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This paper is aimed to provide a comprehensive review on the spectrally efficient transmission techniques applied in multiple antenna systems. Specifically, three state-of-the-art techniques which make use of the spatial domain to convey information bits, including the Vertical Bell-Labs Layered Space-Time (V-BLAST), the source-based spatial modulation (SM) and the media-based modulation (MBM), will be surveyed and their critical advantages as well as limitations will be highlighted.
Review EXPLOITING SPATIAL DOMAIN TO INCREASE SPECTRUM EFFICIENCY FOR WIRELESS COMMUNICATIONS FROM SOURCE TO MEDIA-BASED MODULATION (INVITED PAPER) Tran Xuan Nam* Abstract: Spectrum is a scarce resource for wireless communications During the past decades there have been great research efforts in proposing various transmission techniques which can increase the spectral efficiency of the wireless systems This paper is aimed to provide a comprehensive review on the spectrally efficient transmission techniques applied in multiple antenna systems Specifically, three state-of-the-art techniques which make use of the spatial domain to convey information bits, including the Vertical Bell-Labs Layered Space-Time (V-BLAST), the source-based spatial modulation (SM) and the media-based modulation (MBM), will be surveyed and their critical advantages as well as limitations will be highlighted Finally, technical challenges and open research problems will be given as a guideline for possible future advancement Keywords: Wireless communications, MIMO, Spatial Modulation, MBM, Spectral efficiency INTRODUCTION Together with the growth of the information-centric society, wireless applications have become more popular During the last few decades we have witnessed a very fast development of wireless communications Today it is easy to find a wireless network nearby such as Bluetooth, Wi-Fi, television and mobile cellular communications Many users are equipped with several electronic devices such as smart phones, laptop computers and digital cameras which all can be easily connected to a wireless network The Internet of Things (IoT) is a new network concept which describes a huge network connecting several billions electronic devices which can connect with one another through ubiquitous wireless networks Increase in wireless networks and devices would result in a scarcity of carrier frequencies for transmission due to limited frequency spectrum Various efforts have been paid on the road to search for transmission solutions which can utilize spectrum more efficiently Traditional carrier digital modulation such as Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) has been known as a common method to embed information bits into carrier waves Theoretically, it is possible to increase the number of bits into a carrier wave in order to achieve high spectrum efficiency For example, for 8-PSK modulation it is possible to embed bits into a transmitted symbol to attain the spectrum efficiency of bpcu1 or bits/symbol Using 16-QAM modulation can increase the spectral efficiency to bpcu More high order modulation like 256-QAM which often seen in the digital television systems can achieve the spectrum efficiency of bpcu In general the spectral efficiency of the signal modulation schemes is given by: log2 M bpcu, where M is the modulation order The straight implication here is that increasing the modulation order would lead to improved spectral efficiency However, due to constrain in the transmit power, increasing modulation order means the constellation points are packed closer to each other As a result the increase in spectral efficiency of the signal modulation schemes is significantly affected by their error performance Bit per channel use Journal of Military Science and Technology, Special Issue, No 48A, - 2017 21 Electronics and Automation In the past several decades, there have been large efforts by the information theory and communications research community in order to propose novel transmission techniques to increase the spectral efficiency of the wireless communications systems The aim of this paper is to provide a comprehensive technical overview of the state-of-the-art transmission techniques which exploit the spatial domain to convey information We first introduce the concept of a spatial multiplexing in the multiple-input multiple-output (MIMO) system We then present the principle of the so-called spatial modulation which uses the antenna indexes to convey information bits Next, we will focus on the most advanced modulation technique which is referred to as the media-based modulation Technical challenges and open problems will be highlighted for possible future research INCREASING SPECTRAL EFFICIENCY USING MIMO TRANSMISSION Traditionally, the capacity of a noisy communication channel with bandwidth B Hz and the signal-to-noise power ratio SNR is given by Shannon theorem as follows C B log2 (1 SNR) (bps) (1) Under a static fading wireless communication channel with path gain h the capacity can be expressed as C B log2 (1 h SNR) (bps) (2) The spectral efficiency is then given by C / B log2 (1 h SNR) (bps/Hz) (3) Note that this spectral efficiency increases with SNR of the channel in the logarithmic scale and thus soon becomes saturated when the transmit power is large enough The Shannon capacity was limited for a very long time until the late of the 20th century when Foschini and Gans in [2] and Telatar in another independent work [3] found the capacity of a rich-scattering MIMO fading channel Given a wireless communication system with Nt transmit antennas and N r receive antennas, using the result of [2][3] the spectral efficiency of the MIMO channel H under invariant condition is given by MIMO C MIMO Where: 22 SNR H H H if N r N t log2 det I N t / B SNR HHH if N r N t log2 det I Nt min(N r , Nt ) When the number of antennas is large enough we have MIMO,N t N r log2 (1 SNR) MIMO,N r Nt log2 (1 SNR Nr ) N (4) t Tr.X.Nam, “Exploiting spatial domain to increase spectrum …” Review This means for a MIMO system with a large number of antennas the spectral efficiency is linearly increased with the employed antennas For this reason the spectral efficiency of the MIMO channel has been considered a breakthrough which can break the limit by the Shannon theorem In order to make the MIMO system realistic, the authors in [4] proposed a MIMO architecture called Vertical-Bell Labs Layered Space-Time (V-BLAST) and implemented it in a tested The proposed system multiplexes parallel data streams over multiple transmit antennas and uses linear detection combined with successive interference cancellation (SIC) to estimate transmitted symbols from spatial layers The V-BLAST spatial multiplexing system was demonstrated to achieve spectral efficiencies of 40-50 bps/Hz at the SNRs from 24-34 dB with reasonable detection complexity The V-BLAST system was then adopted as an air-interface standard for various state-of-the-art wireless communication systems such as 3GPP LTE/LTE-Advanced, WiMAX and Wi-Fi Significant efforts were also paid to invent other MIMO systems such as space-time codes [5][6], beam forming [7], however, these approaches aim at improving the transmission quality of MIMO systems and thus are out of scope of the current paper Assume that MPSK/M-QAM modulation is used for signal mapping the spectral efficiency of the VBLAST system in terms of bpcu is given by V-BLAST N t log2 M (5) Although the MIMO V-BLAST system could achieve significant improvement in spectral efficiency, it faces some critical problems Firstly, since multiple data streams are transmitted simultaneously over multiple antennas the problem of inter-channel interference (ICI) should be carefully resolved Although the combined linear estimation and SIC detector proposed in V-BLAST could treat ICI sensibly, the residual interference after each detection iteration still exists, which makes the bit error rate (BER) performance of the V-BLAST system suboptimal Secondly, the V-BLAST system is very sensitive with the problem of Inter-Antenna Synchronization (IAS) Apart from these limitations the V-BLAST system also requires more radio frequency (RF) chains and thus is not energy efficient All these disadvantages of the V-BLAST system have opened an opportunity for a novel transmission system which is called spatial modulation SPATIAL MODULATION USING ANTENNA INDEXES Spatial modulation is a novel transmission technique for wireless communications, which was originally proposed in [8] and then further developed in [9][11] The basic idea of spatial modulation is to exploit spatial domain, i.e antenna indexes to convey information bits When information bits are carried only by antenna indexes we have space-shift keying or generalized space-shift keying Whereas when information bits are conveyed by both antenna indexes and signal symbols we have spatial modulation or generalized spatial modulation 4.1 Space-Shift Keying/Generalized Space-Shift Keying The Space-Shift Keying (SSK) [10] and Generalized Space-Shift Keying (GSSK) [11] use only spatial domain for modulation A typical system configuration of SSK/GSSK is illustrated in Figure Journal of Military Science and Technology, Special Issue, No 48A, - 2017 23 Electronics and Automation b1 ,b2 , ,bm 1 n1 2 n2 H Nt Nr x nN bˆ1 ,bˆ2 , ,bˆm r y Figure A typical SSK/GSSK system In the SSK/GSSK system the transmit bit sequence b1, b2, , bm does not modulate the carrier wave but selects the transmit antennas For SSK systems only one transmit antenna, i.e Na , is activated at an instant time for transmission Similarly, in GSSK systems there are Na 1,(N t Na 1) , active antennas used for transmission For implementation convenience, the number of transmit antennas Nt should be a power of two As an illustrative example, let us assume that we have bits 00110110 to be transmitted at the spectral efficiency of bpcu In the SSK system, we need to use transmit antennas and map each combination of bits to a specific antenna Specifically, the first bits 00 is mapped to the first antenna, 11 to the fourth, 01 to the second, and 10 to the third Since each time an antenna is activated there are a combination of two bits transmitted, the spectral efficiency is clearly bpcu In general, the spectral efficiency of SSK is given by SSK log2 N t (6) Note that when higher spectral efficiency is required more transmit antennas need to be used and the SSK system demands a complex antenna system In such a case, using GSSK with an appropriate number of N a allows the transmitter to reduce the number of transmit antenna For a GSSK system with Nt transmit and N a active antennas we have Nt Na different active antenna combinations For example, let N t 5, N a we have 10 combinations of active antennas and we can select out of the 10 for transmission The achieved spectral efficiency is bpcu If SSK is used the transmitter need to have antennas so it is clear that GSSK can save transmit antennas at the cost of an additional RF unit In general, the achievable spectral efficiency of GSSK is given by: N GSSK log2 t N RF (7) Where: denotes rounding down to the nearest power of two 24 Tr.X.Nam, “Exploiting spatial domain to increase spectrum …” Review The receiver in the SSK/GSSK uses an SSK/GSSK decoder to detect the activated antennas during each transmission period Specifically, upon reception of the received ˆ as follows: vector y the decoder finds the transmitted vector x ˆ arg y - Hxk x k (8) F Where: xk denotes the transmit vector which contains 1s in the elements corresponding to activated antennas and 0s elsewhere 4.2 Spatial Modulation/Generalized Spatial Modulation Although they were independently proposed, spatial modulation (SM) [8] and generalized spatial modulation (GSM) [9] can be regarded as extensions of SSK and GSSK, respectively Figure shows a typical configuration of SM/GSM systems ma bits m bits ms bits 1 n1 2 n2 H Nt Nr x nN m bits r y Figure A typical SM/GSM system The difference between SM and SSK and between GSM and GSSK is that the activated antennas transmit modulated symbols in SM/GSM systems but not in SSK/GSSK As shown in Figure 3, at the input to the SM/GSM transmitter m ma ms data bits are divided into two branches in which ma bits are used for spatial modulation (antenna selection) as in SSK/GSSK systems while the remaining ms bits are used for signal modulation as in the conventional wireless communication systems Either M-PSK or MQAM can be used for signal modulation For example, given a SM/GSM system which transmits signal at the spectral efficiency of bpcu If SM is used and the transmitter has antennas, ma bits can be used for spatial modulation In order to achieve bpcu by signal modulation one can use 16-QAM In case GSM is used with N a antennas and N t transmit antennas it is possible to achieve bpcu by spatial modulation while the remaining spectral efficiency of bpcu can be obtained by 16-QAM modulation In general, the spectral efficiency of SM/GSM is given, respectively, by: SM log2 Nt log2 M N GSM log2 t log2 M N RF Journal of Military Science and Technology, Special Issue, No 48A, - 2017 (9) 25 Electronics and Automation At the receiver, in order to detect the transmitted bits the SM/GSM decoder needs to perform joint estimation of both the activated antennas and modulated symbols Denote k , i the index of transmit a receive antennas, respectively Also, let q denote the index of a modulated symbol from the signal alphabet The joint estimation performed by the decoder is given by Nr kˆ, qˆ arg max p y | x, H arg y h x y i k ,i q k ,q k ,q i 1 Note that although they can achieve higher spectral efficiency, SM and GSM suffers from error performance degradation due to joint detection compared with SSK and GSSK 4.3 High-Rate Spatial Modulation High rate spatial modulation (HR-SM) [12] is an enhanced spatial modulation scheme which achieves higher spectral efficiency over GSM systems In the HR-SM scheme all transmit antennas are activated for transmission, i.e Na N t Configuration of the HRSM scheme is shown in Figure ma bits m bits ms bits s x 1 n1 2 n2 m bits H Nt Nr x nN r y Figure Configuration of HR-SM system The idea of the HR-SM scheme is not simply to send the transmit vector x but optimize it before transmission By decomposing the transmit as x s x where x denotes the modulated symbol while s is a complex vector which is referred to as the spatial constellation codeword In the GSM scheme each element of s has the following properties: s s ={0,1}, sum(s) N a In the HR-SM scheme the authors design the SC code words s such that s s ={ 1, j }, sum(s) N a N t Since the SC code words receive complex values for an HR-SM system with Nt transmit antennas there are 4N t different combinations of the SC vectors However, in order to achieve full diversity the first element of s is fixed to As a result there are 4N t 1 combinations of s , which can be used for spatial modulation The authors proposed to select 2Nt 1 combinations which satisfy the minimum Euclidean distance as the SC code words As a result, the spectral efficiency of the HR-SM is given by 26 Tr.X.Nam, “Exploiting spatial domain to increase spectrum …” Review HR-SM 2(N t 1) log2 M (10) It is noted that the achievable spectral efficiency of the HR-SM is linearly increased with the number of transmit antennas and thus much higher than that of the previous SSK, GSSK, SM, and GSM schemes In order to detect the transmitted bits, the HR-SM decoder uses a reduced-complexity maximum-likelihood detector which can achieve optimal bit error rate performance Figure compares the spectral efficiency of various spatial modulation schemes including: V-BLAST, SSK, GSSK, SM, GSM and HR-SM for Na 2, M 16 100 V-BLAST SSK GSSK SM GSM HR-SM 90 Spectral Efficiency (bpcu) 80 70 60 50 40 30 20 10 10 12 14 16 18 Number of Transmit Antennas 20 22 24 Figure Spectral efficiency of spatial modulation schemes: Na 2, M 16 We can see from the figure that the V-BLAST scheme achieves the highest spectral efficiency Next, the HR-SM scheme outperforms all remaining schemes due to the linear increase of spectral efficiency similar to V-BLAST Among the other schemes, SM and GSM are superior to SSK and GSSK respectively At N t 24 the spectral efficiency of SM is shown to coincide with that of GSSK This is a special case where both the total spectral efficiencies due to spatial modulation and signal modulation of SM equal the spectral efficiency due to spatial modulation of GSSK In reality, there may be other similar cases Moreover, when designing an SM system for a given spectral efficiency it is possible to choose either of the above schemes according to the specific requirements and constrains such as the number of transmit antennas, the number of employed RF chains and signal modulation For example, given the spectral efficiency of bpcu, one can flexibly choose: V-BLAST with N t and BPSK, SSK with N t 16 , GSSK with N t 8, Na , SM with N t and 4-QAM, GSM with N t 4, Na and 4PSK, and HR-SM with N t 2, N a and 4-QAM 4.4 Other spatial modulation schemes Apart from the above mentioned schemes, many other works have also been successful in inventing spatial modulation schemes with different types of merits such as obtaining Journal of Military Science and Technology, Special Issue, No 48A, - 2017 27 Electronics and Automation full diversity [13], low computational complexity, low power consumption A more detailed review on these schemes can be found in a tutorial review in [14] MEDIA-BASED MODULATION USING PERTURBED CHANNELS Media-based modulation (MBM) [15]-[17] is the latest technology related to spatial modulation The idea of MBM is similar to that of SM in which each unique channel is utilized to convey signal symbols In SM systems each transmission channel corresponds to the natural wireless channel between the receiver and an activated transmit antenna Under assumption that the propagation channel is affected by rich scattering multipath fading and that the space between transmit antennas is large enough the channels from transmit antennas are independent and uncorrelated This allows the SM transmitter to utilize unique channels to bear information bits in the form of antenna index In the MBM system, however, antenna index is not used as the transmitter has only single antenna In contrast, unique channels are created by perturbing propagation environment intentionally by RF elements called RF mirrors Figure presents a typical configuration of an MBM system As shown in the figure the RF mirrors with K elements are placed around a dipole antenna to generate unique channel states to convey information bits With K mirrors which can be turned on or off it is possible to generate 2K channel states to achieve spectral efficiency of K bpcu Theses channel states are selected by ma data bits Together with signal modulation the achievable spectral efficiency of the MBM system is given by MBM K log2 M (11) It is also interesting to note that the spectral efficiency of MBM is linearly increased with the number of RF mirrors The more RF mirrors are used the higher spectral efficiency can be achieved However, the difficulty lies in designing the RF mirrors so that they can generate unique channel states but not too complex in size and estimating channel states A design example of a transmit antenna with RF mirrors can be found in [16] More discussions about channel estimation for MBM are provided in [17] ms bits n1 n2 nN m bits r y ma K bits Figure Configuration of MBM TECHNICAL CHALLENGES AND OPEN RESEARCH PROBLEMS While the V-BLAST system has been well researched and implemented in various advanced wireless systems such as Wi-Fi, WiMAX or 3GPP LTE, the implementation of the SM and MBM still requires further investigations For the SM systems including SSK/GSSK and SM/GSM effective channel estimation methods which can balance the 28 Tr.X.Nam, “Exploiting spatial domain to increase spectrum …” Review transmission efficiency and error performance need further efforts For hardware implementation, RF switches which can smoothly switch from one antenna to another are expected to be soon completed For the MBM system, as cited above the design of the RF mirrors is challenging and needs further proposals Similar to the SM, channel estimation is also an interesting topic for research CONCLUSION Increasing spectral efficiency is an important task for making efficient use of radio spectrum in order to meet the increasing demand in transmission rate of advanced wireless communications systems In this paper, various state-of-the-art transmission techniques which can increase the spectral efficiency have been introduced The V-BLAST system has the highest spectral efficiency, however, has significant problems with IAS and ICI The source-based SM systems can alleviate those problems faced by V-BLAST but at the cost of reduced spectral efficiency The MBM system is similar to the SM system but may face difficulty in fabricating the antenna system with RF mirrors Thus the employment of which system for practical implementation depends on the requirements for each specific case REFERENCES [1] T X Nam L M Tuấn, “Xử lý tín hiệu khơng gian-thời gian,” NXB Khoa học Kỹ thuật (2013) [2] G J Foschini and M J Gans "On limits of wireless communications in a fading environment when using multiple 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European Trans on Telecom., Vol 10, No (1999), pp 585-595 [4] P W Wolniansky, G J Foschini, G D Golden and R A Valenzuela, "V-BLAST: an architecture for realizing very high data rates over the rich-scattering wireless channel," Proc 1998 URSI Int’l Symp on Signals, Systems, and Electronics, Pisa, (1998), pp 295-300 [5] V Tarokh, N Seshadri, and A R Calderbank "Space-time codes for high data rate wireless communication: Performance criterion and code construction," IEEE Trans on Infor Theory, Vol 44, no.2 (1998), pp 744-765 [6] S M Alamouti, "A simple transmit diversity technique for wireless communications," IEEE J on Select Areas in Commun., Vol 16, No.8 (1998), pp 1451-1458 [7] D J Love, R.W Heath, and T Strohmer "Grassmannian beamforming for multipleinput multiple-output wireless systems," IEEE Trans on Inform Theory, Vol.49, No.10 (2003), pp 2735-2747 [8] R Y Mesleh, H Haas, S Sinanovic, C W Ahn and S Yun, "Spatial modulation," IEEE Trans on Vehicular Tech., Vol 57, No (2008), pp 2228-2241 [9] A Younis, N Serafimovski, R Mesleh and H Haas, "Generalised spatial modulation," in Proc 2010 Conference Record of the 44th Asilomar Conf on Signals, Systems and Computers, Pacific Grove, CA, (2010), pp 1498-1502 [10].J Jeganathan, A Ghrayeb, L Szczecinski and A Ceron, "Space shift keying modulation for MIMO channels," IEEE Trans on Wireless Commun., Vol 8, No (2009), pp 3692-3703 [11].J Jeganathan, A Ghrayeb and L Szczecinski, "Generalized space shift keying modulation for MIMO channels," in Proc 2008 IEEE 19th Int’l Symp on Personal, Indoor and Mobile Radio Commun., Cannes, (2008), pp 1-5 Journal of Military Science and Technology, Special Issue, No 48A, - 2017 29 Electronics and Automation [12].T P Nguyen, M T Le, V D Ngo, X N Tran and H W Choi, "Spatial Modulation for High-Rate Transmission Systems," in Proc 2014 IEEE 79th Vehicular Tech Conf (VTC Spring), Seoul, (2014), pp 1-5 [13].M.-T Le, V.-D Ngo, H.-A Mai, X N Tran, and M Di Renzo, "Spatially modulated orthogonal space-time block codes with non-vanishing determinants," IEEE Trans on Commun., Vol 62, No 1, (2014), pp 85-99 [14].M Di Renzo, H Haas, A Ghrayeb, S Sugiura and L Hanzo, "Spatial Modulation for Generalized MIMO: Challenges, Opportunities, and Implementation," Proceedings of the IEEE, Vol 102, No (2014), pp 56-103 [15].A K Khandani, "Media-based modulation: A new approach to wireless transmission," in Proc 2013 IEEE Int’l Symp on Infor Theory, Istanbul, (2013), pp 3050-3054 [16].A K Khandani, "Media-based modulation: Converting static Rayleigh fading to AWGN," in Proc 2014 IEEE Int’l Symp on Infor Theory, Honolulu, (2014), pp 1549-1553 [17].E Seifi, M Atamanesh, and A K Khandani "Media-Based MIMO: A New Frontier in Wireless Communications," arXiv preprint arXiv:1507.07516 (2015) TÓM TẮT TĂNG HIỆU SUẤT SỬ DỤNG PHỔ NHỜ KHAI THÁC MIỀN KHÔNG GIAN CHO THÔNG TIN VÔ TUYẾN TỪ ĐIỀU CHẾ TẠI NGUỒN ĐẾN ĐIỀU CHẾ TẠI MÔI TRƯỜNG (BÀI BÁO MỜI) Phổ tần nguồn tài nguyên khan cho thông tin vô tuyến Trong vài thập kỷ vừa qua có nhiều nỗ lực việc tìm kiếm kỹ thuật truyền dẫn có khả tăng hiệu suất sử dụng phổ hệ thống vô tuyến Bài báo cung cấp đánh giá tổng quan kỹ thuật truyền dẫn hiệu phổ tần hệ thống đa ăng-ten Cụ thể, ba kỹ thuật tiên tiến sử dụng miền không gian để chuyển tải bit thơng tin phân tích gồm V-BLAST, kỹ thuật điều chế không gian nguồn kỹ thuật điều chế môi trường Các ưu điểm hạn chế quan trọng kỹ thuật làm sáng tỏ Cuối cùng, thách thức vấn đề mở đưa định hướng cho nghiên cứu tương lai Từ khóa: Thơng tin vơ tuyến, MIMO, Điều chế khơng gian, Điều chế môi trường, Hiệu suất sử dụng phổ Received date, 11th April 2017 Revised manuscript, 25th April 2017 Published on 26th April 2017 Author affiliations: Military Technical Academy ; *Corresponding author: namtx@mta.edu.vn 30 Tr.X.Nam, “Exploiting spatial domain to increase spectrum …” ... opportunity for a novel transmission system which is called spatial modulation SPATIAL MODULATION USING ANTENNA INDEXES Spatial modulation is a novel transmission technique for wireless communications, ... which can balance the 28 Tr.X.Nam, Exploiting spatial domain to increase spectrum …” Review transmission efficiency and error performance need further efforts For hardware implementation, RF switches... shown to coincide with that of GSSK This is a special case where both the total spectral efficiencies due to spatial modulation and signal modulation of SM equal the spectral efficiency due to spatial