Advanced Wireless Networks - 4G Technologies

38 PHYSICAL LAYER AND MULTIPLE ACCESS

AP17F3

1

0.8

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

–0.8
–10	–8	–6	–4	–2	0	2	4	6	8	10
				Time	(nanoseconds)

Figure 2.16 A collection of received pulses in different locations. (Reproduced by permission of IEEE [53].)

Figure 2.17 A collection of channel delay profiles. (Reproduced by permission of IEEE [52].)

A typical time-hopping format used in this case can be represented as

str(k) t(k) =	∞
	ωtr t(k) − j Tf − c(jk) Tc − δd[(kj/) Ns]	(2.11)
	j=−∞

where t(k) is the kth transmitter’s clock time and Tf is the pulse repetition time. The transmitted pulse waveform ωtr is referred to as a monocycle. To eliminate collisions due to multiple access, each user (indexed by k) is assigned a distinctive time-shift pattern {c(jk)} called a time-hopping sequence. This provides an additional time shift of c(jk) Tc seconds to the jth monocycle in the pulse train, where Tc is the duration of addressable time delay bins. For a fixed Tf the symbol rate Rs determines the number Ns of monocycles that

ULTRAWIDE BAND SIGNAL

39

are modulated by a given binary symbol as Rs = (1/Ns Tf) s−1. The modulation index δ is chosen to optimize performance. For performance prediction purposes, most of the time the data sequence {d(jk)}∞j=−∞ is modeled as a wide-sense stationary random process composed of equally likely symbols. For data a pulse position data modulation is used.

When K users are active in the multiple-access system, the composite received signal at the output of the receiver’s antenna is modeled as

r (t) =	K
	Ak srec(k) (t − τk ) + n (t)	(2.12)
	k=1

The antenna/propagation system modifies the shape of the transmitted monocycle ωtr (t) to ωrec (t) on its output. An idealized received monocycle shape ωrec (t) for a free-space channel model with no fading is shown in Figure 2.14.

The optimum receiver for a single bit of a binary modulated impulse radio signal in additive white Gaussian noise (AWGN) is a correlation receiver

‘decide d0(1) = 0’ i f
		Pulse correlator output = α j				(u)
Ns−1
		τ1+( j+1)Tf	t − τ1 − j Tf − c(j1) Tc dt > 0					(2.13)
j=0		r (u, t) υ
		τ1+ j Tf

Test statistic α(u)

=

where υ (t) = ωrec (t) − ωrec (t − δ).

The spectra of a signal using TH are shown in Figure 2.18. If instead of TH a DS signal is used, the signal spectra is shown in Figure 2.19(a) for pseudorandom code and Figure 2.19(b) for a random code. The FCC (Frequency Control Committee) mask for indoor communications is shown in Figure 2.18. Possible options for UWB signal spectra are given in Figures 2.21 and 2.22 for single band and Figure 2.23 for multiband signal format. For more details see www.uwb.org and www.uwbmultiband.org.

	10
Power	0				Random TH
(dB)	-10
	-20
	-30
	-40
	-50
	-60
	-700	2	4	6	8	10	12

Figure 2.18 Spectra of a TH signal.

40 PHYSICAL LAYER AND MULTIPLE ACCESS

	10
	0					Pseudo
	-10					random DS
Power
(dB)	-20
	-30
	-40
	-50
	-60
	-70	0	2	4	6	8	10	12

Frequency (GHz)

	10
	0				Random
	-10				DS
Power	-20
(dB)	-30
	-40
	-50
	-60
	-70 0	2	4	6	8	10	12

Frequency (GHz)

Figure 2.19 Spectra of pseudorandom DS and random DS signal.

	-
	-
Power	-
(dBm)	-
							GPS band
	-
	-						802.11a
	-						802.11b
	-
	-	0	2	4	6	8	10	12

Frequency (GHz)

Figure 2.20 FCC frequency mask.

The optimal detection in a multiuser environment, with knowledge of all time-hopping sequences, leads to complex parallel receiver designs [2]. However, if the number of users is large and no such multiuser detector is feasible, then it is reasonable to approximate the combined effect of the other users’ dehopped interfering signals as a Gaussian random process. All details regarding the system performance can be found in Glisic [54].

MIMO CHANNELS AND SPACE TIME CODING

41

	-40
	-45		FCC mask for indoor
	-50
Power	-55
	-60
(dBm)
	-65
	-70
	-75
	-80			Gaussian pulse: 0.36 ns
				Gaussian pulse: 0.72 ns
	-85
	-90
	-95
	0	2	4	6	8	10	12

Frequency (GHz)

Figure 2.21 FCC mask and possible UWB signal spectra.

	-40
	-45	FCC		Possible spectra
Power		mask
(dBm)	-50
	-55
	-60
	-65
	-70
	-75
	-80 0	2	4	6	8	10	12

Frequency (GHz)

Figure 2.22 Single-band UWB signal.

2.6 MIMO CHANNELS AND SPACE TIME CODING

In order to increase the capacity, 4G networks use the previously described signal formats in MIMO (multiple input multiple output) channels combined with space-time coding. More details on these technologies can be found in Glisic [54].

42 PHYSICAL LAYER AND MULTIPLE ACCESS

-40

Power -45 (dBm)

-50

-55

1

2

3

4

5

6

7

8

9

10

11

12

Frequency (GHz)

Figure 2.23 Multiband UWB signal.

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3

Channel Modeling for 4G

3.1 MACROCELLULAR ENVIRONMENTS (1.8 GHz)

In this section we briefly present statistical properties of azimuth and delay spread in macrocellular environments. For more details see Glisic [1]. The analysis is based on data reported from a measurement campaign in typical urban, bad urban and suburban (SU) [3] areas. In the experiment a base station (BS) equipped with an eight-element uniform linear antenna array and a mobile station (MS) with an omnidirectional dipole antenna are used. The MS is equipped with a differential global positioning system (GPS) and an accurate position encoder so its location is accurately known by combining the information from these two devices. MS displacements of less than 1cm can, therefore, be detected. The system is designed for transmission from the MS to the BS. Simultaneous channel sounding is performed on all eight branches, which makes it possible to estimate the azimuth of the impinging waves at the BS. The sounding signal is a maximum length linear shift register sequence of length 127 chips, clocked at a chip-rate of 4.096 Mbs. This chip rate was initially used in WCDMA proposals in Europe.The testbed operates at a carrier frequency of 1.8 GHz. Additional information regarding the stand-alone testbed can be found in Pedersen et al. [2], Frederiksen et al. [3] and Algans et al. [4]. A summary of macrocellular measurement environments is given in Table 3.1.

Channel azimuth-delay spread function at the BS is modeled as

L
h (φ, τ ) = αl δ (φ − φl , τ − τl )	(3.1)
l=1

where the parameters αl , τl and φl are the complex amplitude, delay and incidence azimuth of the lth impinging wave at the BS. In general, h (φ, τ ) is considered to be a time-variant function, since the constellation of the impinging waves is likely to change as the MS moves

Advanced Wireless Networks: 4G Technologies Savo G. Glisic

C 2006 John Wiley & Sons, Ltd.