Advanced Wireless Networks - 4G Technologies

28	PHYSICAL LAYER AND MULTIPLE ACCESS
	Table 2.3 Parameters of UWC-136 and EDGE signal
			GSM radio interface
Key characteristic		TIA UWC-136	(for reference only)
Multiple access		TDMA	TDMA
Band width		30/200/1600 kHz	200 kHz
Bit rate		48.6 kb/s	270.8kb/s
		72.9 kb/s	for EDGE 812.5 kb/s
		270.8 kb/s
		361.1 kb/s
		722.2 kb/s
		2.6 Mb/s
		5.2 Mb/s
Duplexing		FDD/TDD	FDD
Carrier spacing		30/200/1600 kHz	200 kHz
Inter BS timing		Asynchronous	Asynchronous
		(synchronization possible)	(synchronization
			possible)
Inter-cell synchronization		Not required	Not required
Base station synchronization		Not required	Not required
Cell search scheme		L1 power-based, L2	L1 power-based,
		parameter-based,	L2 parameter-based,
		L3 service/network/	L3 service/network/
		operator-based	operator-based
Frame length		40/40/4.6/4.6 ms	4.6 ms
HO		HHO	HHO
DL	Data modulation	π/4 DPSK	GMSK
		π/4 coherent	8 PSK
		QPSK
		8 PSK
		GMSK
		Q-O-QAM
		B-O-QAM
DL	Power control	Per slot and per carrier	Per slot
DL	Variable rate	Slot aggregation	Slot aggregation
	accommodation
UL	Data modulation	π/4 DPSK	GMSK
		π/4 coherent QPSK	8 PSK
		8 PSK
		GMSK
		Q-O-QAM
		B-O-QAM
UL	Power control	Per slot and per carrier	BS-directed MS power
			control

(Continued )

	CODE DIVISION MULTIPLE ACCESS		29
	Table 2.3 (Continued )
		GSM radio interface
Key characteristic	TIA UWC-136	(for reference only)
UL Variable rate	Slot aggregation	Slot aggregation
accommodation
Channel coding	Punctured convolutional code	Convolutional coding
	(R = 1/2, 2/3, 3/4, 1/1)	Rate dependent on
	Soft or hard decision coding	service
Interleaving periods	0/20/40/140/240 ms	Dependent on service
Rate detection	Via L3 signaling	Via stealing flags
Other features	Space and frequency	MRC
	diversity; MRC/
	‘MRC-like’
	Support for hierarchical
	structures
Random access mechanism	Random access with shared	Random
	control feedback (SCF),
	also reserved access
Power control steps	4 dB	2 dB
Super frame length	720/640 ms (hyperframe is	720 ms
	1280 ms)
Slots/frame	6 per 30 kHz carrier 8 per	8
	200 kHz carrier 16–64 per
	1.6 MHz carrier
Focus of backward	AMPS/IS54/136/GSM	GSM
compatibility

HHO, hard handoff; DL, downlink; UL, uplink.

Despreading, represented by operator D( ), is performed if we use ε( ) once again and bandpass filtering, with the bandwidth proportional to 2/ Tm represented by operator BPF( ), resulting in

D(Sw) = BPF[ε(Sw)] = BPF(cc b cos ωt) = BPF(c2 b cos ωt) = b cos ωt

(2.3)

The baseband equivalent of Equation (2.3) is

D Swb = LPF ε Swb = LPF[c(t, Tc)c(t, Tc)b(t, Tm )] = LPF[b(t, Tm )] = b(t, Tm ) (2.3a)

where LPF( ) stands for low pass filtering. This approximates the operation of correlating the input signal with the locally generated replica of the code Cor(c, Sw). Nonsynchronized despreading would result in

Dτ ( ); Cor(cτ , Sw) = BPF[ετ (Sw)] = BPF(cτ cb cos ωt) = ρ(τ )b cos ωt

(2.4)

30 PHYSICAL LAYER AND MULTIPLE ACCESS

In Equation (2.4) BPF would average out the signal envelope cτ c, resulting in E(cτ c) = ρ(τ ). The baseband equivalent of Equation (2.4) is

	Tm			Tm
Dτ ( ); Cor cτ , Swb	=		cτ Swb dt = b(t, Tm ) cτ c dt = bρ(τ )		(2.4a)
	0			0
				=
This operation extracts the useful signal b as long as τ 0, otherwise the signal will be
=		≥		Tc. Separation of multipath components in a RAKE
suppressed because ρ(τ ) 0 for τ

receiver is based on this effect. In other words, if the received signal consists of two delayed replicas of the form

					r = Swb (t) + Swb (t − τ )
	the despreading process defined by Equation (2.4a) would result in
				Tm	Tm
	Dτ ( );	Cor(c, r) =			cr dt = b(t, Tm ) c(c + cτ ) dt = bρ(0) + bρ(τ )
	=		≥	0	0
		0 for τ		Tc, all multipath components reaching the receiver with a delay
	Now, if ρ(τ )

larger then the chip interval will be suppressed. If the signal transmitted by user y is despread in receiver x the result is

Dx y( ); BPF[εx y (Sw )] = BPF(cx cy by cos ωt) = ρx y (t)by cos ωt

(2.5)

So in order to suppress the signals belonging to other users (multiple access interference, MAI), the cross-correlation functions should be low. In other words if the received signal consists of the useful signal plus the interfering signal from the other user:

r = Swb x (t) + Swb y (t) = bx cx + by cy			(2.6)
the despreading process at receiver of user x would produce
Tm	Tm	Tm
Dx y ( ); Cor(cx , r) = cx r dt = bx	cx cx dt + by	cx cy dt = bx ρx (0) + by ρx y (0)
0	0	0	(2.7)
	=
			=

When the system is properly synchronized ρx (0) 1 , and if ρx y (0) 0 the second compo-

nent representing MAI will be suppressed. This simple principle is elaborated in WCDMA standard, resulting in a collection of transport and control channels. The system is based on 3.84 Mcips rate and up to 2 Mb/s data rate. In a special downlink high data rate shared channel the data rate and signal format are adaptive. There will be mandatory support for QPSK and 16 QAM and optional support for 64 QAM based on UE capability, which will proportionally increase the data rate. For details see www.3gpp.com. CDMA is discussed in detail in Glisic [54, 55].

2.3 ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

In wireless communications, the channel imposes the limit on data rates in the system. One way to increase the overall data rate is to split the data stream into a number of

ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING				31
		cos w1t
Gate	Filter	Mod
Gate	Filter	Mod
		sin w1t
		cos w2t
Gate	Filter	Mod	Add
Gate	Filter	Mod
Clock Clock		sin w2t

Figure 2.3 An early version of OFDM.

f

Figure 2.4 Spectrum overlap in OFDM.

parallel channels and use different subcarriers for each channel. The concept is presented in Figure 2.3 and represents the basic idea of the orthogonal frequency division multiplexing (OFDM) system. The overall signal can be represented as

x(t)	=	N −1	D	e j2π (n/N ) fst	;	k1		< t <	N + k2	(2.8)
						− fs			fs
		n=0	n

In other words complex data symbols [D0, D1, . . . , DN −1] are mapped in OFDM symbols [d0, d1, . . . , dN −1] such that

dk =	N −1
	Dn e j2π (kn/N )	(2.9)
	n=0

32 PHYSICAL LAYER AND MULTIPLE ACCESS

Data in Block into N complex numbers

Rate 1/T

Channel

IFFT Filter

Rate N/T

Channel

Filter Sample Block

Synch	FFT	Equalize	Data out
			Unblock

Figure 2.5 Basic OFDM system.

The output of the FFT block at the receiver produces data per channel. This can be represented as

D˜ m =		1		N −1
				rk e− j2π m(k/2N )
			N
				k=0
rk =		N −1
				Hn Dn e j2π (n/2N )k + n (k)		(2.10)
		n=0
D˜ m	=		Hn Dn + N (n), n = m
			N (n) , n		=
					m

The system block diagram is given in Figure 2.6. In order to eliminate residual intersymbol interference, a guard interval after each symbol is used as shown in Figure 2.7. An example of OFDM signal specified by IEEE 802.11a standard is shown in Figure 2.8. The signal parameters are: 64 points FFT, 48 data subcarriers, four pilots, 12 virtual subcarriers, DC component 0, guard interval 800 ns. Discussion on OFDM and an extensive list of references on the topic are included in Glisic [54].

2.4 MULTICARRIER CDMA

Good performance and flexibility to accommodate multimedia traffic are incorporated in multicarrier (MC) CDMA, which are obtained by combining CDMA and OFDM signal formats. Figure 2.9 shows the DS-CDMA transmitter of the jth user for binary phase shift keying/coherent detection (CBPSK) scheme and the power spectrum of the transmitted signal, respectively, where GDS = Tm / Tc denotes the processing gain and

C j (t) = [C1j C2j · · · CGj DS ] the spreading code of the jth user. Figure 2.10 shows the MCCDMA transmitter of the jth user for CBPSK scheme and the power spectrum of the

transmitted signal, respectively, where GMC denotes the processing gain, NC the number of subcarriers, and C j (t) = [C1j C2j · · · CGj MC ] the spreading code of the jth user. The MCCDMA scheme is discussed assuming that the number of subcarriers and the processing gain are the same.

However, we do not have to choose NC = GMC, and actually, if the original symbol rate is high enough to become subject to frequency selective fading, the signal needs to be first S/P-converted before spreading over the frequency domain. This is because it is

					MULTICARRIER CDMA		33
Data		Map	Block into
in			N	Rate N/T cos wct
			complex
			Rate	Real	X
			1/T		+	BPF
			IFFT
				Imaginary
					X
					sin wct
			Transmitter
			cos wc t
			X		Block
			Sample
	BPF
			rate	N/T
			X		FFT	Demap
					Rate 1/T

sin wc t

Sync

Receiver

Figure 2.6 System with complex transmission.

1/T

Guard interval

f

Figure 2.7 OFDM time and frequency span.

crucial for the multicarrier transmission to have frequency nonselective fading over each subcarrier.

Figure 2.11 shows the modification to ensure frequency nonselective fading, where TS denotes the original symbol duration, and the original data sequence is first converted into P parallel sequences, and then each sequence is mapped onto GMC subcarriers (NC =

P × GMC).

34 PHYSICAL LAYER AND MULTIPLE ACCESS

Figure 2.8 802.11a/HIPERLAN OFDM.

C1j	C3j		f0	Frequency
		Power spectrum of transmitted signal
		Time
Time	j	C j(t) COS(2πfot)	C j(t)
		j
	C2	CG	Scanning	j
Data		DS		C (t) Path gain 1
			correlator
stream
			Tc	LPF
			2Tc	LPF
				Combiner
			Path selector	C j(t) Path gain 2
			GDST	LPF
			Rake receiver	Path gain GDS

Figure 2.9 DS-CDMA scheme.

j

j		j
		C1
	Fr	j
j	eq	j
CGMC	ue	C2
	ncy
Data
stream		Copier
a j		NC=GM
	Time
		a j
		j
		CGMC
		Time

f1 f2 f3	Frequency
cos(2πf 1t)	cos(2πf 1t)	q j
		1
		LP
cos(2πf 2t)	cos(2πf 2t)	q j
Σ		2	Σ
		LP
			D j
		LP
cos(2πfGMC t)	cos(2πfG MC t)	j
		CGMC

Figure 2.10 MC-CDMA scheme.

		MULTICARRIER CDMA	35
		C1j
		cos(2πf1t)
	a1j
Data	Serial/parallel	Σ
stream	converter
	converter
	1:P	CGj MC cos[2πf1+(GMC-1)/TS)]
	a j
	p
1	2

Frequency

NC = P×GMC

Figure 2.11 Modification of MC-CDMA scheme: spectrum of its transmitted signal.

The multicarrier DS-CDMA transmitter spreads the S/P-converted data streams using a given spreading code in the time domain so that the resulting spectrum of each subcarrier can satisfy the orthogonality condition with the minimum frequency separation. This scheme was originally proposed for an uplink communication channel, because the introduction of OFDM signaling into the DS-CDMA scheme is effective for the establishment of a quasi-synchronous channel.

Figure 2.12 shows the multicarrier DS-CDMA transmitter of the jth user and the power spectrum of the transmitted signal, respectively, where GMD denotes the processing gain, NC the number of subcarriers, and C j (t) = [C1j C2j · · · CGj MD ] the spreading code of the jth user.

The multitone MT-CDMA transmitter spreads the S/P-converted data streams using a given spreading code in the time domain so that the spectrum of each subcarrier prior to spreading operation can satisfy the orthogonality condition with the minimum frequency separation. Therefore, the resulting spectrum of each subcarrier no longer satisfies the orthogonality condition. The MT-CDMA scheme uses longer spreading codes in proportion to the number of subcarriers, as compared with a normal (single carrier) DS-CDMA scheme; therefore, the system can accommodate more users than the DS-CDMA scheme.

Figure 2.13 shows the MT-CDMA transmitter of the jth user for the CBPSK scheme and the power spectrum of the transmitted signal, respectively, where GMT denotes the processing gain, NC the number of subcarriers, and C j (t) = [C1j C2j · · · CGj MT ] the spreading code of the jth user. All these schemes will be discussed in details in Glisic [54].

36 PHYSICAL LAYER AND MULTIPLE ACCESS

C1j	C3j	Power spectrum of trasmitted

	Time	j			j
				cos(2πf1t)	C (t)
	CGjMD	C (t)	cos(2πf1t)
C2j

		j	cos(2πf 2t)
		C (t)
	Data	Serial-	Σ
		to-
	stream
		parallel
		converter
	Time
		C j(t)	cos(2πf NCt)
		Time

	LP
cos(2πf 2t) C j(t)
	LP	Parallel
		-to-serial
		converter
	LP
cos(2πfNCt)	j
	C (t)

Figure 2.12 Multicarrier DS-CDMA scheme.

C1j	C3j C5j	C7j	f1 f 2 f 3 f4 • • • f NC	Frequency

			Time C j(t)	cos(2πf 1t)
j	j	j	CGj MT = CGjDS ×NC
C2	C4	C6
			C j(t)	cos(2πf 2t)
			Serial-
	Data		-to-	Σ
	stream		parallel
			converter
		Time
			C j(t)	cos(2π f NCt)
			Time

cos(2πf 1t)

Rake

cos(2πf 2t)

cos(2π f NCt)

combiner 1

Rake combiner 2

Rake combiner NC

Parallel -to-serial converter

Figure 2.13 MT-CDMA scheme.

2.5 ULTRAWIDE BAND SIGNAL

For the multipath resolution in indoor environments a chip interval of the order of few nanoseconds is needed. This results in a spread spectrum signal with the bandwidth in the order of few GHz. Such a signal can also be used with no carrier, resulting in what is called impulse radio (IR) or ultrawide band (UWB) radio. A Typical form of the signal used in this case is shown in Figure 2.14. A collection of pulses received on different locations within the indoor environment is shown in Figure 2.16. UWB radio is discussed in detail in Glisic [54]. In this section we will define only a possible signal format.

Received monocycle w (t ) rec

1

0.8

0.6

0.4

0.2

–0.2

–0.4

0.1

0.2

0.3

0.4

0.5

0.6

t (ns)

Figure 2.14 A typical ideal received monocycle ωrec (t) at the output of the antenna subsystem as a function of time in nanoseconds.

Amplitude

(a)

	1.5
	1
	0.5
	0
	–0.5	Zeroth order
		First order
		Second order
	–1	Third order
		–4	–3	–2	–1	0	1	2	3	4	5
	–5

Time (ns)

Amplitude

(b)

0.8

0.6

0.4

0.2
0
–0.2
–0.4
–0.6
–0.8	Zeroth order
	First order
–1	Second order
–1.2–5	Third order
	–4	–3	–2	–1	0	1	2	3	4	5

Time (ns)

Amplitude

Derivative

(c)			(d)
3	order 0		3	order 0
	1			1
2	2		2	2
	3			3
1		Amplitude	1
0			0
−1			−1
−2			−2
−30	0.1 0.2 0.3 0.4 0.5 0.6 0.7		−30	0.1 0.2 0.3 0.4 0.5 0.6 0.7
	Time (ns)			Time (ns)

Derivative

Figure 2.15 Modified Hermite pulse with Gram-Schimidt orthogonalization. (a) Generated pulses; (b) transmit pulses.