Next generation wireless systems and networks phần 7 ppt

296 MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS As its name suggests, the system is based on OFDM, however, OFDMA is much more than just a physical layer solution. It is a cross-layer-optimized technology that exploits the unique physical properties of OFDM, enabling significant higher layer advantages that contribute to very efficient packet data transmission in a cellular network. Packet-switched air interface The telephone network, designed basically for voice, is an example of circuit-switched systems. Circuit-switched systems exist only at the physical layer that uses the channel resource to create an end-to-end bit pipe. They are conceptually simple as the bit pipe is a dedicated resource, and the pipe does not need to be controlled once it is created (some control may be required in setting up or tearing down the pipe). Circuit-switched systems, however, are very inefficient for burst data traffic. Packet- switched systems, on the other hand, are very efficient for data traffic but require that the upper layers be controlled in addition to the physical layer that creates the bit pipe. The MAC layer is required for the many data users to share the bit pipe. The data link layer is needed to take the error-prone pipe and create a reliable link for the network layers to pass packet data flows over. The Internet is the best example of a packet-switched network. Because all conventional cellular wireless systems, including 3G, were fundamentally designed for circuit-switched voice, they were designed and optimized pri- marily at the physical layer. Some people suggested that the choice of CDMA as the physical layer multiple access technology was also dictated by voice requirements. OFDMA, on the other hand, is a packet-switched scheme designed for data and is optimized across the physical, MAC, data link, and network layers. The choice of OFDM as the multiple access technology is based not only on physical layer consideration, but also on the MAC layer, data link layer, and network layer requirements. Physical layer advantages: OFDMA As discussed earlier, most of the physical layer advantages of OFDM are well understood. Most notably, OFDM creates a robust multiple access technology to deal with the impairments of the wireless channel, such as multipath fading, delay spread, and Doppler shifts. Advanced OFDM-based data systems typically divide the available spectrum into a number of equally spaced tones. For each OFDM symbol duration, information carrying symbols (based on modulation such as QPSK, QAM, etc.) are loaded on each tone. The OFDMA can also use fast hopping across all tones in a predetermined pseudorandom pattern, making it an SS technology. With fast hopping, a user that is assigned one tone does not transmit every symbol on the same tone, but uses a hopping pattern to jump to a different tone for every symbol. Different BSs use different hopping patterns, and each uses the entire available spectrum (thus to realize frequency reuse of 1). In cellular deployment, this adds to the advantages of CDMA systems, including frequency diversity and out of cell (intercell) interference averaging spectral efficiency benefit that narrowband systems such as conventional TDMA do not have. As discussed earlier, different users within the same cell use different resources (tones) and hence do not interfere with each other. This is similar to TDMA, where different users in a cell transmit at different time slots and do not interfere with one another. In contrast, CDMA users in a cell do interfere with each other, increasing the total interference in the system. OFDMA therefore has the physical layer benefits of both CDMA and TDMA and is at least three times (3times) more efficient than CDMA. In other words, at the physical layer, OFDMA creates the biggest pipe of all cellular technologies. Even though the 3times advantage at the physical layer is a huge advantage, the most significant advantage of OFDMA for data is at the MAC and link layers. MAC and link layer advantages OFDMA exploits the granular nature of resources in OFDM to come up with extremely efficient control layers. In OFDM, when designed appropriately, it is possible to send a very small amount MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS 297 (as little as one bit) of information from the transmitter to the receiver with virtually no overhead. Therefore, a transmitter that is previously not transmitting can start transmitting as little as one bit of information, and then stop, without causing any resource overhead. This is unlike CDMA or TDMA, in which the granularity is much coarser, and merely initiating a transmission wastes a significant resource. Hence, in TDMA, for example, there is a frame structure, and whenever a transmission is initiated, a minimum of one frame (a few hundred bits) of information is transmitted. The frame structure does not cause any significant inefficiency in user data transmission, as data traffic typically consists of a large number of bits. However, for the transmission of control-layer information, the frame structure is extremely inefficient, as the control information typically consists of one or two bits but requires a whole frame. Not having a granular technology can therefore be very detrimental from a MAC layer and link layer point of view. OFDMA takes advantage of the granularity of OFDM in its control-layer design, enabling the MAC layer to perform efficient packet switching over the air and at the same time provide all the hooks to handle QoS. It also supports a data link layer that uses local (as opposed to end-to-end) feedback to create a very reliable link from an unreliable wireless channel, with very low delays. The network layer’s traffic therefore experiences small delays and no significant delay jitter. Hence, interactive applications such as (packet) voice can be supported. Moreover, Internet protocols such as TCP/IP run smoothly and efficiently over an OFDMA air link. As discussed in Chapter 3, TCP/IP performance on 3G networks is very inefficient because the data link layer introduces significant delay jitter so that channel errors are misinterpreted by TCP as network congestion and TCP responds by backing off to the lowest rate. Packet switching leads to efficient statistical multiplexing of data users and helps the wireless operators to support a much greater number of users for a given user experience. This desirable feature in OFDMA, together with QoS support and a three times bigger pipe, allows the operators to profitably scale their wireless networks to meet the burgeoning data traffic demand in an all-you-can-eat pricing environment. 7.6 Ultra-Wideband Technologies As mentioned in Section 2.2.3, the UWB technology can be viewed as a derivative from the spreading spectrum technology, in particular, the time hopping spread spectrum (THSS) technique, which is also considered as a multiple access technology, being particularly suited for extreme narrow pulse transmissions. Before discussing the technical details about the UWB technologies, we would like to review briefly the history as well as the recent research activities carried out in this area. Since the introduction of UWB technology to commercial applications in the early 1990s [674], much of its initial research has been focused on the application of the THSS [675], where sev- eral pulses in each symbol duration are sent with a particular time offset pattern determined by a unique signature code for multiple access. The implementation of a THSS-UWB system requires a precise network-wise synchronization clock. This inevitably increases overall hardware complexity at a transceiver, which used to be a major concern in realizing a feasible UWB system at its early stage. On the other hand, DS techniques can also work jointly with UWB systems to provide multiple access among different users within the same wireless personal area network (WPAN). The operation of a DS-UWB system does not need an accurate synchronization clock and the use of antipodal pulses in DS modulation can boost up effective transmission power, which is very important to improve the detection efficiency of a UWB receiver, due to the severe emission constraints imposed on the power spectral mask specified in the FCC Part15.209, in which the maximal transmitting power for a UWB transmitter should be lower than −41.3 dBm within the bandwidth from 3.1 to 10.6 GHz. The UWB technologies have been standardized in IEEE 802.15.3a as a technology for WPANs. Figure 7.18 shows all IEEE 802 standards, including those for WLANs as IEEE 802.11 standards, 298 MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS Figure 7.18 Various IEEE 802 standards, in which UWB technologies have been covered in IEEE 802.15.3a standard for WPAN applications. wireless metropolitan area networks (WMANs) as IEEE 802.16 standards, WPANs as IEEE 802.15 standards, and so on. It is noted that IEEE 802.15.4.a is emerging as the standard for low-data-rate transmission. The FCC issued a notice of inquiry (NOI) in September 1998 and within a year the Time Domain Corporation, US Radar, and Zircon Corporation had received waivers from the FCC to allow limited deployment of a small number of UWB devices to support continued development of the technology, and the University of Southern California (USC) UltRa Lab had an experimental licence to study UWB radio transmissions. A notice of proposed rule making (NPRM) was issued in May 2000. In April 2002, after extensive commentary from the industry, the FCC issued its first report and order on UWB technology, thereby providing regulations to support deployment of UWB radio systems. This FCC action was a major change in the approach to the regulation of RF emissions, allowing a significant portion of the RF spectrum, originally allocated in many smaller bands exclusively for specific uses, to be effectively shared with low-power UWB radios. The FCC regulations classify UWB applications into several categories (see Table 7.5) with differ- ent emission regulations in each case. Maximum emissions in the prescribed bands are at an effective Table 7.5 The application categories specified by FCC UWB regulations Application Frequency band for operation User limitations at Part 1 limit Communications and measurement systems 3.1 to 10.6 GHz (different out-of-band emission limits for indoor and outdoor devices) No Imaging: ground penetrating radar, wall, medical imaging <960 MHz or 3.1 to 10.6 GHz Yes Imaging: through wall <960 MHz or 1.99 to 10.6 GHz Yes Imaging: surveillance 1.99 to 10.6 GHz Yes Vehicular 24 to 29 GHz No MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS 299 −40 −45 −50 −55 −60 −65 −70 −75 0.96 1.61 1.99 3.1 10.6 GPS 10 0 10 1 Frequency in GHz UMB EIRP Emission Level in dBm −40 −45 −50 −55 −60 −65 −70 −75 0.96 1.61 1.99 3.1 10.6 GPS 10 0 10 1 Frequency in GHz UMB EIRP Emission Level in dBm Indoor Limit Part 15 Limit Outdoor Limit Part 15 Limit Figure 7.19 FCC regulated spectral masks regarding the indoor and outdoor UWB communications applications. Figure 7.20 Other communications applications in the vicinity of UWB operating bands. isotropic radiated power (EIRP) of −41.3 dBm per MHz, and the −10 dB level of the emissions must fall within the prescribed band, as shown in Figure 7.19, which should be compared with Figure 7.20 to know other communication applications in the vicinity of the UWB operating bands. 7.6.1 Major UWB Technologies There are four major UWB technologies that have been proposed in the literature. The first type is Time Hopped (TH) UWB or Time-modulated (TM) UWB, 1 which is a traditional UWB scheme 1 The traditional impulse radio technology can be called as either time hopped (TH) UWB or time modulated (TM) UWB. It should be noted that both names are used very often. 300 MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS and is often called impulse radio (IR) UWB. The TH-UWB is by far the earliest version of UWB technology and remains an important solution even today. The TH-UWB can be further divided into two subcategories, that is, analog impulse radio multiple access (AIRMA) and digital impulse radio multiple access (DIRMA), which were suggested and studied in [613, 624, 637]. The second UWB technology is called direct-sequence CDMA-based UWB and can be implemented with a multi-carrier CDMA architecture. The DS-CDMA UWB scheme will be discussed in detail in Subsections 7.6.1, 7.6.2, 7.6.3, 7.6.4, and 7.6.5. Another UWB scheme that has gained much popularity is based on OFDM technology, namely, OFDM-UWB, which can be implemented on a multiband (MB) OFDM scheme. The MB-OFDM UWB technology is particularly useful when cognitive radio technology is used, as discussed in Chapter 9. In addition, some people also proposed frequency-modulation (FM) based UWB systems, which can be implemented by swept frequency technology. Figure 7.21 shows a family tree for all possible UWB technologies that have been proposed so far. Because of limited space, we will only focus on the discussions on TH-UWB (or TM-UWB) and DS-CDMA UWB in this subsection. TH-UWB technology The basic concept of a TH-UWB system is shown in Figure 7.22, where the system consists of four major parts, namely, modulator 2 , delay unit, transmission time controller, and a pseudorandom sequences generator. Obviously, in such a TH-UWB system, the data is sent in bursts and transmission time is controlled by the pseudorandom sequences generator. Understandably, the bandwidth of such a TM-UWB system is determined by the width and shape of impulses, which usually takes some special waveforms, such as “monocycle.” The design of the monocycles suitable for IR applications is a very interesting research topic in that the shape of the monocycles should provide a very good time ACF for a better detection efficiency and fit FCC spectral mask as illustrated in Figure 7.19. There are many pulse waveforms that have been proposed, such as Gaussian pulse and its derivative functions, Hermite pulse and its modified versions, prolate spheroidal waveforms, Laplacian monocycle, cubic monocycle, wavelets, and so on. For more information on these popular impulses suitable for UWB applications, please refer to the large number of references given at the end of this book [604–691]. Figure 7.21 Family tree for various UWB technologies proposed so far. 2 The most commonly used modulator scheme in an IR (or time hopping UWB) is pulse position modulation (PPM), although many other modulation schemes can also be used, such as pulse amplitude modulation (PAM), on-off-keying (OOK), pulse shape modulation (PSM), and so on. MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS 301 Figure 7.22 Block diagram for a TH-UWB IR transmitter. The data signal should be sent out from an IR system, as shown in Figure 7.22, using carrier- less transmission. The base band signal can be converted directly from the received signal and no intermediate frequency unit is required, thus reducing the implementation complexity. The TM-UWB scheme can provide a relatively large PG due to the fact that it has a very narrow impulse (whose width is of the order of a nanosecond). This large PG also entails several other operational advantages, which are explained as follows. First of all, it offers an excellent multipath immunity because of its very high so time resolution that almost all multipath components can be separated and combined coherently at a receiver. If the time between two pulses is longer than the channel delay spread, there will be no ISI between two consecutive pulses, nor between two symbols. 3 Second, it gives a good resistance to external interference based on the same reasons as any SS system. The big PG also ensures a relatively low-power spectral density, which helps in not causing interference to other existing wireless applications, as shown in Figure 7.20. It is to be noted that the data-carrying modulation in an IR-UWB system is usually PPM, which controls the appearance position of a pulse in a certain duration to represent different data-information. On the other hand, the multiple access capability of an IR-UWB system is implemented through time hopping schemes, as briefly discussed in Subsection 2.2.3. Different users in a pico-cell can be assigned different PN sequences that control the timing of pulses, as shown in Figure 7.23, where only three users are present for simplicity of illustration and 13 hopping slots are shown in one symbol duration. In this case, there is no overlapping in the hopping slots among the three users, implying that there will be no MAI. A TH-UWB can offer a very good time diversity gain if multiple hopping patterns can be assigned to a single user. Therefore, it is intuitively true that it can be made very robust against time-selective fading, especially suitable for the applications where fast mobility is present. 4 DS-CDMA UWB technology The direct-sequence CDMA UWB scheme is the focus of discussion in this subsection. The analysis of a DS-CDMA UWB system is given in the following subsections. A DS-CDMA UWB scheme works like a conventional DS-CDMA system. The pulse trains are used to perform DS modulation to spread the signal. A PN code is assigned to a particular user and will be used to spread its data bit into multiple chips. In the same way as in IR, various data modulation schemes, such as PAM, OOK, PSM, and so on, can also be used in the DS-CDMA UWB system. Figure 7.24 shows an example of the PAM-modulated DS-CDMA UWB scheme. 3 This is particularly true if a UWB system is operating in an indoor environment where the delay spread is relatively small. 4 Because of the fact that most UWB systems are operated in an indoor environment, this advantage may not be well exploited. 302 MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS U U U Figure 7.23 Multiple access capability provided by a TH-UWB IR system. Figure 7.24 Conceptual diagram of a DS-CDMA UWB system with PAM. Many results have been reported on the performance of the DS-CDMA-based UWB systems, as shown in [594–673]. Srinivasa [677] presents a comparison between a TH-PPM UWB and a TH DS spreading with antipodal signaling (TH/DS-BPSK) in terms of their multiple access performance, where the study was limited to an AWGN channel only. Foester [678] characterized the performance of a direct sequence UWB system in the presence of multipath and narrowband interferences. It was shown that the code design that tries to minimize sequence autocorrelation sidelobes as well as cross correlation among spreading codes is critical for a good performance under multipath, multiuser, and narrowband interferences at the same time. A comprehensive review on almost all possible multiple access techniques suitable for UWB- based WPANs or piconet was given in [679]. It was suggested that, among all multiple access schemes (i.e. FDMA, TDMA, and CDMA), CDMA is the most suitable for UWB applications. The use of CDMA allows multiple piconets to be relatively independent, and it is able to produce the highest aggregate data rate. It was also pointed out that CDMA is completely compatible with high QoS, MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS 303 video streaming capable MAC layer protocols, such as the TDMA-based IEEE 802.15.3. On the implementation side, to map to high-speed low-voltage low-power IC technologies, UWB systems must use low peak-to-average pulse trains with a relatively high chip rate. These high chip rates are perfectly suited for building UWB CDMA systems. Qinghua Li and Rusch [680, 681] studied the effectiveness of an adaptive MMSE multiuser detection for a DS-CDMA-based UWB system, particularly under the interference of an IEEE 802.11a OFDM transmitter, as shown in Figure 7.20. Extensive simulations were performed using channel sounding techniques in the 2- to 8-GHz band in a residential environment, which was characterized by a high level of multipath fragmentation. It was demonstrated that the adaptive MMSE is able to reject intersymbol and interchip interference for those channels much more effectively than by using a RAKE receiver with four to eight fingers. It was also shown that the same receiver setting can reject a narrowband interferer generated from an adjacent IEEE 802.11a transmitter. The majority of the work was carried out on the basis of computer simulations. Sadler and Swami [682] investigated a DS-UWB system with so-called episodic transmission, that is, the system should send n pulses per information bit and allow for off time separation between pulses. Several issues on the design of a DS-UWB system, such as PG, jamming margin, coding gain, and multiple access interference, power control, and so on, were investigated. The BER performance was studied using a Chernoff bound and considering a single-user matched-filter receiver in an AWGN channel scenario. The comparison between two UWB techniques for implementing multiple access communications, specifically TH-PPM and DS-BPSK schemes, was made by Canadeo et al. [683]. They carried out a spreading-code-dependent study on both UWB schemes. A generic channel model based on a very simple delay tapped line was used. The coefficients in this multipath channel model were constants, implying that no fading was considered. Boudaker and Letaief [684] outlined the attractive features of DS-UWB multiple access systems employing antipodal signaling and compared it with the TH scheme. An appropriate DS-UWB trans- mitter and receiver were designed, and the system signal processing formulation was investigated. The performance of such communication systems in an AWGN channel in terms of multiple access capability, error rate performance, and achievable transmission rate were evaluated without MI. Only a single matched-filter detector was considered. An interesting method for implementing a DS-UWB system based on a new multi-carrier pulse waveform was proposed in [685]. A unique frequency domain processing technique was used at the receiver side to exploit diversity in the frequency domain and provide resistance against intersymbol interference and multiple access interference. The performance of such a frequency domain processing DS-UWB scheme was compared with a DS-UWB system using traditional time-domain processing techniques. An UWB system with PPM for data modulation and DS spreading for multiple access in an indoor fading environment was considered in [686]. A RAKE receiver was used to combine a subset of the resolvable multipath components using MRC technique. In the following subsections, we will consider a multipath environment, modeled by a discrete-time linear filter with an impulse response whose coefficients are lognormally distributed random variables. Runkle et al. [688] compared a multi-carrier UWB with a DS-CDMA UWB. The results illustrated that a significant advantage can be obtained if a UWB system is implemented by DS-CDMA tech- niques. The multi-carrier UWB was implemented by a MB OFDM architecture. The authors explained how the DS-CDMA UWB architecture could support robust and flexible multiuser capabilities, pro- tect against in-band interference, and provide high resolution ranging capability for safety-of-life applications. A comparison of the average BER and outage probability performance of the three UWB multiple access and modulation combinations for a single-user UWB radio was reported by [689]. The three schemes are TH with bit flipping modulation, TH-PPM, and DS with bit flipping modulation. The authors used the channel models recommended for use in the IEEE 802.15.3a evaluation. The results 304 MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS showed that direct sequence multiple access coding was more likely to achieve the lowest BER for a fixed channel. Unfortunately, most of the currently reported researches on UWB have separated the issues on pulse waveform design from system-level performance, such as bit error probability, and so on. In other words, the previous system-level analysis on BER performance seldom considered the characteristic features of UWB pulses used in the system, as seen from all the papers referred in the preceding text [675–689]. On the other hand, most of the current research on UWB pulse waveforms was focused on the requirements concerning their spectral shapes and has little to do with the overall system BER performance. In the following subsections, we demonstrate a BER performance analysis that is associated with the characteristic feature of UWB pulse waveforms. We give a unified approach to derive a closed form BER expression by taking into account major factors of a UWB system, such as noninteger chip asynchronous transmission of the signals, multiple access interference, MI, and so on, as well as their impact on the BER performance. In particular, we introduce a merit parameter, namely, normalized mean squared autocorrelation function (NMSACF) of the pulse waveform denoted by σ 2 mp normal . It will be used to characterize different UWB pulses in terms of their ACF. In fact, σ 2 mp normal measures the average interchip interference level associated with the autocorrelation side lobes of the pulse waveforms. We will illustrate from the analysis that σ 2 mp normal should be made as small as possible to ensure a desirable BER performance. 7.6.2 DS-CDMA UWB System Model Let us consider a DS-CDMA UWB radio system with K users. An ultranarrow pulse waveform g(t) defined over ( 0,T c ) is used to directly modulate the binary data stream {b (k) j } k=1, ,K j =−∞, ,∞ without using a sinusoidal carrier. The k-th user is assigned a signature sequence {a (k) n } k=1, ,K n=0, ,N−1 to modulate antipodal pulses. Presumably, a pulse covers just a chip duration T c , and a signature code has N chips such that T b = NT c ,whereN is the PG. The block diagram of this generic DS-CDMA UWB transceiver is shown in Figure 7.25, where each transmitted signal will experience fading in the channel with its impulse response being h k (t) for the k-th user. The receiver model is tuned to the first user’s transmitted signal with its signature code being {a (1) n } N−1 n=0 . The received signal will be processed by signature code matched filtering as well as pulse waveform–correlation before making a decision for the j-th bit, or at time t = (j + 1)T b . The transmitted signal from the k-th user can be written as s k (t) = ∞  j =−∞ N−1  n=0 b ( k ) j a ( k ) n g(t −jT b − nT c ) (7.6) where g(t) is defined as    g(t) = 0, 0 ≤ t ≤ T c g(t) = 0,t<0,t>T c max  g(t)  = 1, 0 ≤ t ≤ T c (7.7) We are considering an asynchronous DS-CDMA UWB system and its pulse waveform–dependent bit error performance analysis. The k-th user’s channel impulse response is h k (t) = αδ(t −τ k ),where α is a fading coefficient, which may obey any distribution dependent on a particular environment, and δ(t −τ k ) is an impulse function being unit at t = τ k and zero elsewhere. The received signal can be expressed as r(t) = K  k=1 s k (t) ⊗ h k (t) + n(t) = K  k=1 αs k (t −τ k ) + n(t) (7.8) where symbol ⊗ denotes the convolution operation, {τ k } K k=1 is the delay of the k-th user, n(t) obeys Gaussian Distribution N(0,σ 2 n ) or can simply be denoted as n(t) ∼ N(0,σ 2 n ), which specifies a MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS 305 Figure 7.25 A block diagram of a DS-CDMA UWB transceiver. (a) Transmitter model; (b) Receiver model, where the receiver is intended for user k and a flat fading channel is used. relationship between n(t) and a Gaussian distribution with zero mean and variance σ 2 n . Here, the receiver intends to detect the first user’s transmission. Without loss of generality, let τ 1 = 0andτ k be the relative delay between the first and k-th users’ transmissions. Inserting s k (t) and h k (t) into Equation (7.8), we obtain r(t) = ∞  j =−∞ N−1  n=0 αb ( 1 ) j a ( 1 ) n g(t −jT b − nT c ) + K  k=2 ∞  j =−∞ N−1  n=0 αb ( k ) j a ( k ) n g(t −τ k − jT b − nT c ) + n(t) (7.9) The decision variable at the receiver can be written as y  ( j + 1 ) T b  =  ( j +1 ) T b jT b r(t) N−1  n=0 a ( 1 ) n g(t −jT b − nT c )dt = S +I +η (7.10) [...]... σmp Tc λTc (7. 79) Effect of lognormal fading The conditional variances of MI and MAI terms from the RAKE receiver, as given in Equations (7. 71) and (7. 79), are still conditioned on variables wk,q,l and αk,q,l , where wk,q,l is a discrete random variable uniformly distributed over (−1, 1) and αk,q,l is a Lognormal random variable To apply MRC to the RAKE receiver, we let βp = w1,p α1,p , (7. 80) which... B3G WIRELESS where 1 X (IK ) = N (2 − Tc ) q Qk , q Lk , 1 2 u Qk , Tc − q Lk , 1 λγ 1 λγ − 2 1 q Qk , λTc u Lk , 1 λγ (7. 89) The functions q (L1 , δ) and u (L1 , α) have been defined in Equation (7. 87) After having obtained all means and variances of the four variables in the decision variable, defined in Equation (7. 57) , or v (j ) = S + IL + IK + η, as shown in Equations (7. 81), (7. 82), (7. 85), and (7. 88),... = w1,p α1,p , (7. 80) which could be inserted into Equations (7. 59), (7. 63), (7. 71) and (7. 79) to obtain the useful signal component The distribution of the noise term, the variances of MI and MAI terms after the MRCRAKE receiver, respectively, can be expressed as L1 (1) S = Nbj Emp p=1  η∼ 2 α1,p ,  L1 2 N 0, σn NEmp (7. 81) 2 α1,p  , (7. 82) p=1 V ar IL |αq,l = E V ar IL |wq,l , αq,l  L1 2 α1,p... the q-th cluster and the first ray in the first cluster is The channel gain αq,l 1 1 (7. 47) λ is a Lognormal random variable, whose relation can be written as E Tq + τq,l = (q − 1) + (l − 1) 20 log10 αq,l ∼ N µq , σ 2 (7. 48) Also, αq,l in Equation (7. 43) is always positive and its second order moment obeys a dualexponential distribution as 2 (7. 49) E αq,l = 1 e−Tq / e−τq,l /γ where and γ are the attenuation... arrival rates for the clusters and rays are 1/11 (1/ns) and 1/0.35 (1/ns), respectively σ = 4.8 dB The mean attenuation power of the first ray 1 = 1, and all other rays decay exponentially with its decay factor being 16 γ = 8.5 particular pulse waveform function g(t), which is defined in Equation (7. 7), before we actually apply it to a real system Figures 7. 33, 7. 34, and 7. 35 show the BER performance... antenna and a single receiver antenna, as shown in Figure 8.1(a) The time-variant impulse response of the channel can be written as h(τ, t) = p(τ, t) ∗ g(τ ) (8.1) where p(τ, t) is the impulse response of the time-variant propagation channel, and g(τ ) is the combined effect of the transmitter pulse-shaping and the receiver matched-filtering Thus, the received Next Generation Wireless Systems and Networks. .. rays The arrival rates for the clusters and rays are 1/11 (1/ns) and 1/0.35 (1/ns), respectively Channel parameter σ is 4.8 dB The average power of the first ray 1 = 1, and all other rays decay exponentially with their decay factor and γ are being 16 The path attenuation factor γ is equal to 8.5 All the variables σ , 1 , defined in Equations (7. 49) and (7. 50) Figure 7. 33 shows that the BER of a DS-CDMA... signature codes and ACF of the pulse waveform can be calculated explicitly if we have the knowledge of user signature codes and pulse waveforms, and thus the variance of MAI can also be determined Random sequences In this subsection, purely random sequences will be used as spreading sequences, whose chips (k) k=1, ,K (k) {an }n=0, ,N−1 will take “−1” and “+1” equally likely In addition, ai(k) and aj should... clusters and rays obey Poisson distributions and thus their interarrival times are exponentially (instead of uniform, as specified in the original S–V model [690]) distributed The modified S–V channel model can be expressed mathematically by Q L wq,l αq,l δ t − Tq − τq,l h (t) = q=1 l=1 (7. 43) 314 MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS q=1, ,Q where q ∈ (1, Q) and l ∈ (1, L) stand for cluster and ray...        p=1    = Q ξ  L1 1 L1 1 p=1 2  α1,p  (7. 90) where NEmp ξ= 2 2 σmp R (IL ) + (K − 1) σmp X (IK ) + = 2 σn NEmp 1 1 2 σmp normal R (IL ) N 2 Emp + 2 (K−1)σmp normal X (IK ) N 2 Emp (7. 91) + 1 1 1 SNR T 2 Emp = 0 c g 2 (t) dt denotes the energy contained in a single pulse, and σmp normal is defined in Equations (7. 37) and (7. 42), representing normalized mean squared ACF of the pulse . Subsections 7. 6.1, 7. 6.2, 7. 6.3, 7. 6.4, and 7. 6.5. Another UWB scheme that has gained much popularity is based on OFDM technology, namely, OFDM-UWB, which can be implemented on a multiband (MB). applications. wireless metropolitan area networks (WMANs) as IEEE 802.16 standards, WPANs as IEEE 802.15 standards, and so on. It is noted that IEEE 802.15.4.a is emerging as the standard for low-data-rate transmission. The. the operators to profitably scale their wireless networks to meet the burgeoning data traffic demand in an all-you-can-eat pricing environment. 7. 6 Ultra-Wideband Technologies As mentioned in Section