38

CHAPTER 3

REVIEW OF RESEARCH OUTCOMES IN MILLIMETER

WAVE MIMO

39

CHAPTER 3

REVIEW OF RESEARCH OUTCOMES IN MILLIMETER

WAVE MIMO

The advantage of MIMO is twofold link reliability and high data rate. These features when

combined with MMW address the problem of penetration losses and multipath fading.

Spatial diversity in transmitter and receiver is feasible in MMW as the size of the antenna is

of the order of millimeter. This is not the case for low frequency where diversity is exploited

only in fixed base station and not in the subscriber unit (SU). The reason being large sized

multi-antenna in the SU affects portability of the device. The data rate of the order of gigabits

per second is achieved owing to the unlicensed bandwidth and combining spatial

multiplexing with MMW. This chapter deals with survey of reported results of MMW

MIMO. The chapter is organized as follows: section 3.1 presents the MMW features and

challenges, section 3.2 presents MMW MIMO, section 3.3 discusses the need for

beamforming for 60 GHz system, section 3.4 presents the need for multibeam in MIMO,

section 3.5 presents the 60 GHz channel sounder and section 3.6 presents the research

outcomes in outdoor MMW MIMO.

3.1 Millimeter Wave features favoring use of MIMO

The major features in brief and technical challenges in MMW that necessitate the need for

MIMO is presented in the following section. The indoor channel in 57-64 GHz band

experiences oxygen absorption at 60 GHz and also high free space path loss. The oxygen

absorption limits the range to 10m indoor and 100m outdoor and this facilitates frequency

reuse with interference free operation between the MMW systems. The propagation loss is

overcome using high gain directional antenna. The directivity of an antenna scales inversely

as the square of carrier wavelength. As a result of these directive antennae, the multipath

environment is much sparser compared to lower carrier frequencies. Analysis of directional

transmission and reception with electronically steerable beams using compact antenna arrays

resulted in the compensation of propagation loss. Due to this, rich scattering environment

was then reduced to sparse scattering leading to moderate antenna separation. This has led to

40

achieve spatial multiplexing in LOS. Capacity computations indicate data rate of gigabits is

possible using channel bandwidths of the order of GHz (Torkildson, E. et al., 2010).

Indoor channel model

The channel model for WPAN is different from that of WLAN as the distance ranges are

smaller, influence of human presence close to antenna device. These characteristics motivate

the need for generalizing classical LOS MIMO model based on the assumption that antenna

channel experience different small scale statistics and different channel gain. Hence MIMO

channel model for WPAN is proposed and the simulation results on capacity, eigenvalue

distribution and antenna correlation validated with measured data (Karedal, J. et al., 2010).

License free huge bandwidth

Making effective use of the unlicensed bandwidth, Ariza,A.P.G., proposed a method for high

data rate polarimetric radio wave processing that finds use in short range communications

and radar (Ariza, A.P.G., 2013). Tap-wise polarimetric filtering in delay domain enhances

the 60 GHz link budget. This filtering enhances multipath from scatterers around the link and

reduces multipath due to shadowing. In addition to this, MIMO polarimetry reduces heavy

clutter for MMW radar. This help in capturing the complete information of the target and

clutter (Ariza, A.P.G., 2013).

Small Scale fading effects

Small scale fading accounting for multipath effects dominates the signal propagation in

indoor environment. MIMO small scale fading channel parameters in an underground gold

mine analyzed by Mabrouk,I.B et al., in two scenarios, one in physically static environment

and other in the presence of miners (Mabrouk, I.B. et al., 2013). As the propagation loss at 60

GHz is high, 2x2 MIMO antenna based on microstrip patch array was designed and

fabricated. Using this experimental setup, various parameters like RMS delay spread, path

loss, K-factor, channel correlation and capacity were analyzed. Table 3.1 shows the results

(Mabrouk, I.B. et al., 2013).

41

Measurement

Scenario

RMS

(ns)

K-factor

(dB)

Path loss

exponent

Correlation Capacity

(bits/s/Hz) Transmit Receive

LOS 11.22 9.39 1.48 0.82 0.78 5.4

Human effect 24.86 7.90 2.4 0.75 0.58 7.44

Table 3.1 MIMO channel parameters analyzed in underground mine (Mabrouk, I.B. et al.,

2013)

Results in Table 3.1 indicate the presence of miners substantially affect both the received

power and time dispersive parameters and hence should be considered while developing

underground mine wireless networks.

The overview of the MMW was discussed in length in chapter 2. The features that affect the

channel capacity, link budget and channel modeling are summarized paving way for

alternative solutions. As data rate of the order of gigabits with reliability is required in MMW

system, researchers consider MIMO technique to meet the demand.

3.2 MMW MIMO

This section presents the review of reported work in LOS and NLOS. The MMW research

lab, University of California, Santa Barbara (UCSB) has proposed and developed 2x2

prototype hardware architecture that can be scaled to higher configurations. Similar such

contributions are reviewed in this section.

Capacity analysis for LOS

Operating wireless links at optical speeds is demonstrated in 60 GHz band full duplex

wireless Ethernet link. Speed of 1.25 Gbps is achieved in point to point link (Ohata, K. et al.,

2003). This work stimulated wider interest of achieving data rate of the order of 10 to 40

Gbps. The MMW MIMO using MMW spectrum in the E-Band covering 71 -95 GHz was

considered as large spectrum was made available on semi-unlicensed basis by the FCC. Also

this spectrum, avoided oxygen absorption at 60 GHz providing range of the order of

kilometers. Owing to the fact of small wavelengths, high directive beams enabled spatial

multiplexing with moderate antenna separation even in LOS link. Eigenmode beamforming

using water filling and beam-steering with MMSE receiver using fixed constellation

analyzed. The spatial multiplexing gain is robust to LOS blockage even with a small

42

transmit power per antenna element achieved using low cost CMOS process (Torkildson, E.

et al., 2010). The authors extended the work using adaptive beamforming and spatial

multiplexing to obtain capacity improvement. Adaptive beamforming using adaptive

antenna array called subarray, enabled highly directive beam steerable over a larger angle

improving the directivity gain in both transmitter and receiver. Subarrays were interpreted as

single virtual elements in MIMO system with antenna spacing satisfying the Rayleigh

criterion for realizing uniform array. This characteristic claimed the possibility of attaining

high data rates of the order of 40 Gbps (Torkildson, E. et al., 2006). The concept extended to

analyzing MMW in 57-64 GHz band for indoor environment.

Authors (Torkildson E. et al., 2006) analyzed the performance fixing the range as 1km,

which might not hold good if the range varies. The performance degradation is attributed to

correlation between the spatial responses for different transmitters. Hence non-uniform

arrays were proposed that reduced correlation over large set of ranges (Torkildson, E. et al.,

2009).

Capacity of short range systems using channel transfer function with angle of arrival (AoA)

proposed. AoA data measured with the experimental setup used to calculate the 2x2 MIMO

static channel capacity for varying antenna spacing of 1 mm to 20mm and transmit and

receive distance of 27 cm (Liu, C. et al., 2010).

Hardware prototype

Hardware prototype demonstrating two channel spatial multiplexing operating at 1.2 Gbps in

indoor office environment developed (Sheldon,C. et al., 2008). This was tested in indoor with

6.14 m link range and 12.4cm antenna element separations. Diffraction limited optics was

used to establish multiple parallel data channels. This system can be scaled to longer link

range, larger one and two dimensional arrays supporting data rate greater than 160 Gbps

(Sheldon,C. et al., 2008).

Scalable architecture proposed for four channel spatial multiplexing in LOS link using 1x4

linear arrays in transmitter and receiver. The system was tested in indoor office environment

at 5m link range with antenna element spacing of 7.9cm. BER of less than 10-5

reported with

signal to interference ratio of 11 dB (Sheldon, C. et al., 2009). This replaces the manually

tuned channel separation at Intermediate Frequency (IF) reported in (Sheldon, C. et al., 2008)

with automatically tuned baseband channel separation. Enhancing the performance of

43

channel separation electronics in (Sheldon, C. et al., 2009) resulted in data rate of 2.4 Gbps

(Sheldon, C. et al., 2010).

Spatial degree of freedom

Spatial degree of freedom is limited by the antenna array geometry, number of antennas and

the scattering environment. Lower frequency systems rely on multipath for obtaining an

uncorrelated channel leading to high multiplexing gain. But MMW systems, experience high

propagation loss due to multipath that reduces the probability of uncorrelated channel. This is

overcome by placing antenna with moderate separation with spatial degree of freedom

preserved. Optimal array configuration has been analyzed such that LOS channel capacity is

maximum with minimum number of antenna elements (Torkildson, E. et al., 2009).

Capacity analysis for NLOS

Minimum antenna spacing effects on capacity were analyzed. Antenna spacing of the order

of the wavelength or fraction of wavelength was required for MIMO channels with both LOS

and reflected path while few tens of wavelength was required for MIMO channel with only

LOS. The reason attributed is LOS path suffers from blockages. Hence channel capacity of

MIMO channel with LOS and reflected path was higher compared to MIMO channel with

LOS (Lee, S.J. et al., 2010).

Capacity with multiple elements at both antenna terminals at 60 GHz was studied for SIMO,

MISO and MIMO configurations. The channel model utilized the geometric characteristics of

the environment, angle of arrival, angle of departure of the propagation paths, the antenna

elements and their spacing. Capacity of 4 bits/s/Hz was estimated for transmit and receive

distance of 15m (Arvanitis, A. et al., 2007).

Study of improving the directivity and spatial multiplexing gain in the presence of multipath

and LOS blockage led to the proposal of MMW MIMO architecture using array of subarrays

(Torkildson, E. et al., 2011). Ray tracing model was used to simulate the indoor environment.

Authors have compared the performance of the eigenmode transmission using waterfilling

power allocation with beamsteering transmission and MMSE reception for a fixed signal

constellation. The study provides insights of spatial variations of attainable capacity within a

room. Promising multiplexing gain achieved by electronic beam steering for each subarray

that makes the link robust to node placement and LOS blockage (Torkildson, E. et al., 2011).

44

3.3 Need for beamforming for 60 GHz system

MMW radio suffers severe path loss, oxygen absorption and limited link budget. Methods to

overcome the losses and improve link budget suggest using high gain directional antenna and

beamforming technique. Blockage by metal objects deters the use of high gain directional

antenna. Fixed and adaptive beamforming using MIMO antenna array form the best alternate

in diffraction limited environment. The analysis with the objective of improving the link

robustness, channel capacity, array gain, mixed beamforming techniques are discussed in

length in the following sections.

Beamforming analysis

The influence of beamforming in LOS and NLOS scenario analyzed based on narrowband

and wideband direction based beamformers for 5x5 and 7x7 planar array (Wyne, S. et al.,

2011). Narrowband beamformer operating on 200 MHz bandwidth and wideband

beamformer operating on 4 GHz full bandwidth were used to investigate in terms of

improving the channel metrics i.e. delay spread, excess delay and SNR. The direction based

beamformer was compared with various theoretical bounds. RMS delay spread and excess

delay in beamformed LOS channel was around 1 ns and 0.5 ns. The array gain or SNR

improvement was MN, with M and N representing the antenna elements in transmitter and

receiver. The performance of direction based beamformer LOS channel was comparable to

dominant eigenmode and statistical beamformer. Beamformed NLOS channel showed

performance degradation relative to dominant eigenmode (Wyne, S. et al., 2011). The

reported results are tabulated in Table 3.2.

Channel metric Direction beamformed LOS Direction beamformed

NLOS

RMS Delay spread 1 ns 5 ns

Excess Delay 0.5 ns 0.5 ns

Array gain MN 2( )M N

Table 3.2 Channel metrics using 5x5 planar antenna array (Wyne, S. et al., 2011).

45

Results in Table 3.2 indicate that the NLOS channels have few strong reflected components.

Also influence of array size on beamforming was analyzed in addition to the channel metric.

The 7x7 planar array had the same performance as the 5x5 array, thus setting limits for

practically realizable array size.

Weight vector for beamforming

Beamforming vectors using iterative antenna training algorithm proposed (Xia, P. et al.,

2008). This algorithm computes the Tx/Rx beamforming vector for both training and payload

phase with reduced training overhead and is found efficient compared to antenna training. In

antenna training entire channel co-efficients are estimated while in iterative algorithm two

matrices one each in transmit and receive side are updated based on previous beamforming

vectors (Xia, P. et al., 2008).

Conventional baseband beamforming depends on channel matrix and thereupon determining

the weight vectors with exact amplitude and phase. This method is quite impractical as it

requires multiple RF chains that lead to high power consumption and also increases the

complexity of the phase shifter and gain controller to cover a wider range of phases and

amplitudes. Hence to minimize power consumption of RF devices, codebook based

beamforming proposed (Wang, J. et al., 2009). The elements of the codebook are specified

by one of the four phase shifts (0, 90, 180 and 270 degrees) without any amplitude

adjustment.

Multilevel training and antenna selection was considered in order to reduce the beamforming

setup time (Lee, H-H. and Ko, Y-C. 2011). In every level transmit and receive antennas are

selected based on inter-element spacing. This is followed with transmitting training

sequences with different weight vectors from a pre-defined codebook. This helps the receiver

to select the best transmit and receive weight vector to optimize the SNR. These vectors

determine the codebook for the next level. This is computationally less complex relative to

conventional codebook based beamforming that uses an exhaustive search transmitting all

the training sequences before the best beam pair is selected (Lee, H-H. and Ko, Y-C. 2011).

Efficient codebook –based MIMO beamforming training scheme proposed for estimating

antenna weight vectors (Zhou, L. and Ohashi, Y. 2012). Initially Discrete Fourier Transform

(DFT) based codebook entries were used that had equal amplitude with low power

consumption. The training algorithm then selects the best transmit weight vector and receive

46

weight vector as compared with multiple selection of weight vector in IEEE 802.11ad.

Further refinement of weight vectors is performed by angular rotation around the best

transmit and receive weight vector to improve the beam gain. The proposed work

outperforms the IEEE 802.11ad standard in terms of efficient beamforming gain with less

setup time (Zhou, L. and Ohashi, Y. 2012).

Joint transmit and receive analog beamforming (ABF) proposed for systems with one scalar

weight per antenna (Nsenga, J. et al., 2009). This is aimed at realizing low cost, low power

MMW system. The scheme targets maximizing the average SNR at the input of the equalizer

and analytically derive the close to optimal transmit/receive (Tx/Rx) scalar weights. The joint

Tx/Rx ABF weights computed using channel state information (CSI). CSI is obtained from

the inner product between all Tx/Rx channel impulse response (CIR) pairs. The BER

performance of the joint Tx/Rx ABF outperforms the ABF scheme (Nsenga, J. et al., 2009).

Low cost multi-antenna architectures with a lower number of analog front end chains than

antenna elements proposed (Nsenga, J. et al., 2010). This enables mixed analog/digital

beamformers that aim at maximizing the received average SNR. The performance found to

be better than antenna selection techniques and digital beamforming (Nsenga, J. et al., 2010).

Beamforming and Precoding attempted with large antenna arrays. Precoding in traditional

MIMO systems is implemented in the baseband (Pi, Z. and Khan, F. 2011; Doan, C. et al.,

2004). Baseband processing requires a dedicated radio frequency (RF) chain. This increases

the cost of MMW RF hardware (Pi, Z. and Khan, F. 2011; Doan, C. et al., 2004). Thus

precoding is shared between the analog and digital domain. Precoding in analog domain is

done to reduce high cost of mixed -signal and RF chains. However precoding in the RF

domain after upconversion is implemented using analog phase shifters (Zhang, X. et al.,

2005; Nsenga, J. et al., 2010), that place a constant modulus constraint on the elements of

the precoding matrix. Antenna selection, equal gain transmissions were proposed to address

the problem of limited number of transmit/receive chains. These solutions do not account for

large MMW systems, where transmit correlation is to be analyzed. Single user beamforming

and precoding in MMW systems using large arrays is considered. The structure of MMW

channel was exploited to design the precoder using sparsity constrained least square method.

Principle of basis pursuit was used to develop the precoding algorithm that approximates

unconstrained precoder using low dimensional basis representation. This results in low

47

complex RF hardware and the capacity of proposed algorithm was equal to that of MMW

system using waterfilling method (Ayach, O.E. et al., 2012; Ayach, O.E. et al., 2012).

MIMO postamble, used to reduce the complexity of implementation in MIMO beamformed

system with SVD proposed (Tiraspolsky, S. et al., 2010). The complexity reduction is

achieved by extracting CSI from the MIMO postambles that avoids the need for feedback.

This leads to simple MIMO system architecture (Tiraspolsky, S. et al., 2010).

Conventional beamforming with open loop transmit diversity studied and performance

evaluated based on theoretical calculations. The hybrid scheme provides gain improvement

over conventional beamforming with the same complexity (Pedersen, K.I. and Mogensen,

P.E. 2001).

Beamforming and closed loop transmit diversity in downlink proposed (Goransson, B. et al.,

2004). The transmitted signal phase and amplitude on the diversity antennas is adjusted using

feedback from each mobile user. With fixed antenna array, energy is radiated in fixed

direction that may lead to interference to the mobile user other than the intended mobile user.

To meet the Quality of Service (QoS), high power overcoming the interference is required. In

an attempt to reduce the interference adaptive antenna array capable of steering the radiated

energy to or from the mobile user is used (Goransson, B. et al., 2004).

Near field beamforming addressing beampointing error is analyzed (Yan-ping, L. et al.,

2012). The problem is formulated for uniform linear array. Convex optimization is applied to

update the steering vector error of the beam, by maximizing the signal to interference and

noise ratio subject to the constraint of minimum output variance of the source signal (Yan-

ping, L. et al., 2012).

Continuous Aperture Phased (CAP) MIMO based on the concept of beamspace MIMO that

enables efficient access to the p communication modes of an n-dimensional MMW link is

proposed, where p << n (Song, G-H. et al., 2013; Brady, J. et al., 2013). This is possible by

exploiting the spatial dimension at MMW using high gain directional antenna proportional to

n that leads to sparsity of propagation paths. This reduces the dimension of the

communication subspace. Discrete lens array used to capture the beamspace channel matrix.

The channel matrix is nearly diagonal indicating orthogonal Fourier spatial basis vectors used

for beamforming. This is an indication of spatial multiplexing of data streams over p

dimensional communication subspace. Thus using CAP MIMO enables near-optimal

48

communication with reduction in transceiver complexity. Mutual coupling is a matter of

concern between closely spaced feed antennas which requires further investigation (Song, G-

H. et al., 2013; Brady, J. et al., 2013).

MIMO beamforming in multiuser environment

Opportunistic beamforming in the transmitter that induces random beamforming in the

network is reviewed. In order to have maximum opportunism, the system depends on perfect

CSI. The authors Ozdemir, O. and Torlak, M., at University of Texas, Dallas have

investigated the benefit of partial CSI at an opportunistic transmitter. The partial CSI

information was basically the signal to noise ratio (SNR) relative to channel gain feedback by

the user. The effect of relative SNR was then used to improve the throughput of the system

(Ozdemir, O. and Torlak, M., 2008). The authors have extended the work by developing an

optimum SNR quantization method that is performed at the mobile terminal. This

development is later used to analyze the system throughput. Furthermore, the analysis

indicates when the number of users in the system approach infinity, the performance of the

opportunistic beamforming approaches to that of true beamforming (Ozdemir, O. and Torlak,

M., 2010).

Optimum zero forcing downlink beamforming analysed. The scheduling of the multiuser

studied using greedy algorithm and water filling power allocation with average power

constraint at the transmitter. The authors have derived joint probability density function for

the scheduled users. The sum rate performance is evaluated and results indicate that the

subchannel gains of the previously scheduled users remain unaffected. This is due to the

cutoff value for water filling power allocation is suitably expressed (Ozyurt, S. and Torlak,

M. 2012). The authors have extended the analysis in evaluating the zero forcing (ZF) Vertical

Bell Labs Layered Space-Time (V-BLAST) algorithm using greedy decoder ordering in

Rayleigh fading channels. The joint probability density function (pdf) of the squared layer

gains assuming no error propagation is derived and is found applicable to any number of

transmit and receive antennas. Based on this, joint distribution of the ordered gains from the

joint distribution of the unordered gains is determined. This results in optimal power

allocation using water filling algorithm avoiding extensive simulations to be carried out

(Ozyurt, S. and Torlak, M. 2013).

49

3.4 Need for Multibeam in MIMO

Multibeams are generated by antenna array. They form an integral part in MIMO systems.

The capacity and performance of a system can be improved in two ways (i) At the baseband

or at the intermediate frequency (IF) level by spatial multiplexing and diversity (ii) At the RF

level by having antenna array and beamforming network (Godara, L.C., 1997). The diversity

combining enhance the signal-to-noise ratio (SNR) of the received signal, while the

beamforming network steers the beam to a direction where maximum gain is achieved (Yazd,

M. H. and Faraji-Dana, R., 2005; Bellofiore, S. et al., 2002). Combining the above two

methods may result in increase in both capacity and performance. This also leads to

reduction in interference in multiuser environment (Tsunekawa, K. 2005). The multibeam

generated by multi-antenna system is shown in Figure 3.1 (Tsunekawa, K. 2005). In Figure

3.1, the antenna element and multi-antenna system are shown.

Figure 3.1 Multi-antenna system with baseband, IF and RF subsystems (Tsunekawa, K.

2005)

50

The baseband unit, RF and antenna element together form the multi-antenna system in Figure

3.1. High gain in antenna element is achieved by adding a parasitic element or integrating an

RF circuit. The radiation pattern is controlled by baseband signal processing and hence

pattern of each wireless channel can be controlled by each baseband channel. Thus, multi-

antenna systems are generally called smart or active antennas (Alexiou, A. and Haardt, M.

2004).

Multibeam selection scheme, selecting the best subset of all beams to maximize capacity

using orthonormal random beamforming with partial CSI is proposed (Zhang, K. and Niu, Z.

2006). The scheme is compared with dirty paper coding that uses full CSI and is found to

have performance improvement under low SNR (Zhang, K. and Niu, Z. 2006).

Diversity and Multiplexing gain for multi-user MIMO using orthogonal multiple random

beams proposed (Oho, D.-C. and Lee, Y.-H. 2005). Orthogonal beams enable users to

experience multiple channels at the same time realizing multi-user diversity. Also

multibeams increase the multiplexing gain. Both MISO and MIMO configuration benefit

from the scheme (Oho, D.-C. and Lee, Y.-H. 2005). Multibeam assisted MIMO (MBA-

MIMO) proposed as an alternative for fixed beamfomer that suffers from inter-beam

interference proposed (Papadogiannis, A. and Burr, A.G. 2011). MBA-MIMO is applied in

the beam domain to mitigate interference and enable the provision of required capacity

density (Papadogiannis, A. and Burr, A.G. 2011).

3.5 60 GHz channel sounder

Measurement of capacity and double directional parameters with 16 x16 MIMO systems was

performed in LOS, LOS obstructed and NLOS (Ranvier, S. et al., 2005; Foschini, J.G. 1996).

Direction of Arrival (DOA) and Direction of Departure (DOD) were calculated from LOS

measurements and channel capacity was calculated from NLOS. MIMO capacity obtained in

NLOS is less compared to i.i.d Rayleigh channel (Ranvier, S. et al., 2005; Foschini, J.G.

1996). 2x2 MIMO capacity measured using 4-port Agilent Network Analyzer. Open-ended

waveguide antennas with different orientations are used in the study. The study confirms the

achievement of enhanced data rate assessed using the Shannon capacity theorem (Ahmadi-

Shokouh, J. et al., 2012).

51

3.6 Outdoor MMW MIMO

Outdoor deployment of MMW was analyzed, investigating the robustness of the link using

ray tracing channel model (Zhang, H. et al., 2010). This study indicated the sensitivity of the

received power to small variations in propagation geometry, which has led the analysis to

employ multiple antennas. Robust performance was reported based on antenna separation,

transmit precoding and space time coding (Zhang, H. et al., 2010).

A two channel hardware prototype operating at 1.2 Gbps over a link range of 41m

highlighted the potential of spatial multiplexing in LOS link (Sheldon, C. et al., 2008). The

prototype achieved a BER of less than 2x10-6

and data rate of 600 Mbps per channel. The

antenna spacing as a function of wavelength and link range determined in interference

limited propagation (Sheldon, C. et al., 2008).

Multilevel beamforming in outdoor MMW communication systems using beam searching

algorithm and codebook design proposed (Hur, S. et al., 2011). The codebook controls the

beamwidth and steering direction. The study is used in the design of MMW backhaul

systems supporting picocell data traffic. The codebook is designed based on adaptive

beamwidth beamforming technique developed on subarray using squinting angles. As

beamwidth is inversely proportional to the array size (Trees, H.L.V. 2002), sharp beam is

obtained using the entire array, while this beam can be broadened using subarraying and

squinting. Each level of the codebook uses a different subarray size resulting in varied

beamwidth. Directional gain issue arising out of squinting and subarraying is avoided using

spectral windowing. The performance of the exhaustive search technique when compared

with proposed scheme offers better beamforming gain at the cost of increased complexity.

But the proposed joint search method displays larger gain compared to one-side search with

the same complexity (Hur, S. et al., 2011).

52

Summary of the comparative description of various reported results of MMW MIMO

worldwide.

a) Ariza,A.P.G., et al., (2013), IEEE Transactions on Antennas and Propagation.

(a)

b) Mabrouk,I.B. et al., (2013), IEEE Transactions on Antennas and Propagation

(b)

c) Song, G-H et al., (2013),International Conference on Acoustics, Speech and Signal

Processing

d) Brady, J. et al., (2013), IEEE Transactions on Antennas and Propagation

(c)

Parameter Methodology/Formulation Outcome

Multipath effects

resulting from

scatterer and

shadowing

Polarimetric radio wave processing.

Polarimetry is used to analyze the

characteristics of electromagnetic waves.

Tap-wise polarimetric filtering in delay

domain enhancing the 60 GHz link budget.

Enhanced multipath from

scatterers around the link

and reduces multipath due to

shadowing. In addition to

this, MIMO polarimetry is

proposed to reduce heavy

clutter for MMW radar.

Parameter Methodology/Formulation Outcome

MIMO channel

parameters in an

underground gold

mine (RMS delay

spread, K-factor,

path loss

exponent, Tx/Rx

correlation,

capacity)

2x2 MIMO antenna based microstrip

patch array designed.

Parameters were studied in two

scenarios: one in physically static

environment and other in the presence of

miners. Table 3.1 lists the measured data

for the above two cases.

The presence of miners

substantially affects both

received power and time

dispersion parameters and

the effects to be considered

when developing

underground mine wireless

networks in the unlicensed

60-GHz

band.

Parameter Methodology/Formulation Outcome

Channel matrix Continuous Aperture Phased (CAP)

MIMO based on the concept of

beamspace MIMO that enables efficient

access to the p communication modes of

an n-dimensional MMW link, where p

<< n.

Exploiting the spatial dimension at

MMW using high gain directional

antenna proportional to n that leads to

sparsity of propagation paths. This

reduces the dimension of the

communication subspace.

Channel matrix is nearly

diagonal indicating

orthogonal Fourier spatial

basis vectors used for

beamforming

53

e) Ozyurt, S. and Torlak, M. (2013), IEEE Transactions on Wireless Communications

(d)

f) Ayach, O.E. et al., (2012) IEEE International Conference on Communications.

g) Ayach,O.E. et al., (2012), IEEE International Workshop on Signal Processing Advances in

Wireless Communications.

(e)

h) Liu. C. et al., (2010), IEEE Transactions on Wireless Communications.

(f)

i) Ozdemir, O. and Torlak, M. (2010), IEEE Transactions on Wireless Communications

(g)

Parameter Methodology/Formulation Outcome

Sum rate and

Subchannel gain

Downlink Beamforming in multiuser

environment

Zero Forcing V-BLAST using greedy

decoder ordering in Rayleigh fading

channel.

The joint probability density

function assuming no error

propagation derived. This

results in optimal power

allocation using water filling

algorithm.

Parameter Methodology/Formulation Outcome

Channel capacity

and RF hardware

complexity

Beamforming and precoding in MMW

systems with large arrays.

Principle of basis pursuit was used to

develop the precoding algorithm that

approximates unconstrained precoder

using low dimensional basis

representation.

Capacity is equal to that

achieved using water filling

algorithm.

Parameter Methodology/Formulation Outcome

Channel Capacity

Angle-of-Arrival information used to

predict the

static channel capacity.

An Anritsu 37397 Vector Network

Analyzer (VNA) used to measure the

channel transfer function.

LOS

signal contributes

significantly to the spatial

MIMO channel

capacity for short distance

applications.

Parameter Methodology/Formulation Outcome

SNR and

Throughput

Optimum SNR quantization performed

at the mobile terminal

Opportunistic beamforming in the

transmitter for flat and slow fading

channel.

System throughput is

improved. The performance

of opportunistic

beamforming approaches

true beamforming when

number of users in the

system approach infinity.

54

j) Sheldon, C. et al., (2009), IEEE- MTTS International Microwave Symposium.

(h)

k) Wang, J. et al., (2009), IEEE Vehicular Technology Conference.

(i)

l) Papadogiannis, A and Burr, A.G. (2011), Future Network & Mobile Summit.

(j)

m) Oho,D.-C., Lee,Y.-H., (2005), IEEE Vehicular Technology Conference.

(k)

Parameter Methodology/Formulation Outcome

Four channel

spatial

multiplexing

Scalable system architecture is proposed

and demonstrated for spatial

multiplexing over MMW line-of-sight

communication.

The system was tested in indoor office

environment at 5m link range with

antenna element spacing of 7.9cm

Increased communication

link data capacity without

increased channel bandwidth.

BER of less than 10-5

reported with signal to

interference ratio of 11 dB

Parameter Methodology/Formulation Outcome

Power

Minimization of

RF devices.

Codebook based beamforming used.

The elements of the codebook are

specified by one of the four phase shifts

(0, 90, 180 and 270 degrees) without any

amplitude adjustment.

Power minimization

achieved compared to

conventional baseband

beamforming.

Parameter Methodology/Formulation Outcome

Interference and

Capacity

MIMO techniques are

applied in the beam domain.

Linear detection technique such as Zero

forcing (ZF), Minimum Mean Square

Error (MMSE) are applied in beam

domain instead of the antenna domain.

Capacity density for future

systems beyond 4G.

Parameter Methodology/Formulation Outcome

Diversity and

Multiplexing

Gain

Orthogonal multiple random beams in

MIMO.

Orthogonal beams enable users to

experience multiple channels at the same

time realizing multi-user diversity. Also

multibeams increase the multiplexing

gain.

Applicable to multi-input

single-output (MISO)

schemes as well as MIMO

schemes, enabling the use of

flexible antenna structures in

the receiver

55

n) Zhang,K. and Niu,Z. (2006), IEEE International Conference on Communications

(l)

o) Tsunekawa, K. (2005), NTT Technical Review.

(m)

Table 3.3 (a – m) Survey of reported results in Millimeter Wave MIMO

Parameter Methodology/Formulation Outcome

Computational

Complexity

Multibeam selection scheme, selecting

the best subset of all beams to maximize

capacity using orthonormal random

beamforming with partial CSI is

proposed.

Orthonormal

random beamforming which only

requires partial

CSI feedback at the transmitter. This

technique results in less computational

complexity than dirty paper coding.

Efficient under low SNR

Parameter Methodology/Formulation Outcome

Antenna Gain

High-efficiency antenna created by

integrating it with a monolithic

microwave integrated circuit (MMIC) in

a millimeter-wave frequency-band

system,a multi-antenna system for the

MIMO technique that enables multiple

data transmission for land mobile

communication systems.

The baseband unit, RF and antenna

element together form the multi-antenna

system. High gain in antenna element is

achieved by adding a parasitic element

or integrating an RF circuit.

High-efficiency antenna can

be made using an active

integrated antenna, which

integrates an

MMIC with the antenna

element.

56

3.7 General observations based on present Research work

The chapter has extracted the contributions of researchers in MMW MIMO with the

following concluding points in addition to the salient features presented in Table 3.3 and

spatial multiplexing in LOS possible by high gain directional antenna leads to enhanced data

rate. The losses at 60 GHz, small wavelength, oxygen absorption, unlicensed bandwidth,

poor link budget and LOS blockage motivate the use of antenna beamforimg.

Beamformer reduces the interference and ensures increased received signal power. The phase

and amplitude of the weight vectors of the beamformer are fixed or can be adapted based on

the CSI fedback to the transmitter. Implementation of fixed antenna array is preferred

compared to adaptive array as this requires complex phase control network.

Solutions optimizing the cost of RF chain have been reported. Dedicated RF chain per

antenna increases the cost and complexity of RF components, hence antenna selection and

subarray technique are reported. Methods of diagonalizing the channel matrix are analyzed

with an objective of reducing the computational complexity. Continuous Aperture Phased

(CAP) MIMO based on the concept of beamspace MIMO enables efficient access to the p

communication modes of an n-dimensional MMW link is reported, where p << n.

Conventional beamforming with open loop transmit diversity studied and performance

evaluated based on theoretical calculations. The hybrid scheme provides gain improvement

over conventional beamforming with the same complexity.

Experimental studies on channel parameters have considered Ray Tracing based on

geometric optics, Saleh Valenzuela (SV) a cluster based model for estimating the channel

parameters such as the RMS delay spread, Angular spread, Rician factor. These parameters

have been studied for indoor environment such as conference room, desktop, hospital and

underground gold mine.

Various studies indicate the indoor system using MMW technology possess time dispersive

and doppler sparse environment. Also using directional antenna causes sparsity of

propagation paths that enables compressive sensing technique for channel estimation.

Multilevel beamforming in outdoor MMW communication systems using beam searching

algorithm and codebook design proposed. The codebook controls the beamwidth and steering

direction. The study is used in the design of MMW backhaul systems supporting picocell

data traffic.

57

Multibeam MIMO proposed with an objective of reducing beam interference most likely in

fixed beamformer using MBA-MIMO. Orthogonal multiple random beams in MIMO enables

improvement in diversity and multiplexing gain. Also antenna gain and efficiency

improvement using multibeam MIMO reported.

MMW-MIMO needs more refined system components in terms of antenna systems, channel

parameters etc. Survey report presents wide scope of research as no complete system design

with fully satisfied results reported. The proposed research work, we have carried out in this

tenure is not found in any of the reports that proves it may be unique in nature. Future 5G

systems, propose to use MMW as the last mile solution that is cost-effective alternative to the

existing standards. Samsung Electronics announced that it has successfully developed the

world‘s first adaptive array transceiver technology operating in the millimeter wave Ka bands

for cellular communications. The new technology sits at the core of 5G mobile

communication system and will provide data transmission up to several hundred times faster

than current 4G networks. Samsung plans to accelerate the research & development of 5G

mobile communication technologies, including adaptive array transceiver at the MMW

bands, to commercialize those technologies by 2020 (http://global.samsungtomorrow.com/).

Based on this survey, it is very clear that MMW MIMO still needs to be refined to achieve its

full potential. The main challenge of integrating the RF front end system with baseband

signal processing system is still a formidable task due to the complexity in the antenna design

and associated systems for mobile devices. The free space path loss and atmospheric loss

added constraints to be tackled using beamforming concept. Omnidirectional antenna

systems are not a good solution in terms of power optimization. Since indoor communication

environments are considered, the rain attenuation need not be accounted.

In summary, MMW MIMO is a wonderful pair of technology that will tend to have

extraordinary potential for establishing wireless networks with gigabit capacity.

Thus summarizing on the research outcomes, MIMO for MMW was analyzed for LOS and

NLOS conditions, need for beamforming and multibeam MIMO. Ray tracing channel model,

empirical model was used to model the indoor channel and study the system under test. As

this model is site-specific and throws constraints in evaluating system performance, statistical

channel models have to be considered. Also both open and closed loop beamforming

techniques proved to yield better results throws scope for extending the idea to multibeam

58

MIMO. The challenges brought out in the reported results lays path for exploring the cluster

based statistical channel models in LOS supporting multi-antenna configuration that is

discussed in detail in chapter 4.