Microwave Engineering Europe - October 2007 - (Page 34)

34 DIRECT SYNTHESIS Direct synthesis of UWB-WiMedia signal generation By Iqbal Bawa, K A Muralidharan, Tektronix and Joan Mercadé, Arbitrary Resources A s new applications utilizing wireless transmission and digital RF systems proliferate, engineers need better ways to create intricate RF signal behaviors and interactions. This paper discusses the challenges to generate frequency hopping Ultra-Wide Band (UWB) signals and the various options that RF design engineers have while creating UWB-WiMedia signals by use of an Arbitrary Waveform Generator (AWG). Although UWB promises high data rates it also involves high complexity to create these signals in the lab and to preserve the signal integrity. Three options that are discussed are IQ base band signal generation, IF signal generation and direct RF synthesis generation techniques. This article focuses on the test and measurement needs for direct generation of WiMedia based UWB signals. It discusses the different architectural possibilities for generation of wideband frequency hopped signals. To provide high data rates, the FCC in 2002 approved the unlicensed usage of UWB devices in the spectrum of 3.1 GHz to 10.6 GHz as long as the bandwidth of the signal is greater than 25 percent of the carrier frequency i.e., Fractional bandwidth = (FH - FL)/FC > 25 percent or the total BW > 1.5 GHz. three FFI. For the ﬁfth band group three FFI are deﬁned, thus taking the total number of channels to 53. The transmitter WiMedia RF signal is deﬁned as: where Re(-) represents the real part of the signal, TSYM is the symbol length, Npacket is the number of symbols in the packet, fc(m) is the centre frequency for the mth frequency band, q(n) is a function that maps the nth symbol to the appropriate frequency band, and sn(t) is the complex baseband signal representation for the nth symbol, which must satisfy the following property: sn(t) = 0 for t is not an element of [0, TSYM]. The exact structure of the nth symbol depends on its location within the packet. Channelization Unique logical channels are deﬁned by using up to ten different TFC codes for each band group. TFCs and the associated base sequences for band group 1 are shown in Table 1. The symbolic representation of TFC # 1 with 3 bands can be shown as below. Both of these methods have their own advantages & disadvantages. In the ﬁrst method, although it gives all the advantages of an arbitrary waveform generator such as creating real world signals which include distortions and impairments, frequency hopping at RF, which is mandatory as per Wimedia speciﬁcations, it involves the use of costly and complex arrangements such as external frequency hoppers. The second method provides the ability to band hop signals, but it doesn’t offer the advantages of AWG-like ﬂexibility and maneuverability. The challenge here is to have the best of both worlds. The scope of this paper is devoted to determining the best way to do this using arbitrary waveform generation techniques. Implementation The synthesis of signals with bandwidths exceeding 1.5 GHz at carrier frequencies up to 10 GHz exceeds the capability of traditional vector signal generation One approach to this is UWB-WiMedia which uses a multiband OFDM technique. The WiMedia speciﬁcation divides the UWB frequency spectrum into six band groups with ﬁve (1 to 4 and the 6th) band groups consisting of three bands and the ﬁfth band group of two bands. Each of the bands has a bandwidth of 528 MHz. The physical layer uses OFDM technology with 122 tones in each of the 528 MHz bands. The OFDM packets are then spread using a time-frequency code (TFC). Two types of spreading are deﬁned: one uses frequency hopping over the three bands and is referred to as Time-Frequency Interleaving (TFI) and the other is transmitted in a single band and is referred to as Fixed Frequency Interleaving (FFI). For the ﬁve band groups (1 to 4 and the 6th) ten TFCs are deﬁned, with seven TFI and Challenges It is a challenge to generate WiMedia signals to test the various devices, not only for conformance but also for the purposes of establishing operating margin. Currently there are two ways to generate WiMedia signals Table 1. Microwave Engineering Europe ● October 2007 ● www.mwee.com 034-036-038-040-042-044_MWEE.ind34 34 20/09/07 17:33:48 http://www.mwee.com

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Microwave Engineering Europe - October 2007 Contents Comment News CMOS RF: Si-On-Sapphire Goes Mainstream Cover Feature: New Data Protection Concept for UHF RFID Tags CMOS RF: RF Design Team Touts CMOS Spin for 3G PAs Wireless HID – Are You Following the Standard to Another “Average” Product Development? Phase Optimisation of the RF Front-End Direct Synthesis of UWB-WiMedia Signal Generation 4G Chips to Target 700 MHz Applications Femtocells Mobilize to Fight Wi-Fi in the Home Products Product Feature: AXIEM Pioneers the Future of EM Technology Calendar

Microwave Engineering Europe - October 2007

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