Solutions_2nd_ed

Solution Manual for Mobile Communications 2nd ed. Jochen H. Schiller, Freie Universität Berlin, Germany schiller@computer.org, www.jochenschiller.de 1. Introduction 1.1 Good sources for subscriber numbers and other statistics are, e.g., www.gsmworld.com, www.3gpp.org, www.3gpp2.org, www.emc-database.com, www.3g.co.uk, www.regtp.de … 1.2 Today’s GSM operators add the new 3G air interfaces of UMTS to their existing GSM/GPRS infrastructure networks. Current GSM/GPRS networks already offer packet and circuit switched data transmission following the Release 99 of UMTS. The operators have to install new radio access networks, i.e., antennas, radio network controller etc. as described in chapter 4. The situation is similar for operators using cdmaOne (IS-95) technology. However, these operators go for cdma2000 as this system allows them to reuse their already existing infrastructure. Thus, based on the separation of the mobile phone systems into (very roughly) CDMA and GSM operators will lead to two different major 3G systems, cdma2000 and UMTS (and their future releases). Right now, it does not seem that there is a place for a third 3G system. Current TDMA operators might move to EDGE enhanced systems or join the UMTS system. However, it is still open what will happen in China – the Chinese system TD-SCDMA was pushed by the government, but networks and devices are still missing. Currently, the majority of Chinese subscribers use GSM, some operators offer CDMA. 2. Wireless Transmission 2.1 Check also the WRCs that try to harmonize global frequency plans. 2.2 Below 2 MHz radio waves follow the ground (ground wave propagation). One factor for this is diffraction (waves are bound towards obstacles that have sizes in the order of the wavelength), another factor is the current induced in the Earth’s surface, which slows the wavefront near the earth, causing the wavefront to tilt downward. Several reasons make low frequencies unusable in computer networks: • Lower frequencies also mean lower data rates according to Nyquist/Shannon as the available bandwidth is less. • Lower frequencies also require large antennas for efficient transmission and reception. This might work for submarines, not for mobile phones. • Lower frequencies penetrate material more easily. Thus SDM is more difficult – cell size would increase dramatically and frequency reuse would be almost impossible. 2.3 Frequencies in the THz range, e.g., infrared, visible light, are easily blocked by obstacles and, thus, do not interfere with other transmissions. In this case, only the standard safety regulations apply (e.g., laser emission). Most radio systems stay well beyond 100 GHz as it is not that simple to generate higher frequencies (in the lower THz range). 2.4 The classical European approach was based on standardisation and regulation before any products were available. The EU governments founded ETSI to harmonize all national regulations. ETSI created the standards, all countries had to follow. In the US companies develop systems and try to standardize them or the market forces decide upon success. The FCC, e.g., only regulates the fairness among different systems but does not stipulate a certain system. The effects of the two different approaches are different. Many “governmental” standards in Europe failed completely, e.g., HIPERLAN 1, some succeeded only in Europe, e.g., ISDN, and however, some soon became a worldwide success story, e.g., GSM. For most systems the US approach worked better, first some initial products, then standards. One good example is the wireless LAN family 802.11, a good counter example is the mobile phone market: several different, incompatible systems try to succeed, many features, well established in Europe since many years, are not even known in the US (free roaming, MMS, GPRS roaming, no charges for being called etc.). 2.5 Computers, in contrast to, e.g., TV sets, travel around the world as laptops, PDAs etc. Customers want to use them everywhere. Thus it is very important to be able to use built-in WLAN adapters around the globe without reconfiguration and without licensing. Furthermore, it is much cheaper for WLAN manufacturers to design a single common system compared to many different systems for probably small markets. 2.6 No. Loss-free transmission of analogue signals is not possible. Attenuation, dispersion etc. will always distort the signal. Additionally, each digital signal is transmitted as “bundle” of analogue sine waves (think of Fourier!). A perfect digital signal with rectangular shape requires an infinite number of sine waves to be precisely represented (the digital signal can be represented as infinite sum of sine waves according to Fourier). However, no medium can transmit infinite high frequencies. Thus, the digital signal can never be transmitted without any loss. 2.7 Without any additional “intelligence” directional antennas are not useful in standard mobile phones as users may not want to direct the phone to a certain antenna. Users move, rotate, flip the phones etc. Phones are in bags, pockets, … while operated hands-free. There is no chance of directed transmission. However, new developments comprising fast signal processors and multiple antennas may exploit directed characteristics of antennas (beam forming). There are several ways of improving the gain of an antenna: right dimensioning (e.g., half the wavelength), multiple antennas plus a signal processor combining the signals, active and passive components attached to the antenna (compare with traditional TV antennas, satellite dishes etc.). 2.8 Problems: attenuation, scattering, diffraction, reflection, refraction. Except for attenuation all other effects can divert the waves from a straight line. Only in vacuum and without gravitational effects radio waves follow a straight line. Without reflection radio reception in towns would be almost impossible. A line.-of-sight almost never exists. However, reflection is the main reason for multipath propagation causing ISI. 2.9 ISI mitigation: large enough guard spaces between symbols/low symbol rate (used in OFDM: distribute the symbol stream on many different carriers), channel estimation/calculate the n strongest paths and adapt the receiver accordingly. Using higher frequencies reduces the effects of multipath propagation and thus ISI (waves more and more behave like light). The higher the symbol rate the stronger the ISI. If senders and/or receivers move fast the chances for ISI are higher because the location of obstacles changes, hence the number, magnitude, and timing of the secondary pulses – it is difficult to follow the signals and adjust the delays for recombination. ISI lowers the bandwidth of a TDM scheme as the guard spaces require some time. 2.10 Several mechanisms exist to mitigate narrowband interference (which might be caused by other senders, too): • Dynamic Frequency Selection: Senders can sense the medium for interference and choose a frequency range with lower/no interference. HiperLAN2 and 802.11h use this scheme. Network operators can also this scheme to dynamically assign frequencies to cells in mobile phone systems. DFS has a relatively low complexity. • Frequency hopping: Slow frequency hopping (several symbols per frequency) may avoid frequencies with interference most of the time with a certain probability. This scheme may be used in GSM. Furthermore, wireless systems can use this principle for multiplexing as it is done in Bluetooth systems (still slow hopping as Bluetooth sends many symbols, indeed a whole packet, on the same frequency). Fast hopping schemes transmit a symbol over several frequencies, thus creating a spread spectrum. FH systems have medium complexity. Main topic is synchronisation of the devices. • Direct sequence spread spectrum: Data is XORed with a chipping sequence resulting in a spread signal. This is done in all CDMA systems, but also in WLANs using, e.g., Barker sequences for spreading (e.g., 802.11b). The signal is spread over a large spectrum and, thus, narrowband interference only destroys a small fraction of the signal. This scheme is very powerful, but requires more powerful receivers to extract the original signal from the mixture of spread signals. 2.11 Worldwide regulation always uses FDM for separating different systems (TV, WLAN, radio, satellite, …). Thus, all radio systems must modulate the digital signal onto a carrier frequency using analogue modulation. The most prominent system is the traditional radio: all music and voice use frequencies between, e.g., 10 Hz and 22 kHz. However, many different radio stations want to transmit at the same time. Therefore, all the original signals (which use the same frequency range) must be modulated onto different carrier frequencies. Other motivations for digital modulation are antenna and medium characteristics. Important characteristics for digital modulation are spectral efficiency, power efficiency and robustness. Typical schemes are ASK, PSK, FSK. 2.12 The receiver may “check” the distance between the received point and the neighbouring points. The receiver then chooses the closest neighbour and assumes that the sender originally transmitted data represented by the chosen point. The more points a PSK scheme uses the higher are chances that interference (noise) shifts a transmitted “point” onto another. If the gaps between the points are too small, in particular smaller than noise added during transmission, chances are very high that the receiver will map received data onto the wrong point in the constellation diagram (please note: data is coded using PSK, the points in the constellation diagram represent codes, these codes are then transmitted – it is just simpler to think in “points”…). 2.13 Main benefits: very robust against interference, inherent security (if the spreading code is unknown it is very difficult to tap the transmission), basis for CDMA technologies, can be used in the “background” of existing systems if the signal level is low enough. Spreading can be achieved by XORing a bit with a chipping sequence or frequency hopping. Guard spaces are now the orthogonality of the chipping sequences or hopping patterns. The higher the orthogonality (well, that is not very mathematical, but intuitive), the lower the correlation of spread signals or the lower the collision probability of frequency hopping systems. DSSS system typically use rake receivers that recombine signals travelling along different paths. Recombination results in a stronger signal compared to the strongest signal only. 2.14 The main reason is the support of more users. Cellular systems reuse spectrum according to certain patterns. Each cell can support a maximum number of users. Using more cells thus results in a higher number of users per km². Additionally, using cells may support user localisation and location based services. Smaller cells also allow for less transmission power (thus less radiation!), longer runtime for mobile systems, less delay between sender and receiver. Well, the downside is the tremendous amount of money needed to set-up an infrastructure with many cells. Typically, each cell holds a certain number of frequency bands. Neighbouring cells are not allowed to use the same frequencies. According to certain patterns (7 cluster etc.) cellular systems reuse frequencies. If the system dynamically allocates frequencies depending on the current load, it can react upon sudden increase in traffic by borrowing capacity from other cells. However, the “borrowed” frequency must then be blocked in neighbouring cells. 2.15 TDM/FDM-systems have a hard upper limit of simultaneous users. The system assigns a certain time-slot at a certain frequency to a user. If all time-slots at all frequencies are occupied no more users can be accepted. Compared to this “hard capacity” a CDM system has a so-called “soft-capacity” (compare filling a box with bricks or tissues). For CDM systems the signal-to-noise-ratio typically limits the number of simultaneous users. The system can always accept an additional user. However, the noise level may then increase above a certain threshold where transmission is impossible. In TDM/FDM systems additional users, if accepted, do not influence other users as users are separated in time and frequency (well, there is some interference; however, this can be neglected in this context). In CDM systems each additional user decreases transmission quality of all other users (the space for the tissues in the box gets tighter). 3. Medium Access Control 3.1 Stations in a wired network “hear” each other. I.e., the length of wires is limited in a way that attenuation is not strong enough to cancel the signal. Thus, if one station transmits a signal all other stations connected to the wire receive the signal. The best example for this is the classical Ethernet, 10Base2, which has a bus topology and uses CSMA/CD as access scheme. Today’s wired networks are star shaped in the local area and many direct connections forming a mesh in wide area networks. In wireless networks, it is quite often the case that stations are able to communicate with a central station but not with each other. This lead in the early seventies to the Aloha access scheme (University of Hawaii). So what is CS (Carrier Sense) good for in wireless networks? The problem is that collisions of data packets cause problems at the receiver – but carrier sensing takes place at the sender. In wired networks this doesn’t really matter as signal strength is almost the same (ok, within certain limits) all along the wire. In wireless networks CS and CD at the sender doesn’t make sense, senders will quite often not hear other stations’ signals or the collisions at the receiver. 3.2 In case of Aloha stations do not care about other stations but simply access the medium if they have to send data. There are no stations exposed as stations do not perform carrier sensing. Hidden stations may cause collisions. The same is true for slotted Aloha the only difference being the slotted character of medium access. Reservation schemes typically work with a central reservation station which can be heard by all others. Without this condition or equivalent means of distributing reservations the whole scheme will not work. Thus, there are no hidden or exposed terminals. MACA is designed to handle hidden and exposed terminals in a distributed WLAN without central reservation station. However, MACA may fail in case of asymmetric communication conditions or highly dynamic topologies (stations may move fast into collision range). 3.3 As long as a station can receive a signal and the signal arrives at the right time to hit the right time-slot it does not matter in TDMA systems if terminals are far or near. In TDMA systems terminals measure the signal strength and the distance between sender and receiver. The terminals then adapt transmission power and send signals in advance depending on the distance to the receiver. Terminals in CDMA systems have to adapt their transmission power very often (e.g., 1500 times per second in UMTS) so that all signals received, e.g., at a base station, have almost the same strength. Without this one signal could drown others as the signals are not separated in time. 3.4 Typically, SDMA is performed or supported by a network provider. The provider plans the network, i.e., places the base stations according to certain topologies, geographic situations, capacity planning etc. If the system is running, base stations support the infrastructure in the decision of assigning a certain base station to a terminal. This is often based on received signal strength or the current capacity. The mobile terminal supports the infrastructure by transmitting information about the received signal strengths. The terminal can furthermore initiate the change of the access point. 3.5 Modulation – Transmitters must shift all baseband signals to a carrier frequency. This is typically an analogue process and requires analogue components. Classical receivers also need filters for receiving signals at certain frequencies. Depending on the carrier frequency different antennas may be needed. Pure TDMA systems stay on one frequency, all receivers can wait on the same frequency for data. In FDMA systems receivers have to scan different carrier frequencies before they can receive signals. MAC is performed on many different layers. The WRCs (World Radio Conferences) are used for worldwide frequency assignments such as the 2 GHz range for IMT-2000. ITU controls worldwide frequency usage. National authorities regulate frequencies in different nations. On the next lower layers network operators perform MAC: frequencies usage is controlled by network planning and current load. Finally, base stations in mobile phone systems assign frequencies to terminals depending on the current availability. In WLANs network administration assigns frequencies thus forming cells. 3.6 Wireless networks can use different frequencies, different time slots or even different codes to implement duplex channels. Typical wired networks simply use different wires (however, more elaborated schemes such as echo cancellation are feasible, too). 3.7 If communication systems use fixed TDM patterns terminals can be very simple. The only requirement is to stay synchronised to be able to receive the right data. This is the standard system in classical telecommunication networks (e.g., ISDN, PCM-30 systems, SDH etc.). Ethernets, the Internet, wireless LANs etc. work demand driven. Here the advantage is the low overhead when starting communication: terminals don’t have to setup connections reserving time slots prior to communication. However, users transmit more and more data compared to voice. Most networks of today are data dominated (if the amount of data is considered, not the revenue). Thus, data transmission should be optimised. While WLANs are optimised for data from the beginning (and isochronous audio transmission causes some problems), wide area mobile phone systems started as almost voice only systems. The standard scheme is circuit switched, not packet switched. As more an more data is transmitted these networks have to integrate more and more data oriented technologies: GPRS in GSM, IP in the core network of UMTS etc. 3.8 Interference and countermeasures in: • SDMA: Interference happens if senders are too close to each other. Terminals or base stations have to keep a minimum distance. • TDMA: Interference happens if senders transmit data at the same time. Countermeasures are tight synchronisation and guard spaces (time gap between transmissions). • FDMA: Interference happens if senders transmit data at the same frequency. Thus, different frequencies have to be assigned to senders by organisations, algorithms in base stations, common frequency hopping schemes etc. Furthermore, guard bands between used frequency bands try to avoid interference. • CDMA: Interference happens if senders transmit data using non-orthogonal codes, i.e., the correlation is not zero. Thus, senders should use orthogonal or quasi-orthogonal codes. 3.9 Even in vacuum radio waves have limited velocity: the speed of light. As soon as matter is in the way waves travel even slower. Thus, it can happen that a sender senses the medium idle, starts the transmission and just in a moment before the waves reach another sender this second sender senses the medium idle and