Exploiting spatial domain to increase spectrum efficiency for wireless communications from source to media based modulation

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.

1. 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 3 bits into a transmitted symbol to
attain the spectrum efficiency of 3 bpcu1 or 3 bits/symbol. Using 16-QAM modulation can
increase the spectral efficiency to 4 bpcu. More high order modulation like 256-QAM
which often seen in the digital television systems can achieve the spectrum efficiency of 8
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.
1

Bit per channel use.

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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.
2. 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
2

C  B log2 (1  h SNR)

(bps).

(2)

The spectral efficiency is then given by
2

  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.
4. 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 1.

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b1 ,b2 ,...,bm

1

1

n1

2

2

n2

H


Nt

Nr

x



nN

bˆ1 ,bˆ2 ,...,bˆm

r

y

Figure 1. 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  1 , 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 8 bits 00110110 to be
transmitted at the spectral efficiency of 2 bpcu. In the SSK system, we need to use 4
transmit antennas and map each combination of 2 bits to a specific antenna. Specifically,
the first 2 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 2 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  2 we have 10

combinations of active antennas and we can select 8 out of the 10 for transmission. The
achieved spectral efficiency is 3 bpcu. If SSK is used the transmitter need to have 8
antennas so it is clear that GSSK can save 3 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.

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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 min y - Hxk
x
k

2

(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 2 shows a typical configuration of SM/GSM systems.

ma bits

m bits

ms bits

1

1

n1

2

2

n2

H


Nt

Nr

x



nN

m bits

r

y

Figure 2. 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 7 bpcu. If SM is used and the transmitter has 8
antennas, ma  3 bits can be used for spatial modulation. In order to achieve 4 bpcu by
signal modulation one can use 16-QAM. In case GSM is used with N a  2 antennas and

N t  5 transmit antennas it is possible to achieve 3 bpcu by spatial modulation while the
remaining spectral efficiency of 4 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  

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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 min  y  h  x
y
i

k ,i q


k ,q
k ,q

2

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 3.

ma bits
m bits

ms bits

s

x

1

1

n1

2

2

n2

m bits

H


Nt

Nr



x

nN

r

y

Figure 3. 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 1. 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

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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 4 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
0

4

6

8

10

12
14
16
18
Number of Transmit Antennas

20

22

24

Figure 5. 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 4 bpcu, one can
flexibly choose: V-BLAST with N t  4 and BPSK, SSK with N t  16 , GSSK with

N t  8, Na  2 , SM with N t  4 and 4-QAM, GSM with N t  4, Na  2 and 4PSK, and HR-SM with N t  2, N a  2 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

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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].
5. 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 5 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

1

n1

2

n2



nN

m bits

r

y

ma  K bits

Figure 5. Configuration of MBM.
6. 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

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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.
7. 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.
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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 là một nguồn tài nguyên khan hiếm cho thông tin vô tuyến. Trong vài
thập kỷ vừa qua đã có nhiều nỗ lực trong việc tìm kiếm các kỹ thuật truyền dẫn có
khả năng tăng hiệu suất sử dụng phổ trong các hệ thống vô tuyến. Bài báo này
cung cấp một đánh giá tổng quan về các kỹ thuật truyền dẫn hiệu quả về phổ tần
trong các hệ thống đa ăng-ten. Cụ thể, ba kỹ thuật tiên tiến nhất sử dụng miền
không gian để chuyển tải các bit thông tin sẽ được phân tích gồm V-BLAST, kỹ
thuật điều chế không gian tại nguồn và kỹ thuật điều chế tại môi trường. Các ưu
điểm cũng như các hạn chế quan trọng của các kỹ thuật này sẽ được làm sáng tỏ.
Cuối cùng, các thách thức và vấn đề mở sẽ được đưa ra như một định hướng cho
các nghiên cứu tương lai.
Từ khóa: Thông tin vô tuyến, MIMO, Điều chế không gian, Điều chế tại 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:

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Tr.X.Nam, “Exploiting spatial domain to increase spectrum …”