Multi carrier and spread spectrum systems phần 9 docx

Uplink return channelUplink return channel Uplink return channel Figure 5-21 DVB-RCT network architecture DVB-T transmitter MAC DVB-T-RCT return channel Downlink interaction path Uplink

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Uplink return channel

Figure 5-21 DVB-RCT network architecture

DVB-T transmitter

MAC

DVB-T-RCT return channel Downlink interaction path

Uplink interaction path

Terminal station (TS) with interactive services

DVB-T broadcast

Base station (BS) with interactive services

MPEG

prog stream

DVB-RCT Receiver

MAC DVB-RCT transmitter

DL interactive messages and synch

Interactive data from/to the user

Figure 5-22 Overview of the DVB-RCT standard

parameters of the DVB-RCT speciﬁcation is to employ the existing infrastructure usedfor broadcast DVB-T services

As shown in Figure 5-22, the interactive downlink path is embedded in the broadcastchannel, exploiting the existing DVB-T infrastructure [7] The access for the uplink inter-active channels carrying the return interaction path data is based on a combination ofOFDMA and TDMA type of multiple access scheme [6]

The downlink interactive information data is made up of MPEG-2 transport streampackets with a speciﬁc header that carries the medium access control (MAC) management

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Interaction Channel for DVB-T: DVB-RCT 223

data The MAC messages control the access of the subscribers, i.e., terminal stations, tothe shared medium These embedded MPEG-2 transport stream packets are carried in theDVB-T broadcast channel (see Figure 5-22)

The uplink interactive information is mainly made up of ATM cells mapped ontophysical bursts ATM cells include application data messages and MAC management data

To allow access by multiple users, the VHF/UHF radio frequency return channel ispartitioned both in the frequency and time domain, using frequency and time division.Each subscriber can transmit his data for a given period of time on a given sub-carrier,resulting in a combination of OFDMA and TDMA multiple access

A global synchronization signal, required for the correct operation of the uplink ulator at the base station, is transmitted to all users via global DVB-T timing signals.Time synchronization signals are conveyed to all users through the broadcast channel,either within the MPEG2 transport stream or via global DVB-T timing signals In otherwords, the DVB-RCT frequency synchronization is derived from the broadcast DVB-Tsignal whilst the time synchronization results from the use of MAC management pack-

demod-ets conveyed through the broadcast channel Furthermore, the so-called periodic ranging signals are transmitted from the base station to individual terminal stations for timing

misalignment adjustment and power control purposes

The DVB-RCT OFDMA based system employs either 1024 (1k) or 2048 (2k) carriers and operates as follows:

sub-— Each terminal station transmits one or several low bit rate modulated sub-carrierstowards the base station;

— The sub-carriers are frequency-locked and power-ranged and the timing of the lation is synchronized by the base station In other words, the terminal stations derivetheir system clock from the DVB-T downstream Accordingly, the transmission modeparameters are ﬁxed in a strict relationship with the DVB-T downstream;

modu-— On the reception side, the uplink signal is demodulated, using an FFT process, likethe one performed in a DVB-T receiver

in any free segment of an RF channel, taking into account existing national and regionalanalog television assignments, interference risks, and future allocations for DVB-T

5.5.3 Multi-Carrier Uplink Transmission

The method used to organize the DVB-RCT channel is inspired by the DVB-T standard.The DVB-RCT RF channel provides a grid of time-frequency slots, each slot usable by

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224 Applications

any terminal station Hence, the concept of DVB uplink channel allocation is based on

a combination of OFDMA with TDMA Thus, the uplink is divided into a number oftime slots Each time slot is divided in the frequency domain into groups of sub-carriersreferred to as sub-channels The MAC layer controls the assignment of sub-channels andtime slots by resource requests and grant messages

The DVB-RCT standard provides two types of sub-carrier shaping, where out of theseonly one is used at any time The shaping functions are:

— Nyquist shaping in the time domain on each sub-carrier to provide immunity against

both ICI and ISI A square root raised cosine pulse with a roll-off factor α=

0.25 is employed The total symbol duration is 1.25 times the inverse of the

sub-carrier spacing

— Rectangular shaping with guard interval T g that has a possible value ofT s /4, T s /8,

T s /16, T s/32, whereT s is the useful symbol duration (without guard time)

5.5.3.1 Transmission Modes

The DVB-RCT standard provides six transmission modes characterized by a dedicatedcombination of the maximum number of sub-carriers used and their sub-carrier spac-ings [6] Only one transmission mode is implemented in a given RCT radio frequencychannel, i.e., transmission modes are not mixed

The sub-carrier spacing governs the robustness of the system in regard to the possiblesynchronization misalignment of any terminal station Each value implies a given maxi-mum transmission cell size and a given resistance to the Doppler shift experienced whenthe terminal station is in motion, i.e., in case of portable receivers The three targetedDVB-RCT sub-carrier spacing values are deﬁned in Table 5-20

Table 5-21 gives the basic DVB-RCT transmission mode parameters applicable forthe 8 MHz and 6 MHz radio frequency channels with 1024 or 2048 sub-carriers Due

to the combination of the above parameters, the DVB-RCT ﬁnal bandwidth is a tion of sub-carrier spacing and FFT size Each combination has a speciﬁc trade-offbetween frequency diversity and time diversity, and between coverage range and porta-bility/mobility capability

func-5.5.3.2 Time and Frequency Frames

Depending on the transmission mode in operation, the total number of allocated carriers for uplink data transmission is 1024 carriers (1k mode) or 2048 carriers (2kmode) (see Figure 5-23) Table 5-22 shows the main parameters

sub-Table 5-20 DVB-RCT targeted sub-carrier spacing for 8 MHz channel

Sub-carrier spacing Targeted sub-carrier spacing

Sub-carrier spacing 1 ≈ 1 kHz (symbol duration ≈ 1000 µs)

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Table 5-21 DVB-RCT transmission mode parameters for the 8 and 6 MHz DVB-T systems

Total number of sub-carriers 2048 (2k) 1024 (1k) 2048 (2k) 1024 (1k)

Sub-carrier spacing 1.116 kHz 1.116 kHz 0.837 kHz 0.837 kHz RCT channel bandwidth 1.911 MHz 0.940 MHz 1.433 MHz 0.705 MHz

168 Unused sub-carriers

Figure 5-23 DVB-RCT channel organization for the 1k and 2k mode

Table 5-22 Sub-carrier organization for the 1k and 2k mode

Overall used sub-carriers

Lower and upper channel guard band 91 sub-carriers 168 sub-carriers

Two types of transmission frames (TFs) are deﬁned:

— TF1: The ﬁrst frame type consists of a set of OFDM symbols which contain several

data sub-channels, a null symbol and a series of synchronization/ranging symbols;

— TF2: The second frame type is made up of a set of general purpose OFDM symbols

which contain either data or synchronization/ranging sub-channels

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Furthermore, three different burst structures are speciﬁed as follows:

— Burst structure 1 uses one unique sub-carrier to carry the total data burst over time,

with an optional frequency hopping law applied within the duration of the burst;

— Burst Structure 2 uses four sub-carriers simultaneously, each carrying a quarter of the

total data burst over time;

— Burst structure 3 uses 29 sub-carriers simultaneously, each carrying one twenty-ninth

of the total data burst over time

These three burst structures provide a pilot-aided modulation scheme to allow coherentdetection in the base station The deﬁned pilot insertion ratio is approximately 1/6, whichmeans one pilot carrier is inserted for approximately every ﬁve data sub-carriers Further-more, they give various combinations of time and frequency diversity, thereby providingvarious degrees of robustness, burst duration and a wide range of bit rates to the system.Each burst structure makes use of a set of sub-carriers called a sub-channel One orseveral sub-channels can be used simultaneously by a given terminal station depending

on the allocation performed by the MAC process

Figure 5-24 depicts the organization of a TF1 frame in the time domain It should benoted that the burst structures are symbolized regarding their duration and not regard-ing their occupancy in the frequency domain The corresponding sub-carrier(s) of burststructure 1 and burst structure 2 are spread over the whole RCT channel

Null symbol and ranging symbols always use rectangular shaping The user symbols ofTF1 use either rectangular shaping or Nyquist shaping If the user part employs rectangularshaping, the guard interval value is identical for any OFDM symbol embedded in thewhole TF1 frame If the user part performs Nyquist shaping, the guard interval value toapply onto the Null symbol and ranging symbols isT s/4 The user part of the TF1 frame

is suitable to carry one burst structure 1 or four burst structure 2 The burst structures arenot mixed in a given DVB-RCT channel

The time duration of a transmission frame depends on the number of consecutive OFDMsymbols and on the time duration of the OFDM symbol The time duration of an OFDMsymbol depends on

— the reference downlink DVB-T system clock,

— the sub-carrier spacing, and

— the rectangular ﬁltering of the guard interval (1/4, 1/8, 1/16, 1/32 timesT s)

Time

Data symbols Ranging symbols

Transmission frame type 1

Null symbol

Ranging symbols

Data symbols carrying burst structure 1 or 2 (not simultaneously)

Figure 5-24 Organization of the TF1 frame

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Table 5-23 Transmission frame duration in seconds with burst structure 1 and with rectangular ﬁltering withT g = T s/4 or Nyquist ﬁltering and for reference clock 64/7 MHz

Shaping scheme Number of consecutive

OFDM symbols

Sub-carrier spacing 1

Transmission frame type 2

User symbols carrying eight burst structure 3

Null symbols User symbols carrying one burst structure 2

Sub-channel

Figure 5-25 Organization of the TF2 frame

In Table 5-23, the values of the frame durations in seconds for TF1 using burst ture 1 is given

struc-Figure 5-25 depicts the organization of the TF2 in the time domain The correspondingsub-carrier(s) of burst structures 2 and 3 are spread on the whole RCT channel TF2 will

be used only in the rectangular pulse shaping case The guard interval applied on anyOFDM symbol embedded in the whole TF2 is the same (i.e., either 1/4, 1/8, 1/16 or1/32 of the useful symbol duration) The user part of the TF2 allows the usage of burststructure 3 or, optionally, burst structure 2 When one burst structure 2 is transmitted, itshall be completed by a set of four null modulated symbols to have a duration equal tothe duration of eight burst structure 3

5.5.3.3 FEC Coding and Modulation

Channel coding is based on a concatenation of a Reed–Solomon outer code and arate-compatible convolutional inner code Convolutional Turbo codes can also be used.Different modulation schemes (QPSK, 16-QAM, and 64-QAM) with Gray mapping areemployed

Whatever FEC is used, the data bursts produced after the encoding and mapping cesses have a ﬁxed length of 144 modulated symbols Table 5-24 deﬁnes the original sizes

pro-of the useful data payloads to be encoded in relation to the selected physical modulationand encoding rate

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Table 5-24 Number of useful data bytes per burst

FEC encoding rate R= 1/2 R= 3/4 R= 1/2 R= 3/4 R= 1/2 R= 3/4 Number of data bytes

in 144 symbols

Under the control of the base station, a given terminal station can use different cessive bursts with different combinations of encoding rates Here, the use of adaptivecoding and modulation is aimed to provide ﬂexible bit rates to each terminal station inrelation to the individual reception conditions encountered in the base station

suc-The outer Reed–Solomon encoding process uses a shortened systematic RS(63, 55,

t = 4) encoder over a Galois ﬁeld GF(64), i.e., each RS symbol consists of 6 bits Databits issued from the Reed–Solomon encoder are fed to the convolutional encoder ofconstraint length 9 To produce the two overall coding rates expected (1/2 and 3/4), the

RS and convolutional encoder have implemented the coding rates deﬁned in Table 5-25.The terminal station uses the modulation scheme determined by the base station throughMAC messages The encoding parameters deﬁned in Table 5-26 are used to produce thedesired coding rate in relation with the modulation schemes It should be noted thatthe number of channel symbols per burst in all combinations remains constant, i.e., 144modulated symbols per burst

Table 5-25 Overall encoding rates

Outer RS encoding rate

Router

Inner CC encoding rate

Rinner

Overall code rate

Rtotal= Router· Rinner

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Pilot sub-carriers are inserted into each data burst in order to constitute the burststructure and are modulated according to their sub-carrier location Two power levels areused for these pilots, corresponding to+2.5 dB or 0 dB relative to the mean useful symbol

power The selected power depends on the position of the pilot inside the burst structure

5.5.4 Transmission Performance

5.5.4.1 Transmission Capacity

The transmission capacity depends on the usedM-QAM modulation density, error control

coding and the used mode with Nyquist or rectangular pulse shaping

The net bit rate per sub-carrier for burst structure 1 is given in Table 5-27 with andwithout frequency hopping (FH)

5.5.4.2 Link Budget

The service range given for the different transmission modes and conﬁgurations can

be calculated using the RF ﬁgures derived from the DVB-T implementation and agation models for rural and urban areas In order to limit the terminal station RFpower to reasonable limits, it is recommended to put the complexity on the base stationside by using high-gain sectorized antenna schemes and optimized reception conﬁgura-tions

prop-To deﬁne mean service ranges, Table 5-28 details the RF conﬁgurations for sub-carrierspacing 1 and QPSK 1/2 modulation levels for 800 MHz in transmission modes with burststructure 1 and 2 The operational C/N is derived from [7] and considers +2 dB imple-mentation margin, +1 dB gain due to block Turbo code/concatenated RS and convolutionalcodes, and +1 dB gain when using time interleaving in Rayleigh channels

Table 5-27 Net bit rate in kbit/s per sub-carrier for burst structure 1 using rectangular shaping Channel spacing, modulation Rectangular shaping Nyquist shaping

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Table 5-28 Parameters for service range simulations

5 dB

QPSK1/2 3.6 dB

5 dB

BS receiver antenna gain 16 dBi (60 degree) 16 dBi (60 degree) Antenna height (user

side)

Outdoor 10 m Indoor 10 m (2nd ﬂoor)

TS Antenna gain 13 dBi (directive) 3 dBi ( ∼omnidir.)

Standard deviation for

location variation

−10 dB for BS1

−5 dB for BS-2 and BS-3 (spread multi-carrier)

−10 dB for BS1

−5 dB for BS-2 and BS-3 (spread multi-carrier)

Reasonable dimensioning of the output ampliﬁer in terms of bandwidth and modulation products (linearity) indicates that a transmit power of the order of 25 dBmcould be achievable at low cost It is shown in [6] that with 24 dBm transmit power,indoor reception would be possible up to a distance of 15 km, while outdoor receptionwould be offered up to 40 km or more

[1] 3GPP (TR25.858), “High speed downlink packet access: Physical layer aspects,” Technical Report, 2001.

[2] Atarashi H., Maeda N., Abeta S and Sawahashi M., “Broadband packet wireless access based on

VSF-OFCDM and MC/DS-CDMA,” in Proc IEEE International Symposium on Personal, Indoor and Mobile

Radio Communications (PIMRC 2002), Lisbon, Portugal, pp 992–997, Sept 2002.

[3] Atarashi H and Sawahashi M., “Variable spreading factor orthogonal frequency and code division

multi-plexing (VSF-OFCDM),” in Proc International Workshop on Multi-Carrier Spread-Spectrum & Related

Topics (MC-SS 2001), Oberpfaffenhofen, Germany, pp 113–122, Sept 2001.

[4] Burow R., Fazel K., H¨oher P., Kussmann H., Progrzeba P., Robertson P and Ruf M., “On the

Per-formance of the DVB-T system in mobile environments,” in Proc IEEE Global Telecommunications

Conference (GLOBECOM’98), Communication Theory Mini Conference, Sydney, Australia, Nov 1998.

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[10] Fazel K., Decanis C., Klein J., Licitra G., Lindh L and Lebret Y.Y., “An overview of the ETSI-BRAN

HA physical layer air interface speciﬁcation,” in Proc IEEE International Symposium on Personal, Indoor

and Mobile Radio Communications (PIMRC 2002), Lisbon, Portugal, pp 102–106, Sept 2002.

[11] IEEE 802.11 (P802.11a/D6.0), “LAN/MAN speciﬁc requirements – Part 2: Wireless MAC and PHY iﬁcations – high speed physical layer in the 5 GHz band,” IEEE 802.11, May 1999.

spec-[12] IEEE 802.16ab-01/01, “Air interface for ﬁxed broadband wireless access systems – Part A: Systems between 2 and 11 GHz,” IEEE 802.16, June 2000.

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— Time diversity: Time interleaving in combination with channel coding provides

repli-cas of the transmitted signal in the form of redundancy in the temporal domain tothe receiver

— Frequency diversity: The signal transmitted on different frequencies induces different

structures in the multipath environment Replicas of the transmitted signal are provided

to the receiver in the form of redundancy in the frequency domain Best examples

of how to exploit the frequency diversity are the technique of multi-carrier spreadspectrum and coding in the frequency direction

— Spatial diversity: Spatially separated antennas provide replicas of the transmitted

sig-nal to the receiver in the form of redundancy in the spatial domain This can beprovided with no penalty in spectral efﬁciency

Exploiting all forms of diversity in future systems (e.g., 4G) will ensure the highestperformance in terms of capacity and spectral efﬁciency

Furthermore, the future generation of broadband mobile/ﬁxed wireless systems willaim to support a wide range of services and bit rates The transmission rate may varyfrom voice to very high rate multimedia services requiring data rates up to 100 Mbit/s.Communication channels may change in terms of their grade of mobility, cellular infras-tructure, required symmetrical or asymmetrical transmission capacity, and whether they

Multi-Carrier and Spread Spectrum Systems K Fazel and S Kaiser

 2003 John Wiley & Sons, Ltd ISBN: 0-470-84899-5

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234 Additional Techniques for Capacity and Flexibility Enhancement

are indoor or outdoor Hence, air interfaces with the highest ﬂexibility are demanded

in order to maximize the area spectrum efficiency in a variety of communication ronments The adaptation and integration of existing and new systems to emerging newstandards would be feasible if both the receiver and the transmitter are reconfigurableusing software-defined radio (SDR)

envi-The aim of this last chapter is to look at new antenna diversity techniques (e.g., space time coding (STC), space frequency coding (SFC) and at the concept of software-deﬁned radio (SDR) which will all play a major role in the realization of 4G.

In conventional wireless communications, spectral and power efﬁciency is achieved byexploiting time and frequency diversity techniques However, the spatial dimension so faronly exploited for cell sectorization will play a much more important role in future wirelesscommunication systems In the past most of the work has concentrated on the design of

intelligent antennas, applied for space division multiple access (SDMA) In the meantime,

more general techniques have been introduced where arbitrary antenna conﬁgurations atthe transmit and receive sides are considered

If we considerM transmit antennas and L receive antennas, the overall system channel deﬁnes the so-called multiple input/multiple output (MIMO) channel (see Figure 6-1) If

the MIMO channel is assumed to be linear and time-invariant during one symbol duration,the channel impulse responseh(t) can be written as

where h m,l (t) represents the impulse response of the channel between the transmit (Tx)

antennam and the receive (Rx) antenna l.

From the above general model, two possibilities exist: i) case M= 1, resulting in asingle input/multiple output (SIMO) channel and ii) case L= 1, resulting in a multipleinput/single output (MISO) channel In the case of SIMO, conventional receiver diversity

.

Figure 6-1 MIMO channel

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General Principle of Multiple Antenna Diversity 235

techniques such as MRC can be realized, which can improve power efﬁciency, especially

if the channels between the Tx and the Rx antennas are independently faded paths (e.g.,Rayleigh distributed), where the multipath diversity order is identical to the number ofreceiver antennas [15]

With diversity techniques, a frequency- or time-selective channel tends to become anAWGN channel This improves the power efficiency However, there are two ways toincrease the spectral efficiency The first one, which is the trivial way, is to increase thesymbol alphabet size and the second one is to transmit different symbols in parallel inspace by using the MIMO properties

The capacity of MIMO channels for an uncoded system in ﬂat fading channels withperfect channel knowledge at the receiver is calculated by Foschini [11] as

C= log2

det I L+E s /N o

∗T (t)

where “det” means determinant, I L is an L × L identity matrix, and (·) ∗T means the

conjugate complex of the transpose matrix Note that this formula is based on the Shannoncapacity calculation for a simple AWGN channel

Two approaches exist to exploit the capacity in MIMO channels The information ory shows that with M transmit antennas and L = M receive antennas, M independent

the-data streams can be simultaneously transmitted, hence, reaching the channel capacity As

an example, the BLAST (Bell-Labs Layered Space Time) architecture can be referred

to [11][20] Another approach is to use a MISO scheme to obtain diversity, where inthis case sophisticated techniques such as space–time coding (STC) can be realized.All transmit signals occupy the same bandwidth, but they are constructed such that thereceiver can exploit spatial diversity, as in the Alamouti scheme [1] The main advan-tage of STCs especially for mobile communications is that they do not require multiplereceive antennas

pos-Two basic variants of BLAST are proposed [11][20]: D-BLAST (diagonal BLAST) andV-BLAST (vertical BLAST) The only difference is that in V-BLAST transmit antenna

m corresponds all the time to the transmitted data stream m, where in D-BLAST the

assignment of the antenna to the transmitted data stream is hopped periodically If the

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236 Additional Techniques for Capacity and Flexibility Enhancement

.

Figure 6-2 V-BLAST transceiver

channel does not vary during transmission, in V-BLAST, the different data streams maysuffer from asymmetrical performance Furthermore, in general the BLAST performance

is limited due to the error propagation issued by the multistage decoding process

As it is illustrated in Figure 6-2, for detection of data stream 0, the signals transmittedfrom all other antennas are estimated and suppressed from the received signal of the datastream 0 In [2][3] an iterative decoding process for the BLAST architecture is proposed,which outperforms the classical approach

However, the main disadvantages of the BLAST architecture for mobile cations is the need of high numbers of receive antennas, which is not practical in asmall mobile terminal Furthermore, high system complexity may prohibit the large-scaleimplementation of such a scheme

communi-6.2.2 Space–Time Coding

An alternative approach is to obtain transmit diversity withM transmit antennas, where the

number of received antennas is not necessarily equal to the number of transmit antennas.Even with one receive antenna the system should work This approach is more suitablefor mobile communications

The basic philosophy with STC is different from the BLAST architecture Instead

of transmitting independent data streams, the same data stream is transmitted in anappropriate manner over all antennas This could be, for instance, a downlink mobilecommunication, where in the base station M transmit antennas are used while in the

terminal station only one or few antennas might be applied

The principle of STC is illustrated in Figure 6-3 The basic idea is to provide through

coding constructive superposition of the signals transmitted from different antennas.

Constructive combining can be achieved for instance by modulation diversity, where

pos-Two basic variants of BLAST are proposed [11][20]: D-BLAST (diagonal BLAST) andV-BLAST (vertical BLAST) The only difference... issued by the multistage decoding process

As it is illustrated in Figure 6-2, for detection of data stream 0, the signals transmittedfrom all other antennas are estimated and suppressed... techniques such as space–time coding (STC) can be realized.All transmit signals occupy the same bandwidth, but they are constructed such that thereceiver can exploit spatial diversity, as in the

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