, 11] as shown in Fig. 7. Also,                http://www.broadcom.com/docs/WLAN/802-11n-WP100-R.pdf.
the BER equations as a function of the SNR (ρ) and the data           [5] L. Zeng, D. O’Brien, H. Minh, G. Faulkner, K. Lee, D. Jung, Y. Oh, and
                                                                          E. T. Won, “High Data Rate Multiple Input Multiple Output (MIMO)
rates as a function of the bandwidth B for all modulation                 Optical Wireless Communications using White LED Lighting,” IEEE J.
techniques are listed in Table I; where Q(·) denotes the Q-               Select. Areas Commun., vol. 27, no. 9, pp. 1654–1662, Dec. 2009.
function. In OSM, a NRZ pulse is transmitted at each time             [6] P. Djahani and J. M. Kahn, “Analysis of Infrared Wireless Links
                                                                          Employing Multibeam Transmitters and Imaging Diversity Receivers,”
instance from a single transmit unit. Hence, for the same SNR,            IEEE Transactions on Communications, vol. 48, no. 12, pp. 2077–2088,
the output optical power is exactly the same for all compared             Dec. 2000.
systems.                                                              [7] D. Takase and T. Ohtsuki, “Optical Wireless MIMO Communications
                                                                          (OMIMO),” in Proc. IEEE Global Telecommunications Conference
   The PPM technique is the best technique in terms of power              GLOBECOM ’04, vol. 2, Texas, USA, 29 Nov.–3 Dec. 2004, pp. 928–
efficiency. For instance, 4-PPM achieves a BER of about 10−4               932.
                                                                      [8] R. Mesleh, H. Haas, S. Sinanovi´ , C. W. Ahn, and S. Yun, “Spatial
                                                                                                             c
at a SNR of approximately 6 dB. For the same BER, 9 dB,                   Modulation,” IEEE Trans. Veh. Technol., vol. 57, no. 4, pp. 2228 –
9.5 dB, and 18 dB SNR are required for OSM, OOK, and                      2241, July 2008.
4-PAM, respectively. However, 4-PPM is the worst in terms             [9] J. Barry, J. Kahn, W. Krause, E. Lee, and D. Messerschmitt, “Simulation
                                                                          of Multipath Impulse Response for Indoor Wireless Optical Channels,”
of bandwidth efficiency. Half the data rate as compared to                 IEEE J. Select. Areas Commun., vol. 11, no. 3, pp. 367–379, Apr. 1993.
OOK and only a quarter of the data rate as compared to               [10] J. Jeganathan, A. Ghrayeb, and L. Szczecinski, “Spatial Modulation:
OSM with Nt = 4 and 4-PAM can be achieved. The proposed                   Optimal Detection and Performance Analysis,” IEEE Commun. Lett.,
                                                                          vol. 12, no. 8, pp. 545–547, 2008.
OSM technique achieves a performance that is slightly better         [11] R. J. Green, H. Joshi, M. D. Higgins, and M. S. Leeson, “Recent
than OOK and enhances the data rate of OOK by a factor of                 Developments in Indoor Optical Wireless,” IET Communications, vol. 2,
log2 (Nt ). The same data rate is achieved by PAM but with a              no. 1, pp. 3–10, Jan. 2008.
significant power penalty.


                                                                 4

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Optical Spatial Modulation with Transmitter-Receiver Alignments

  • 1. NOC 2011 IT - 1 Optical Spatial Modulation with Transmitter-Receiver Alignments (Invited Paper) Raed Mesleh Hany Elgala, Marwan Hammouda, and Irina Stefan Harald Haas University of Tabuk Jacobs University Bremen The University of Edinburgh 47315/4031 Tabuk, Saudi Arabia 28759 Bremen, Germany Edinburgh, EH9 3JL, UK Email:raed.mesleh@ieee.org Email:h.elgala,m.hammouda,&i.stefan}@jacobs-university.de Email:h.haas@ed.ac.uk Abstract—Optical spatial modulation (OSM) is a pulsed modu- (LOS) paths. Imaging receivers can not collect light over lation technique for indoor optical wireless (OW) communication a field of view (FOV) as wide as a non-imaging angle- systems, proposed recently in [1]. In this paper, the performance diversity receivers, since the elements of the latter can be of OSM is significantly enhanced by proper geometrical align- ment of transmit and receive units. In OSM, multiple transmit oriented in any direction. Also, the size of the imaging lens units are considered where only a single transmit unit is activated and the detector array depend on the location and is not at a particular time instance and all others are turned off. The suitable for all applications. Another OW MIMO system with incoming bits are grouped in blocks with length equivalent to subcarrier multiplexing (SCM) is proposed in [7] where a the base-two logarithm of the number of transmit units. A block spatial multiplexing technique with zero forcing (ZF) detection of bits forms a spatial symbol, and the actual transmit units are considered as spatial constellation points. A 4x4 multiple-input is considered. The performance of the system depends on the multiple-output (MIMO) model inside a room is considered to achievable signal to interference plus noise ratio (SINR) values evaluate the performance of the proposed OSM system. It is for different configurations. It is shown that with transmitter shown that the proposed scheme is very efficient in terms of semiangles (φ 1 ≥ 20◦ ), the separation between transmit and 2 power and bandwidth as compared to on-off keying (OOK), pulse receive units should be larger than 1.5 m to achieve a bit- position modulation (PPM), and pulse amplitude modulation (PAM). error-ratio (BER) lower than 10−4 . An alternative OW MIMO system is the OSM system proposed in [1] to increase the data I. I NTRODUCTION rate by adopting the SM concept [8]. OSM enhances the data The key design challenge for OW is to increase the data rate by a factor of log2 Nt , where Nt is the number of transmit rate while maintaining high power efficiency. For instance, units considered. However, the error performance depends on PPM is a power efficient modulation technique, at the expense the locations and the directions of the transmit and receive of an increased bandwidth to achieve higher data rate. The units. It is shown that when the transmit units are directed data rate can be enhanced by considering PAM at the expense toward the floor and the receive units are directed toward the of a significant power penalty. A compromise between power ceiling in a medium-sized room, the performance of the OSM and bandwidth efficiencies can be achieved by considering system deteriorates due to the high optical channel correlation OOK [2, 3]. and a BER of 10−3 requires signal to noise ratio (SNR) above MIMO techniques for radio frequency (RF) systems have 20 dB. The same BER can be achieved at 5 dB and 8 dB SNR been studied intensively over the last decade. As a result, the for PPM and OOK, respectively [2]. IEEE 802.11n amendment is proposed to significantly improve The performance of OSM can be significantly enhanced network throughput over previous wireless local area network by de-correlating the MIMO channel matrix. In this paper, (WLANs) standards by using MIMO [4]. The expected maxi- the optical MIMO channel matrix is de-correlated by properly mum raw physical net bit rate is boosted from 54 Mbps to 130 aligning the transmit and the receive units. The aligned setup Mbps for two parallel streams and 600 Mbps for four parallel creates a full rank MIMO channel matrix which significantly streams. enhances the power efficiency of OSM. A gain of about 14 dB A spatial multiplexing indoor MIMO technique for visible in SNR is reported. The SNR distribution for a practical light communication (VLC) technology using orthogonal fre- aligned scenario supports 10−3 BER on a large area inside quency division multiplexing (OFDM) is considered in [5]. a moderate size room. In addition, OSM performance is com- It is shown that the channel is highly correlated which pared to OOK, PPM, and PAM in terms of power efficiency significantly degrades the bit error performance at different as well as bandwidth efficiency. locations inside the room. The performance can be enhanced The remainder of this paper is organized as follows: The remarkably by considering the imaging diversity receivers OSM system model is presented in Section II. Performance technique to de-correlate the MIMO channel matrix [6]. This analysis along with comparison to OOK, PPM, and PAM are is achieved at the expense of placing a lens at proper locations presented in Section III. Finally, Section IV concludes the between the transmitters and the receivers for line-of-sight paper. 978-1-86135-373-3/11/$25.00 ©2011 IEEE 1
  • 2. NOC 2011 IT - 1 path gain between LED and PD i for the k th path given ∞ ¯ (k) by hi = (k) hi (t) dt. The notation n(t) indicates an Nr −∞ dimensional noise vector. The noise is the sum of the receiver thermal noise and shot noise due to ambient light which can be modeled as independent and identically distributed additive white Gaussian noise (AWGN) with double sided power spectral density σ 2 [2, eqn.(21)]. For the analysis in this paper, H(t) is an Nr × Nt × (K + 1) normalized indoor optical MIMO channel tensor, defined as Fig. 1: OSM communication system model. The LED mapper ⎡ ⎤ maps input bits to LED indices. Each sequence of log2 (Nt ) h11 (t) h12 (t) · · · h1Nt (t) ⎢ h21 (t) h22 (t) · · · h2Nt (t) ⎥ input bits correspond to a certain LED index. ⎢ ⎥ H (t) = ⎢ . . .. . ⎥ (3) ⎣ . . . . . . . ⎦ II. OSM S YSTEM M ODEL hNr 1 (t) hNr 2 (t) · · · hNr Nt (t) The system model of the OSM approach is depicted in The channel paths are obtained numerically via ray tracing Fig. 1. A MIMO system consisting of Nt transmit units (in technique as discussed in details in [9, eqn.(13)]. particular, light emitting diodes (LEDs)) and Nr receive units (in particular, photo diodes (PDs)) is illustrated. The bits At the receiver, the PD converts the optical signal to electrical signal and applies the optimal SM detector [10] to to be transmitted at each time instance, q(t), are grouped detect the active transmit unit as follows, as the row vectors of the matrix x(t). For illustration pur- poses, the bits to be transmitted in three time instances are ˜ = s ¯ arg max py y|¯, H q (t) = 1 0 0 1 1 1 . Assuming Nt = 4, each √ log2 (Nt ) bits are transmitted at one time instance and grouped = arg min ρ h s 2 − 2 yT h s , (4) T as follows, x (t) = 10 01 11 , where (·)T T {t=1} {t=2} {t=3} K K K denotes the transpose. The bits in this matrix are mapped to where h = ¯ (k) h1 ¯ (k) h2 ··· ¯ (k) hN r is an one of the transmitting LEDs. The selected LED ( ) transmits k=0 k=0 k=0 Nr dimensional vector containing the sum of the channel a non-return to zero (NRZ) pulse with an optical power path gains from transmit unit to each receive unit, H = ¯ (intensity) s = Pt at a particular time instance while all other h1 h2 · · · hNt is the channel matrix after the optical LEDs are off. The intensity level carries no information and to electrical conversion at the receiver, ¯ is the transmitted s can be utilized to optimize power consumption and range. In column vector from the matrix s(t) at this time instance, and the considered example, assuming the mapping table in Fig. 1, √ ¯ 2 the resultant matrix is given by, s ¯ py y|¯, H = π −Nt exp − y − ρH¯ F s (5) ⎡ ⎤ 0 0 0 is the probability density function (pdf) of y conditioned on ⎢ 0 s 0 ⎥ ¯ ⎢ ⎥ the transmitted vector ¯ and the channel H. The notation · F s s(t) = ⎢ 0 0 s ⎥. (1) ⎣ ⎦ stands for the Frobenius norm of a vector or a matrix. s 0 0 The estimated transmit unit index ˜ is then used to decode {t=1} {t=2} {t=3} the original information bits by inverse mapping process using Each column of the matrix s(t) is transmitted at a single the same mapping table as used at the transmitter. time instance where each element in the column vector corre- sponds to the respective transmit unit. As can be seen, at any III. P ERFORMANCE A NALYSIS give time only one transmit unit is active, all others transmit For Monte Carlo simulations, the considered 4x4 MIMO zero power. The transmission is made over the optical MIMO system inside a room is depicted in Fig. 2(a). The height of channel H(t). the transmit units from the ground is given by z in meters and The received signal can be written as, the transmitters half power angle is given by φ 1 in degrees. √ 2 The transmit units are placed on the ceilings with the exact y(t) = ρH(t) ⊗ s(t) + n(t) (2) locations shown in Fig. 2(b) and Fig. 2(c) for the unaligned ¯ r2 P 2 where ⊗ denotes time convolution, ρ = σ2r is the aver- and the aligned scenarios, respectively. The receive units are age electrical SNR at each PD, r is the PD responsivity, placed on a desktop with height 1 m from the floor. The exact Nr ¯ 1 (i) locations are shown in Fig. 2(b) and Fig. 2(c) for the unaligned Pr = Nr Pr is the average received optical power at each i=1 and the aligned scenarios, respectively. K For the unaligned scenario, the transmit units are directed PD with Pr (i) = ¯ (k) hi Pt being the average received optical k=0 toward the floor and the receive units are directed toward the power at PD i when LED is active, K is the number of the ceiling. The receivers FOV is 90◦ . In Fig. 3, the average ¯ (k) considered channel reflection paths, and hi is the channel BER is plotted versus the average electrical SNR at each 2
  • 3. NOC 2011 IT - 1 0 10 0 10 −1 −1 10 10 −2 −2 10 10 BER BER −3 −3 10 10 −4 −4 10 10 φ1/2 = 35◦ simulation z = 5m simulation φ1/2 = 35◦ analytical z = 5m analytical Fig. 2: (a) A 4x4 optical MIMO model inside a room. (b) Tx- −5 −5 10 10 ◦ φ1/2 = 40 analytical z = 6.5m analytical φ1/2 = 60◦ analytical z = 8m analytical Rx locations for the unaligned scenario. (c) Tx-Rx locations −6 −6 10 10 for the aligned scenario. Each Tx-Rx pair are directed toward 0 2 4 6 SNR (dB) 8 10 12 0 2 4 6 SNR (dB) 8 10 12 each other. 0 10 0 10 Fig. 4: Aligned OSM performance analysis. −1 10 −1 10 1.00 at Rx1 1 Channel Impulse Response 0.55 at Rx2 1 0.53 at Rx3 Channel Impulse Response 0.55 at Rx4 0.8 −2 10 −2 10 0.8 LOS First reflection 0.6 0.6 0.4 BER BER −3 −3 10 10 0.2 0.4 φ1/2 = 35◦ simulation z = 5m simulation 0 −4 10 φ1/2 = 35◦ analytical −4 10 z = 5m analytical 4 φ1/2 = 40◦ simulation z = 6.5m simulation 0.2 z = 6.5m analytical 2 Rx1 4 φ1/2 = 40◦ analytical z = 8m simulation −5 −5 10 10 2 φ1/2 = 60◦ simulation Y[m] 0 z = 8m analytical 0 0 X[m] 0 10 20 30 40 φ1/2 = 60◦ analytical Time (nsec) −6 −6 10 10 0 5 10 15 20 25 30 0 5 10 15 20 25 30 SNR (dB) SNR (dB) Fig. 5: Channel impulse response for the aligned scenario between Tx2 and Rx1. Fig. 3: Unaligned OSM performance analysis. In the left subfigure, φ 1 values of 35◦ , 40◦ , and 60◦ are considered at 2 Compared to the results in Fig. 3, the aligned OSM sys- z = 6 m. In the right subfigure, transmit units are considered tem performance enhances by at least 14 dB in SNR. This at heights z =5, 6.5, and 8 m for φ 1 = 30◦ . 2 enhancement is not due to lower path loss for the aligned scenario since the SNR is the same for all compared systems. receiver. The results for at least 106 channel realizations are The improvement is rather due to de-correlating the MIMO considered. Analytical and simulation results are presented for channel matrix by direct alignment of the transmit and receive Nt = Nr = 4, z = 6 m for different values of φ 1 , and for 2 units. Further reduction in the FOV creates a diagonal channel φ 1 = 30◦ for different values of z. The analytical results are 2 matrix and further enhances the performance. upper bounds and calculated as discussed in [1, Section III, For the aligned scenario as in Fig. 2(c), the channel impulse eqns. (5) and (6)]. The analytical BER equation is also shown response (LOS) when a single transmitter Tx2 is on and is in Table I, where N (κ, ν) is the number of bit errors when aligned to the location of Rx1 is shown in the left-subplot of choosing κ instead of ν as transmit unit index. Fig. 5 (φ 1 is 60◦ , FOV is 45◦ , z = 6 m). Rx1 is aligned to 2 The performance of the unaligned system is poor and for the location of Tx2 and the other receive units are directed all considered φ 1 angles, see the left sub-plot in Fig. 3, 2 toward the ceiling. The de-correlation is clearly noticed from SNR values above 20 dB are required to achieve 10−2 BER. the obtained amplitudes at the different locations of the four According to the right sub-plot in Fig. 3, at 20 dB SNR, a transmit units. For the same aligned scenario, the channel BER ≤ 10−3 can not be achieved regardless of the considered impulse response (LOS and first reflection path) between Tx2 height, z. and Rx1 is shown in the right-subplot of Fig. 5. It can be The performance can be significantly enhanced by aligning found that the 3 dB bandwidth for this path is 30 MHz and the transmit and receive units as depicted in Fig. 2(c) and that the channel delay spread is approximately 33 ns. At this reducing the receivers FOV to 45◦ . Performance results for delay spread, 96% from the overall power is captured. this scenario are shown in Fig. 4. It can be observed that The SNR distributions on a horizontal plan at 1 m from neither changing the angles, nor changing the heights affect the floor for the unaligned and the aligned scenarios (a single the simulated BER performance. However, slight differences transmit unit, Tx2, is turned on and an AWGN model intro- are noticeable between analytical and simulation results which duced in [2, eqn.(21)] is considered) are shown in Fig. 6(a) and can be attributed to numerical errors caused by the small Fig. 6(b), respectively. For the unaligned scenario, it can be variation between the channel path gains with different heights seen that the maximum achieved SNR is 11 dB. The achieved and angles. maximum SNR would not allow a useful BER as can be found 3
  • 4. NOC 2011 IT - 1 TABLE I: OSM, OOK, PPM, and PAM performance Scheme BER Data rate √ OOK Q ρ B L Llog2 L log2 L L-PPM 2 Q 2 ρ L B 2(L−1) log 2 L L-PAM Llog2 L Q (L−1)2 ρ Blog2 L Nt Nt 1 ρ 2 OSM Nt N (κ, ν)Q 4 hk − hν Blog2 Nt κ=1 ν=1 ν=κ (a) (b) IV. C ONCLUSIONS Fig. 6: SNR distribution. A single transmitter Tx2 is on (15 W OSM performance can significantly be enhanced by a proper optical power), φ 1 is 60◦ , and the receiver is directed toward 2 alignment of transmit and receive units. The resultant scheme the ceiling. (a) unaligned scenario. Tx2 directed toward the is shown to be very efficient in terms of power and band- floor and FOV is 90◦ . (b) aligned scenario. Tx2 is aligned to width efficiency as compared to the existing OW modulation the location of Rx1 and FOV is 45◦ . techniques. The simplicity of the scheme makes it a strong candidate for practical implementations. The considered OSM −1 10 4−PPM, 10 Mbps OSM, 40 Mbps system is shown to achieve a BER slightly better than OOK OOK, 20 Mbps −2 4−PAM, 40 Mbps and a data rate that is twice that of OOK. The power efficiency 10 of OSM is expected to be significantly enhanced by increasing −3 the number of receive units and/or considering channel coding BER 10 −4 10 techniques. In addition, the bandwidth efficiency of OSM −5 can be boosted by increasing the number of transmit units. 10 These will be addressed in detail in future publications and in −6 10 0 5 10 SNR (dB) 15 20 experimental demonstrations. Fig. 7: Performance comparison between aligned OSM, OOK, R EFERENCES 4-PPM, and 4-PAM and assuming a bandwidth of 30 MHz. [1] R. Mesleh, R. Mehmood, H. Elgala, and H. Haas, “Indoor MIMO Optical Wireless Communication Using Spatial Modulation,” in IEEE International Conference on Communications (ICC’10), Cape Town, South Africa, 22–27 May 2010, pp. 1–5. from results in Fig. 3. However, for the aligned case, SNR [2] J. M. Kahn and J. R. Barry, “Wireless Infrared Communications,” values between 7.5 dB and 8.5 dB are achieved in several Proceedings of the IEEE, vol. 85, no. 2, pp. 265–298, Feb. 1997. locations. These SNRs are sufficient to achieve at least 10−3 [3] B. Wilson and Z. Ghassemlooy, “Pulse Time Modulation Techniques for Optical Communications: A Review,” in In the Proceeding of the IEE BER according to the obtained BER results in Fig. 4. on Optoelectronics, vol. 140, no. 6, Dec. 1993, pp. 347–357. Finally, the performance of the aligned OSM (φ 1 is 60◦ , 2 [4] BROADCOM Corporation, “802.11n: Next-Generation Wireless LAN Technology,” White paper, BROADCOM FOV is 45◦ , z = 6 m) system is compared to the performances Corporation, Tech. Rep., Apr. 2006, retrieved Aug. 4, 2006 of OOK, 4-PPM, and 4-PAM [2, 11] as shown in Fig. 7. Also, http://www.broadcom.com/docs/WLAN/802-11n-WP100-R.pdf. the BER equations as a function of the SNR (ρ) and the data [5] L. Zeng, D. O’Brien, H. Minh, G. Faulkner, K. Lee, D. Jung, Y. Oh, and E. T. Won, “High Data Rate Multiple Input Multiple Output (MIMO) rates as a function of the bandwidth B for all modulation Optical Wireless Communications using White LED Lighting,” IEEE J. techniques are listed in Table I; where Q(·) denotes the Q- Select. Areas Commun., vol. 27, no. 9, pp. 1654–1662, Dec. 2009. function. In OSM, a NRZ pulse is transmitted at each time [6] P. Djahani and J. M. Kahn, “Analysis of Infrared Wireless Links Employing Multibeam Transmitters and Imaging Diversity Receivers,” instance from a single transmit unit. Hence, for the same SNR, IEEE Transactions on Communications, vol. 48, no. 12, pp. 2077–2088, the output optical power is exactly the same for all compared Dec. 2000. systems. [7] D. Takase and T. Ohtsuki, “Optical Wireless MIMO Communications (OMIMO),” in Proc. IEEE Global Telecommunications Conference The PPM technique is the best technique in terms of power GLOBECOM ’04, vol. 2, Texas, USA, 29 Nov.–3 Dec. 2004, pp. 928– efficiency. For instance, 4-PPM achieves a BER of about 10−4 932. [8] R. Mesleh, H. Haas, S. Sinanovi´ , C. W. Ahn, and S. Yun, “Spatial c at a SNR of approximately 6 dB. For the same BER, 9 dB, Modulation,” IEEE Trans. Veh. Technol., vol. 57, no. 4, pp. 2228 – 9.5 dB, and 18 dB SNR are required for OSM, OOK, and 2241, July 2008. 4-PAM, respectively. However, 4-PPM is the worst in terms [9] J. Barry, J. Kahn, W. Krause, E. Lee, and D. Messerschmitt, “Simulation of Multipath Impulse Response for Indoor Wireless Optical Channels,” of bandwidth efficiency. Half the data rate as compared to IEEE J. Select. Areas Commun., vol. 11, no. 3, pp. 367–379, Apr. 1993. OOK and only a quarter of the data rate as compared to [10] J. Jeganathan, A. Ghrayeb, and L. Szczecinski, “Spatial Modulation: OSM with Nt = 4 and 4-PAM can be achieved. The proposed Optimal Detection and Performance Analysis,” IEEE Commun. Lett., vol. 12, no. 8, pp. 545–547, 2008. OSM technique achieves a performance that is slightly better [11] R. J. Green, H. Joshi, M. D. Higgins, and M. S. Leeson, “Recent than OOK and enhances the data rate of OOK by a factor of Developments in Indoor Optical Wireless,” IET Communications, vol. 2, log2 (Nt ). The same data rate is achieved by PAM but with a no. 1, pp. 3–10, Jan. 2008. significant power penalty. 4