Spatial Modulation

Spatial Modulation

Ekta balotra1 , Koushik Barman2 Lovely Professional university

ABSTRACT

In this paper we have studied about Spatial Modulation (SM). Spatial modulation is a novel and recently proposed multipleantenna transmission technique. SM provides improved data rates compared to SingleInputSingleOutput (SISO) systems and robust error performance even with a very low system complexity. This is achieved by adopting a simple coding mechanism that establishes a onetoone mapping between blocks of information bits to be transmitted and the spatial positions of the transmitantenna in the antennaarray. This paper also discusses about its advantages and disadvantages over other conventional multiple antenna techniques.

Keywords- Inter Antenna synchronization (IAS), Inter- Channel interference (ICI), Multiple-input Multiple output (MIMO), Spatial modulation(SM).

1. INTRODUCTION

   Multiple antennas in wireless systems offer a practical way to extend next generation communication capabilities. Their new improvements over single antenna systems have given many ways for research in multiple-input multiple-output (MIMO) communications. One way is spatial multiplexing. It exploits multiple antennas to transmit more information. Spatial multiplexing requires synchronizing all antennas to transmit at the same time. It introduces interference from all antennas during reception which makes detection schemes complex. For adequate performance receivers require the number of receive antennas to be larger or equal to the number of transmit antennas, which is not practical for downlink transmission to small mobile devices.

   Diversity transmission is the other way to exploit MIMO system. In this case, antennas are used to increase the reliability of the message. Diversity systems exploit the spatial domain as a coding mechanism to increase reliability (i.e., diversity). These type of systems also requires synchronizing all antennas to transmit at the same time. However, diversity is attained at the expense of transmission rate, which remains unchanged from a single-input multiple-output (SIMO) system. As opposed to spatial multiplexing, diversity schemes provide simpler detection due to certain transmission properties.

   Finally, the third category is hybrid transmission: both spatial multiplexing and diversity concepts are integrated. The first application of hybrid transmission is multilayered space-time coding in, which exploits transmit antennas to increase both diversity and transmission rate. However, these benefits are achieved at the expense of increased detection complexity.

   All of these systems provide their own sets of benefits and restraints, but are flexible enough to accommodate various requirements. However, some common pitfalls amongst

   MIMO systems are:

   1. Inter-channel interference (ICI): Introduced by coupling multiple symbols in time and space.

   2. Inter-antenna synchronization (IAS): The detection algorithms assume that all symbols are transmitted at the same time. Hence, IAS is necessary to avoid performance degradation, consequently increasing transmitter overhead.

   3. Radio frequency (RF) chains: Multiple antenna elements are relatively inexpensive to deploy and the digital signal processing requirements are feasible due to increased industry growth, so the necessary RF elements are not as simple to implement. These RF chains are bulky, expensive, and necessary for each antenna that is used. Although Antenna selection provides some reduction in RF chains, there is no way around avoiding the increase in RF chains compared to that of a single antenna system. As well, Antenna selection (AS) generally increases the overhead at the receiver, and is prone to feed back errors in the case of transmit AS. The above main drawback of any MIMO scheme increases the complexity and cost of the receiver [1].

   Spatial Modulation has been recently proposed as a new modulation concept for MIMO systems. It reduces the complexity and cost of multipleantenna schemes without deteriorating the system performance and still guaranteeing good data rates. The lowcomplexity transceiver design and high spectral efficiency are simultaneously achieved by adopting the simple modulation and coding mechanisms as follows:

   1. Only one transmitantenna is activated for data transmission at any signaling time instance. This allows SM to entirely avoid the ICI, to require no synchronization among the transmitantenna and to need only one RF chain for data transmission. This is in net contrast with respect to conventional MIMO schemes where the multipleantennas are used to simultaneously transmit multiple data streams.

   2. The spatial position of each transmitantenna in the antennaarray is used as a source of information. This is obtained by establishing a onetoone mapping between each antenna index and a block of information bits to be transmitted, which results in a coding mechanism that can be called transmitantenna index coded modulation. This allows SM to achieve a spatial multiplexing gain with respect to conventional singleantenna systems since part of the information is implicitly conveyed by the position of the transmitantenna. Even though just one antenna is active, SM can also achieve high data throughput [2].

   In the upcoming sections we have discussed about spatial modulation, its advantages and disadvantages and recent results.

2. SPATIAL MODULATION

   The basic idea of SM is to map a block of information bits into two information carrying units:

   1. A symbol that is chosen from a complex signalconstellation diagram.

   2. A unique transmitantenna index that is chosen from the set of transmitantenna in the antennaarray (i.e. spatialconstellation diagram). The net result of embedding part of the information to be transmitted into the position of the transmitantenna is a hybrid modulation and MIMO technique in which the modulated signals belong to a tridimensional constellation diagram, which jointly combines signal and spatial information. An example is shown in Fig.1. for a linear antennaarray with Nt (number of transmit antennas) is 4 and a QPSK (Quadrature Phase Shift Keying) modulation .

      Fig.1. Tridimensional constellation diagram: each spatialconstellation point (i.e., the antenna index) defines an independent complex plane of signalconstellation points [2].

      1. THE TRANSMITTER

         At the transmitter, the bit stream emitted by a binary source is divided into blocks containing log2 (Nt)+log2(M ) bits each. Each block is then processed by a SM mapper, which splits each of them into two subblocks of log2(Nt) and log2 (M ) bits each. The bits in the first subblock are used to select the antenna which is switched on for data transmission while all other transmit antennas is kept silent in the current signaling time interval. The bits in the second subblock are used to choose a symbol in the signalconstellation diagram. For example in Fig. 2.1 Tx2 will be activated for data transmission by the first two bits (10) and a -1 binary signal will be sent out corresponding to the third bit (1).

         Fig.2. The Transmitter

      2. THE WIRELESS CHANNEL AS A MODULATION UNIT

         The signal emitted by the active antenna then goes through a wireless channel. Due to the different spatial positions occupied by the transmitantenna in the antennaarray, the signal transmitted by each antenna will experience different propagation conditions. Because only one transmit antenna is active atany time instance, so only one signal will be actually received. The other antennas will radiate no power.

      3. THE RECEIVER

   The receiver detects the signal which is coming from the transmitter. Channel impulse responses (NtNr) need to be estimated which depends upon the number of transmitting and receiving antennas. If ML detector is used at the receiver then according to the ML principle, the receiver will compute the Euclidean distance(MNtNr) between the received signal and the set of possible signals modulated by the wireless channel and chooses the closest one. In this way all the bits in the transmitted block can be decoded and the original bit stream recovered.

   Fig.3. The Receiver[2]

3. SM TRANSMISSION AND RECEPTION

   1. SM Transmission

      The general system model is shown in Fig.4.which consists of a MIMO wireless link with Nt transmit and Nr receive antennas. A random sequence of independent bits b enters the SM mapper, which groups m=log2(MNt) bits and maps

      them to a constellation vector x=[x1 x2 ··· xNt] where we assume a power constraint of unity (i.e. Ex [xHx]=1). In SM, only one antenna remains active during transmission and

      hence, only one of the xj in x is nonzero. The signal is transmitted over an Nr×Nt wireless channel H and experiences an Nr dim additive white Gaussian (AWGN) noise =[1 2 ···Nr]T . The received signal is given by

      y= +

      here is the average signal to noise ratio (SNR) at each receive antenna, and H and have independent and identically distributed (i.i.d) entries according to CN(0,1). As mentioned earlier, SM exploits the antenna index as an additional means to transmit information. The antenna combined with the symbol index make up the SM mapper which outputs a constellation vector of the following form:

      0 0 0 0

      ,where j represents

      the activated antenna, and xq is the qth symbol from the M-ary constellation X. Hence, only the jth antenna remains active during symbol transmission. For example, in 3 bits/s/Hz transmission with Nt =4 antennas, the information bits are mapped to a ±1 binary PSK (BPSK) symbol, and transmitted on one of the four available antennas. The output of the channel when transmitted from the antenna is expressed as

      = + , where denotes the column of H[3].

   2. SM Detection (Sub-Optimal)

      In [9], a sub-optimal detection rule based on MRC is given by:

      = arg max

      =

      and represent the estimated antenna and symbol index, respectively, and is the constellation demodulator function. Since the mapping is one to one, the demapper obtains an estimate of the transmitted bits by taking and inputs.

      Fig.4. Spatial modulation system model [3]

4. ADVANTAGES AND DISADVANTAGES OF SM OVER CONVENTIONAL SCHEMES

   1. Advantages

      1. SM entirely avoids ICI and IAS.

      2. It only requires a single RF chain at the transmitter. This is due to the working mechanism of SM. A single transmitantenna is switched on for data transmission while all the other antennas are kept silent.

      3. Tridimensional constellation diagram in SM introduces a multiplexing gain in the spatial domain that increases logarithmically with the number of transmitantenna.

      4. SM provides a high spectrallyefficient code with an equivalent code rate greater than one.

      5. The receiver design is simple since complicated interference cancelation algorithms are not required to cope with the ICI unlike conventional spatial multiplexing methods for MIMO systems,

      6. SM can attain ML decoding via a simple singlestream receiver.

      7. SM can efficiently work if Nr<Nt since the receiveantenna are used to get only a diversity gain.

      8. SM is able to work in multipleaccess scenarios since different pairs of transmitters and receivers usually occupy different spatial positions. If each intended receiver uses the set of channel impulse responses of all the transmitters for data detection (i.e., multiuser detection), several users might share the same wireless resources for communication.

      9. SM provides a larger capacity than conventional lowcomplexity coding methods for MIMO systems.

   2. Disadvantages

      1. At least two transmit antenna are required to exploit the SM concept.

      2. If the transmittoreceive wireless links are not sufficiently different, the SM paradigm might not be used or might not yield adequate performance. This limitation is somehow similar to conventional spatial multiplexing techniques, which require a richscattering environment to guaranteeing a significant boost in the achievable data rate [10].

      3. The receiver requires perfect channel knowledge for data detection, this may pose complexity constraints on the channel estimation unit.

      4. SM offers only a logarithmic (instead of linear) increase of the data rate with the number of transmitantenna. This might limit SM to achieve very high spectral efficiencies for practical numbers of antennas at the transmitter.

5. SM VERSUS SSK

   When the information carrying unit is only the transmitantenna index, SM reduces to the so called Space Shift Keying (SSK) modulation, which avoids any form of conventional modulation and tradesoff receiver complexity for achievable data rates. In SSK, antenna indices are used as the only means to relay information, which makes it somewhat a special case of SM. However, elimination of

   APM (amplitude phase modulation) provides SSK with notable differences and advantages over SM:

   1. The performance of SSK is almost identical to SM but the detection complexity is lowered.

   2. Because phase and amplitude of the pulse do not convey information, transceiver requirements for SSK are less stringent than for APM.

6. LITERATURE REVIEW

   In the recent period, the main research interest has been focused on the application of the SM concept to MIMO wireless systems, in order to quantify the performance difference with other popular MIMO schemes. The main aim of this section is to summarize the most significant results:

   In [4], the authors have proposed a simple MRCbased receiver design for SM, which independently detects the bits conveyed by the two information carrying units. The performance of this receiver has been analyzed over independent and identically distributed Rayleigh fading channels and compared to conventional schemes. Furthermore, simulation results have been obtained over more realistic propagation environments that take into account Rician fading, channel correlation, and antenna coupling. The results have clearly showcased that SM can offer better error performance than conventional schemes with a lower receiver complexity, while still guaranteeing the same spectral efficiency.

   In [5], the authors have proposed an improved version of the SSK modulation concept by allowing a subset of transmitantenna to be switched on for data transmission at any time instance. The main contribution of this paper is the optimization criterion to design the spatialconstellation diagram, i.e., the set of antennas to be switched on and kept silent, by minimizing the error probability. The proposed method offers performance similar to SM but with lower complexity. However, the price to be paid to implement this scheme is the need of IAS and multiple RF chains.

   In [6], the authors have extended to correlated Nakagamim fading channels the analytical framework to compute the Average Bit Error Probability (ABEP) of the heuristic detector.

   In [7] the MLoptimum receiver based on harddecision decoding has been generalized by using a MLoptimum softdecision decoding algorithm. It has been shown that softdecision decoding can improve the performance of approximately 3 B if compared to harddecision decoding.

   In paper [8], the authors have proposed a new modulation concept that aims at reducing the effect of channel correlation on the performance of SM. As a matter of fact the detector might be unable to distinguish the different transmitantenna since they will appear almost the same at the receiver. The proposed scheme is called Trellis Coded Spatial Modulation (TCSM). It exploits convolutional encoding and MaximumLikelihood Sequence Estimation (MLSE) decoding to increase the free distance between sequences of spatialconstellation points.

7. CONCLUSION

   SM is a technique which combines digital modulation & coding and multipleantenna in a unique fashion to achieve high data rates.SM offers a lowcomplexity alternative to the design of MIMO wireless systems, which avoids multiple Radio Frequency (RF) chains at the transmitter and highcomplexity interference cancelation algorithms at the receiver, but still guarantees a multiplexing gain that only depends on the number of antennas at the transmitter. This makes this technology especially suitable for the downlink with lowcomplexity mobile units.

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