Part IV MULTIPLE ACCESS AND ADVANCED TRANSCEIVER SCHEMES 363
21.8 Underlay Hierarchical Access – Ultra Wide Bandwidth System Communications
In an underlay system, the secondary users have severe restrictions on the transmit PSD, so that the effect on a primary RX is a “minor” increase of the noise floor that the RX sees. Such low PSD can be achieved by keeping the transmit power low (which is feasible only if the secondary users communicate only over short distances) and/or by spreading the signal over a very large bandwidth. Only the part of the secondary signal that falls within the RX bandwidth of a primary user acts as interference. Underlay radios are actually not “cognitive” in the sense that they adapt their transmission parameters to the environment, but because of their use as secondary radios they are still often mentioned in the cognitive category.
Frequency Regulations and Transmit Power Constraints of UWB Signals
The underlay principle is realized in Ultra Wide Bandwidth system (UWB) communications, where signals have extremely large bandwidth. Such a large bandwidth offers the possibility of very large spreading factors: in other words, the ratio of the signal bandwidth to the symbol rate is very large. For a typical sensor network application with 5 ksymbol/s throughput, a spreading factor of 105to 106is achieved for transmission bandwidths of 500 MHz and 5 GHz, respectively.
Spreading over such a large bandwidth means that the PSD of the radiation, i.e., the power per unit bandwidth, can be made very low while still maintaining good SNR at the secondary RX.
A primary (narrowband) RX will only see the secondary-signal power within its own system bandwidth, i.e., a small part of the total secondary transmit power; see Figure 21.6. This implies that the interference to primary (narrowband) systems is small.
Power spectral density
5-GHz band
Limit on power spectral density 802.11a (20 MHz)
UWB (7.5 GHz)
Frequency 10.6 GHz
3.1 GHz
Figure 21.6 Interference between a UWB system and a narrowband (IEEE 802.11a) local area network.
Frequency regulators have defined UWB signals as signals with a minimum of 500 MHz absolute bandwidth.4 They stipulate that UWB can be used as unlicensed underlay systems as
4Signals are also defined as UWB if they have more than 20% relative bandwidth. However, for the subsequent discussion, we assume large absolute bandwidth.
Cognitive Radio 517
long as the PSD is limited to −41.3 dBm/MHz EIRP (Equivalent Isotropically Radiated Power;
see Chapter 3). This PSD is so low that it does not significantly disturb primary RXs that are more than approximately 10 m away from a UWB TX. Take, e.g., a signal with 6 GHz carrier frequency.
It suffers a free-space attenuation of 68 dB at a 10 m distance, resulting in a received PSD of approximately −109 dBm/MHz, which is comparable to the PSD of white noise. Consequently, even a primary RX operating at the thermal-noise sensitivity limit would hardly see an impact on its performance. RXs with nonideal noise figures, and/or impacted by co-channel interference, e.g., from neighboring cells, would not be affected even if the UWB TXs were somewhat closer to the primary RX. It is also noteworthy that transmission at−41 dBm/MHz is allowed only in certain frequency ranges, while others (e.g., the Global Positioning System, GPS band, and also most cellular bands) are more strongly protected.
In the U.S.A., the Federal Communications Commission (FCC) allows emission between 3.1 and 10.6 GHz. Limits for indoor and outdoor communication systems differ as shown in Figure 21.7a. For outdoor systems, UWB devices are required to operate without a fixed infrastructure. In Europe, the Radio Spectrum Committee (RSC) of the European Commission (EC) imposes a spectral mask shown in Figure 21.7b). Emission between 6 and 8.5 GHz with EIRP of−41.3 dBm/MHz is allowed for devices without some type of additional interference mitigation techniques (techniques similar to those of interweaving systems). The same limit is valid for the shaded frequency region (4.2–4.8 GHz) until the end of 2010. UWB systems with interference mitigation techniques or low duty cycle operation are allowed to transmit at −41.3 dBm/MHz in the 3.4–4.8 GHz band. In Japan, operation between 3.4 and 4.8 GHz is admissible as shown in Figure 21.7c), if the UWB TX uses interference mitigation. However, for 4.2 GHz through 4.8 GHz, interference mitigation techniques were not required until the end of December 2008.
Operation between 7.25 and 10.25 GHz is admissible also without special techniques.
The high spreading factor of UWB systems helps not only in mitigating interference to primary users but also enables a UWB RX to suppress narrowband (primary-user) interference by a factor that is approximately equal to the spreading factor. These principles are well understood from the general theory of spread-spectrum systems, see Chapter 18. The distinctive feature of UWB is that it goes to extremes in terms of the spreading factor. It must be kept in mind that the spreading factor is a function of both the transmission bandwidth and the data rate. Consequently, UWB systems with high data rates (>100 Mbit/s) exhibit a rather small spreading factor at and can thus be used to communicate over very short distances.
Methods of UWB Signal Generation
There are a number of different ways to spread signals to large bandwidths.
1. Frequency Hopping (FH): FH uses different carrier frequencies at different times. In slow FH, one or more symbols are transmitted on a given frequency; in fast FH, the frequency changes sev- eral times per symbol; see also Section 18.1. The bandwidth of the resulting signal is determined by the range of the oscillator, not the bandwidth of the original signal that is to be transmitted.
Implementation of an FH TX is fairly simple: it is just a conventional narrowband modulator fol- lowed by a mixer with the output of a frequency-agile oscillator. An FH RX can be constructed in a similar way; such a simple RX is efficient as long as the delay spread of the channel is shorter than the hopping time (otherwise, multipath energy is still arriving on one subcarrier while the RX has already hopped to a different frequency). Consequently, a hopping rate of approximately 1 MHz or less would be desirable. However, such slow FH can lead to significant interference to primary RXs, since – at a given time – a victim RX “sees” the full power of the UWB signal.
For this reason, FH for UWB has been explicitly prohibited by several frequency regulators.
2. Orthogonal Frequency Division Multiplexing (OFDM): in OFDM, the information is modulated onto a number of parallel subcarriers (in contrast to FH, where the carriers are used one after
Spectral mask in U.S.A
Spectral mask in Japan Spectral mask in Europe
–40 –45 –50 –55 –60 –65 EIRP emission level (dBm) –70
–75 –80
1.99
1.61 0.96
100
Frequency (GHz) 101
–40 –45 –50 –55 –60 –65 –70
EIRP emission level (dBm)
–75 –80 –85 –90 –95
100
Frequency (GHz)
101 3 1
10.6
–40 –45 –50 –55 –60 –65 –70
EIRP emission level (dBm)
–75 –80 –85 –90 –95
100
Frequency (GHz) 101 4.2
4.8 6.0
3.8
8.5
10.6
1.6 1.6
2.7
3.4 4.8 7.25 10.25 Outdoor limit Indoor limit Part 15 limit
Figure 21.7 Spectral masks for UWB transmission in different continents.
Cognitive Radio 519
the other); see Chapter 19. For this reason, OFDM has no innate spectral spreading. Rather, spreading can be achieved by low-rate coding, e.g., by a spreading code similar to CDMA, or by a low-rate convolutional code. The bandwidth of the resulting signal is determined by the employed code rate and the data rate of the original (source) signal. In modern implementations, the subcarriers are produced by a fast Fourier transformation; see Section 19.3. However, this implies that signal generation at the TX, as well as sampling and signal processing at the RX, has to be done at a rate that is equal to the employed bandwidth, i.e., at least 500 MHz.
3. Direct Sequence–Spread Spectrum (DS–SS): also known as CDMA, multiplies each bit of the transmit signal with a spreading sequence. The bandwidth of the overall signal is determined by the product of the bandwidth of the original signal and the spreading factor. At the RX, despreading is achieved by correlating the received signal with the spreading sequence; see Section 18.2. The key implementation challenge lies in the speed at which the RX has to sample and process (despread) the signal.
4. Time Hopping Impulse Radio (TH-IR): TH-IR represents each data symbol by a sequence of pulses with pseudorandom delays. The duration of the pulses essentially determines the width of the transmit spectrum; see also Section 18.5. The key implementation challenge lies in building coherent RXs that keep complexity low while still maintaining adequate performance.
Summarizing, we find that there is a strong duality between FH and TH-IR. FH sequentially hops in the frequency domain, while TH-IR hops in the time domain. Similarly, OFDM and DS–SS are dual, in that they perform low-rate coding operations in the frequency and time domains, respectively.
Further Advantages of UWB Transmission
In addition to the low interference to primary users, UWB signals also offer a number of other advantages.
1. A UWB RX can suppress narrowband interference by a factor that is approximately equal to the spreading factor.
2. A large absolute bandwidth can result in a high resilience to fading. First of all, a large absolute bandwidth allows to resolve a large number of (independently fading) Multi Path Components (MPCs), and thus a high degree of frequency diversity, i.e., the fading at sufficiently separated frequencies is independent. We can also give an alternative interpretation that is especially useful for TH-IR and CDMA systems. An RX with a large absolute bandwidth has a fine delay resolution, and can thus resolve many MPCs. The number of resolvable, and independently fading, MPCs can be up toτmax/B, whereτmax is the maximum excess delay of the channel andB is the system bandwidth. By separately processing the different MPCs, the RX can make sure that all those components add up in an optimum way, giving rise to a smaller probability of deep fades. As an additional effect, we have seen in Section 6.6 that in UWB, the number ofactual MPCs that constitute a resolvable MPC is rather small; for this reason, the fading statistics of each resolvable MPC does not have a complex Gaussian distribution anymore, but shows a lower probability of deep fades.
3. A large absolute bandwidth also leads to high accuracy of ranging and geolocation. Most ranging systems try to determine the flight time of the radiation between TX and RX. It follows from elementary Fourier considerations that the accuracy of the ranging improves the bandwidth of the ranging signal. Thus, even without sophisticated high-resolution algorithms for the determination of the time-of-arrival of the first path, a UWB system can achieve centimeter accuracy for ranging.
4. The large spreading factor and low PSD also provide increased protection against eavesdropping.
UWB Dynamic Spectrum Access
Despite the low interference created by a UWB underlay system, the residual interference to nearby victim RXs can still be too large. It is thus often necessary to combine UWB with a detect-and-avoid scheme. Such a strategy is potentially capable of enabling coexistence, compatibility, interference avoidance, and compliance with regulation – in particular European and Japanese frequency regulations that mandate Detect and Avoid (DAA) schemes for UWB TXs in some frequency ranges. The good ranging (and thus geolocation) capabilities of UWB nodes can help to determine whether nodes are close to potential victim nodes, in particular if the location of such victim nodes is kept in a database that can be accessed by the UWB nodes.
Further Reading
The field of cognitive radio is still very much in flux, and new research as well as new books are appearing on an almost monthly basis. At the time of this writing, the following papers can be especially recommended: The edited monographs [Hossain and Barghava 2008], [Xiao and Hu 2008] give a broad overview of all the aspects of cognitive radio, including spectrum sensing, spectrum allocation and management, OFDM-based implementations of cognitive systems, and UWB-based cognitive systems as well as protocol- and Medium Access Control (MAC) design.
Good concise overviews of these topics are also in Akyildiz et al. [2006], Haykin [2005], Zhao and Sadler [2007]. Spectral sensing is discussed in Quan et al. [2008]; spectrum opportunity tracking in Zhao et al. [2007]. Application of game theory to cognitive radio system is reviewed in Han et al.
[2007] as well as in Ji and Liu [2007]. Overlay systems are described in Jovocic and Visvanath [2007] as well as in Devroye and Tarokh [2007]. UWB communications and its many applications are described, e.g., in diBenedetto et al. [2006]; “detect-and-avoid” methods for UWB are described in Zhang et al. [2008].
For updates and errata for this chapter, see wides.usc.edu/teaching/textbook
22
Relaying, Multi-Hop, and
Cooperative Communications