The many frequency blocks detailed earlier are used for a variety of communications services. Higher frequencies (say above 6 GHz) are mostly used for point-to-point services such as dedicated private lines. Lower frequencies are better suited for broader coverage, and are split into geographical cells.
Covering a large geographic area with limited amount of spectrum leads to the reuse of the same frequency in multiple locations; this leads to co-channel interference considerations, meaning interference from different areas (or cells) that use the same frequency channel.1 Co-channel interference considerations are usually approached by considering the following parameters:
The three quantities are linked by the straightforward relation:
The reuse factor K is therefore an important parameter for capacity. The lowest reuse factor (K = 1) maximizes capacity; but this has to be balanced with interference considerations: indeed a higher reuse factor (K = 3, 4, 7, or higher) provides more distance between cells using the same frequency, which lowers interferences.
To quantify interference due to reuse we have to consider how a signal propagates from one cell to another. We will study propagation models later in chapter ??, but we need a few simple notions here. Assume a propagation model using a power path loss exponent n, that is a model where power decays in 1∕Rn (R being the distance separating transmit station from receiver). This means that the ratio of received power to transmit power may be expressed as Pr∕Pt = A∕Rn, where A is some constant.
With this model, signal to interference ratios are estimated as
where i0 is the number of co-channel cells nearest to the cell (called first tier or tier one); that number increases with K. And Di is the distance to the tier-one cells reusing the same frequency (as shown in figure 2.1). In the case of hexagonal cell approximation the expression simplifies to :
We’ll see more details on n further, its values vary typically between 2 and 4 with the types of terrain. We’ll also see that specific wireless technologies require a certain signal to noise and interference ratio (mostly based on data rates); so equation (2.3) leads to a minimal acceptable value for K.
A major requirement of cellular networks is to provide an efficient technique for multiple devices to access the wireless system. These techniques include:
These are the main multiple access techniques, but subtle extensions and combinations can be devised to obtain more efficient schemes, which we will examine in later chapters (including orthogonal frequency division multiplexing - OFDMA).
Wireless communications deal with at least two main concerns: coverage and capacity. We will look at coverage prediction in the next chapters, and start here with a few words on capacity.
One fundamental concept of information theory is one of channel capacity, or how much information can be transmitted in a communication channel. In the 1940’s Claude Shannon invented formal characterization of information theory and derived the well-known Shanon’s capacity theorem (Theorem 17 in , p.628). That theorem applies to wireless communications. A great presentation of this equation can be found in  p.82; it presents a concise derivation of the equation, and includes a good introduction to important information theory concepts such as information and entropy. 2
The Shannon capacity equation gives an upper bound for the capacity in a non-faded channel with added white Gaussian noise:
where C= capacity (bits/s), W=bandwidth (Hz), S∕N= signal to noise (and interference) ratio.
That capacity equation assumes one transmitter and one receiver, though multiple antennas can be used in diversity scheme on the receiving side. The formula will be revisited for multi antenna systems in §9.1.3. The equation singles out two fundamentally important aspects: bandwidth and SNR. Bandwidth reflects how much spectrum a wireless system uses, and explains why the spectrum considerations seen in §1.2 are so important: they have a direct impact on system capacity. SNR of course reflects the quality of the propagation channel, and will be dealt with in numerous ways: modulation, coding, error correction, and important design choices such as cell sizes and reuse patterns.
Practical capacity of many wireless systems are far from the Shannon’s limit (although recent standards are coming close to it); and practical capacity is heavily dependent on implementation and standard choices.
Digital standards deal in their own way with how to deploy and optimize capacity. Most systems are limited by channel width, time slots, and voice coding characteristics. CDMA systems are interference limited, and have tradeoffs between capacity, coverage, and other performance metrics (such as dropped call rates or voice quality).
This simple equation (2.5) gives us a number of voice channels in a CDMA frequency channel 3.
We can already see some hints of CDMA optimization and investigate certain possible improvement for a 3G system. In particular: improving α can be achieved with dim and burst capabilities, β with interference mitigation and antenna downtilt considerations, R with vocoder rate, W with wider band CDMA, Eb∕Nt with better coding and interference mitigation techniques.
Some aspects however are omitted in this equation and are required to quantify other capacity improvements mainly those due to power control, and softer/soft handoff algorithms.
Of course other limitations come into play for wireless systems, such as base station (and mobile) sensitivity, which may be incorporated into similar formulas; and further considerations come into play such as: forward power limitations, channel element blocking, backhaul capacity, mobility, and handoff.
A final note on capacity: voice capacity is often given in Erlang, and refers to trunking efficiency given a certain blocking probability. ( §3.6,  p. 350.)
Modulation techniques are a necessary part of any wireless system, without them, no useful information can be transmitted. Coding techniques are almost as important, and combine two important aspects: first to transmit information efficiently, and second to deal with error correction (to avoid retransmissions).
A continuous wave signal (at a carrier frequency fc) in itself encodes and transmits no information. The bits of information are encoded in the variations of that signal (in phase, amplitude, or a combination thereof). These variations cause the occupied spectrum to increase, thus occupying a bandwidth around fc; and the optimal use of that bandwidth is an important part of a wireless system. Various modulation schemes and coding schemes are used to maximize the use of that spectrum for different applications (voice or high speed data), and in various conditions of noise, interference, and RF channel resources in general.
Classic modulation techniques are well covered in several texts , and we simply recall here a few important aspects of digital modulations (that will be important in link budgets). The main digital modulations used in modern wireless systems are outlined in table 2.1.
Bits encoded by:
Amplitude Shift Keying
Discrete amplitude levels
Frequency Shift Keing
Multiple discrete frequencies
Phase Shift Keying
Multiple discrete phases
BPSK, QPSK, 8-PSK
Quadrature Ampl. Mod.
Both phase and amplitude
16, 64, 256 QAM
Modulation is a powerful and efficient tool used to encode information; a few simple definitions are commonly used:
Higher order modulations can encode multiple bits in a symbol, and require higher SNR to decode error-free. Figure 2.2 illustrates how multiple phases and amplitudes are used to combine multiple bits into one symbol transmission. The tradeoff between bits encoded per symbol is often referred to as a measure in bits per Hertz (b/Hz), its relation to SNR is bounded by Shannon’s theorem seen earlier (§2.2.1).
Efficient coding schemes are the powerful engines behind the growth of the wireless industry. They have allowed wireless systems to be both spectrally efficient and robust in terms of error corrections.
Block coding are the classical approach: blocks of data are used as input to produce usually larger output blocks containing added redundancy.
Second generation wireless systems like cdmaOne introduced the use of convolutional coding. The coding scheme provides an efficient redundant and error-correcting scheme. This is particularly useful for voice transmission where the need for retransmission causes delays and degrades voice quality.
Wireless data systems of higher rates often use turbo coding, which are a combination of two convolutional coders reading each other (the name comes from the turbo-charged engine, which uses some of its output power to compress some air fed to the intake, and is somewhat reminiscent of the turbo coding diagram of figure 2.4).
Convolutional coding and turbo coding are example of continuous coding schemes, where a bit stream is encoded into another bitstream, usually of greater speed (with a multiplier of 2, 3, 4 or more). The added number of bits can be seen as spreading the spectrum, and the information, which requires more data to transmit, but inherently contains useful redundancy properties (a form of time diversity). The decoding of such schemes was historically difficult and has become possible only with recent processing power (see for instance Viterbi algorithms ).
The combination of modulation and coding provides great flexibility between redundancy and throughput. Higher modulation increases spectral efficiency in good propagation condition; when conditions worsen, lower modulation helps, but increased redundancy is sometimes an efficient alternative. Combined, the two schemes can reach impressive efficiencies, close to Shannon’s limit (§2.2).
We first briefly review current mobile digital technologies, how they were initially introduced, and how and they evolved. 4
1 traffic channel per RF channel
30 kHz, 200 kHz in GSM, 1.25 MHz for CDMA
7 (less with frequency hopping), 1 for CDMA
mostly FDD (emergence of TDD)
FDMA, TDMA (8 full-rate time slots for GSM), or CDMA
Digital encoded: GSM full rate 13.4 kbps, CDMA 13 kbps QCELP or 8 kbps EVRC
1.25, 5, 10, 15 MHz
mostly FDD, some TDD
Digital encoded: bit rates 8 kbps and below
Up to several Mbps (3.1 Mbps for EV-DO, 15 Mbps for HSDPA)
generally wider: 10, 20 MHz, more
1-1.5 (OFDMA – see §8.3.3)
FDD or TDD depending on spectrum
IP based, flat architecture, convergence
The introduction of digital wireless systems means that the acoustic voice wavefront is not simply converted to an electrical signal directly transmitted over RF channel. Voice is now digitized, encoded, and the resulting bit stream is transmitted and of course decoded on the receiving side. Although this process requires additional digital signal processing (DSP), it opens the door to many optimization algorithms and is much more efficient than usual analog voice transmission.
Digital voice coding (vocoding) is very important yet very subjective. Voice coding theory is a domain of study of its own; introductory overviews are presented for instance in  ch. 8 or  ch. 15.
Analog vocoders have emerged at Bell Laboratories in the late 1920’s, and have become more elaborate and efficient in dealing with harmonics important to a good understanding of voice (500 Hz to 3400 Hz) while minimizing bandwidth. The digital area brought significant changes. Initial digital systems sampled that range, which at the Nyquist rate leads to a 64 kilobits per second (kbps, kbit/s, or kb/s) bandwidth. This is referred to as pulse-code modulation (PCM). More elaborate algorithms however can achieve reasonably good voice transmission by transmitting a codebook (set of parameters for a given voice coding algorithm) with as little as 2.4 kbps rate: a 26-fold improvement. Usually these algorithms provide acceptable voice quality, but may provide poor performance in specific situations such as in a noisy environment, with background music, or when combined with different voice coding systems (such as PCM or voice mail systems). Several vocoder systems exist and have been chosen in 2G and 3G standards:
Comparing the quality differences between vocoder is usually done by testing a number of standard phrases, and assessing the quality of the transmitted result under various conditions. That assessment is subjective and is usually given a grade called Mean Opinion Score (MOS) between 0 (completely unintelligible) and 4 (perfect quality). Initial tests relied on actual opinion surveys, but test devices now offer algorithms providing a MOS and are regularly used by wireless network operators to benchmark network quality.
Second generation cellular systems certainly achieved major capacity improvements and contributed to the fast adoption of wireless handsets throughout the world. And the growth continues.
Third generation systems focused on increasing capacity yet again, and on introducing efficient high-speed mobile data systems. Given past heavy investments in different 2G networks, adoption of a common 3G standard had tremendous cost implications and competitive advantages.
These efforts from the wireless industry focused on improving widely deployed systems, and migrate them towards a third generation. All major digital technologies proposed an evolution path to a next generation, typically broader band (in throughput and spectrum).
In short two major 3G standards remain in competition, and the choice of any carrier is clear: GSM operators clearly opt for a migration to UMTS (3GPP), and cdmaOne operators to cdma2000 (3GPP2). The latter is certainly initially cheaper, has advantages in equipment availability, and has well-known performances; but the former may benefit from larger economies of scales as GSM carriers migrate to UMTS services.
In 2002, CDMA Americas Congress (San Diego, December 2002) estimated that cdmaOne operators benefited from a smooth transition and a well-known standard, thus giving them a one or two year advance over GSM efforts towards UMTS. Indeed cdma2000 (3G 1X) systems have been available since 2002, IS-856 (3G-1X EV-DO) have been widely available in the US and Asia since 2004. GPRS and UMTS are finally catching up in 2006. High-speed data services (HSPA) still lag in coverage behind EV-DO in 2008, but most dense areas in the US are well covered by both technologies.
Choosing a migration path is only the first step; upgrading the network is of course very costly. Initially service providers had to decide how long to delay network upgrade: voice capacity and time to market for high-speed data services were the driving factors. Now service providers have to decide how much resources to dedicate to voice versus data.
Second generation cellular systems achieved digital voice efficiency, third generation systems focused on increasing capacity and data rates, what more can a fourth generation standard achieve?
According to most definitions (from the ITU in particular), 4G systems are required to achieve throughput rates around 100Mbps for mobility and 1Gbps for fixed wireless access; so the air interface has to be incredibly efficient. There are certainly additional requirements (mostly on the network infrastructure) such as low latency, flat IP architecture, and the use of small cells, heterogeneous networks, and more (which we’ll review in later chapters).
The main 3G standards have an evolution towards a 4G standard, even if not all aspects are et in its early iteration, these 4G standards have evolution lines towards true 4G requirements. They have a number of commonalities:
Oddly enough two different camps seem to emerge again: LTE and WiMAX, each backed up by different suppliers, and different operators, both using very similar technologies (based on OFDMA), and with very few technical reasons why they should not harmonize to a unique standard.
An important argument to consider is that of spectrum: the vast majority of mobile operators operate in FDD spectrum (see sections 1.2.3 and 1.3) LTE provided an evolution first in that mode. WiMAX on the other hand chose to focus first on TDD bands and is the obvious choice for TDD spectrum owners. The overall timeline for evolution is also important: some cellular providers have made significant investments in EV-DO or in HSPA. Newcomers on the other hand who need high data rates today with smooth evolution towards 4G later may be more likely to chose WiMAX. Practically however, since 2010 the vast majority of the mobile industry is following LTE plans, and that standard is becoming the de facto standard for the 4G mobile wireless world.
Recent technology advances aim at increasing capacity further. Technology improvements are sometimes the result of a major standard modification, but sometimes simple schemes that can be added to existing standards and allow for additional improvements with minimal infrastructure changes.
Voice coding algorithms and DSP capabilities have improved, and current voice codecs operate on less power, and with greater processing efficiencies. (Refer to  ch. 15, or  ch. 8 for speech coding details). GSM for instance is improving voice digitization and quantizing from RPE-LPT to a series of AMR standards. IS-95 systems have a parallel evolution, with EVRC, and half-rate EVRC.
Another standard for selectable mode vocoder (SMV) was in the work but never saw any success in the industry; it based requirements on: operation in presence of frame erasures, noise suppression recommended for background noises, reasonable performance with music for on-hold situations, equivalent performances with different languages, multiple quality modes and multiple bit rates, seamless transition from mode to mode. SMV was design to offer four modes of operations:
The resulting capacity vs. quality tradeoffs seem useful and attractive to service providers, yet this standard never took off, which may illustrate that some standard evolutions (even when based on sound requirements and good improvements) may miss their window of opportunity.
For systems primarily designed for voice, latency was a main concern, and modulations were chosen to be reliable and operating well at fairly low SNR (like QPSK). For data systems it is advantageous to take advantage of higher modulation schemes such as 16QAM and 64QAM when the radio link allows it. Higher modulations are more spectral efficient but prone to more bit error rates and may cause more retransmissions, latency, or jitter.
Interferences may be cancelled or mitigated by changing antenna patterns as required. Such systems are sometimes referred to as smart antennas, and are in essence an elaborate extension of sectoring. The aim may be to balance the load, or steer a main lobe toward a user, or create a null in the direction of an interferer. Some systems are static, others are dynamic and change with cell load. Some systems are passive others include active amplification devices. The main types of smart antenna systems may be described as follows:
Smart antenna systems are efficient in dense areas. Their cost of equipment however (sometimes due to the complex transmit aspect) and large antenna sizes are major drawbacks . Smart antennas are now replaced by MIMO systems covered in chapter 9.
Antenna diversity is a wonderful technique to improve link budgets; receiving diversity simply consists in having more than one antenna at the receiving site. Given the power limitations of a mobile handset, receiving diversity has been implemented at cell site from the early days of cellular systems. Good diversity schemes can add 8 to 11dB on the up-link budget, thus significantly improving coverage, quality and capacity on that link. The goal of antenna diversity is to provide two uncorrelated paths and combine the two signals, thus reducing the probability of deep fades. A general guideline is to measure or calculate the correlation coefficient, ρ, and try to achieve the lowest possible correlation between the two paths.
Diversity improvements are of two kinds: improvements on existing receive diversity in the uplink, and introduction of transmit diversity for the forward link.
Receive diversity has been used from the early days of cellular, and is as popular as ever. Classic diversity schemes use two antennas at the base station and some algorithms to combine signals 5
Transmit diversity is an important feature for forward link capacity improvement. Since handsets are rather small, their receive diversity capabilities are limited and there transmit diversity schemes were long ignored, but are now used in many standards.
Technology advances and standard improvements target an increase in capacity, coverage, data rate, or some other system performance aspect. In many cases however some simple optimization techniques can be used to increase performance:
These techniques are very important tools used by operators to optimize capacity and coverage. In some cases optimization may be seasonal due to foliage or different usage patterns. In all cases RF network demand constant tweaking to provide optimal performance. More recently self optimizing networks (SON) have the ability to continually and automatically optimize these parameters.
Fixed wireless access is sometimes referred to as wireless local loop (WLL), and is an alternative to provide Plain Old Telephone Services (POTS) and high-speed data services in remote areas where wired solutions are impractical for various reasons. In most cases, trenching long distances to place communication conduits (for fiber or copper) is very costly, such as in mountainous areas. Cellular service is often scarce too in remote areas.
Radio solutions for wireless local loops were rolled-out extensively since the 1970’s. Some such radio services are still in place, and in use today. Early systems use analog radios to offer voice service over fairly long distances. Newer WLL system need to be cost-effective, reliable, adaptable to a wide range of situations, and compliant with local exchange carrier technical, legal, and regulatory standards. But the demand for WLL services are generally low, and suppliers consequently treat the opportunity as a fairly low priority.
Initially WLL focused on providing extensions of the public switched telephone network (PSTN) to reach remote customers. As the PSTN evolved to digital voice, digital switching, and Class 5 features (such as call waiting, caller ID, 3-way calling, and others), WLL systems evolved to include many of these features. WLL products therefore focused on providing feature parity for these class 5 services. Connectivity to Class 5 switches like Lucent 5ESS or Nortel DMS100 is specified in Telcordia standards such as GR-303 or GR-008; and WLL systems evolved to use these standard interfaces to the PSTN.
Radio frequencies were allocated for wireless local loop applications, and are referred to as Land Mobile Radio (LMR). LMR radio links for telephony use frequencies in the UHF/VHF band (138-512 MHz), which provide great propagation characteristics even in difficult terrain and heavy tree density. These frequencies however are becoming very rare. In fact, they are in such demand that the FCC recently mandated radio systems to increase their spectral efficiencies, and use only a narrow band of spectrum. Many legacy LMR equipment using 20-25 kHz RF channels must migrate to narrowband LMR 12.5 kHz channels by January 1, 2013. In addition, the FCC order mentions the goal to reach 6.25 kHz channelization so new WLL systems are urged to deploy these narrow RF channels. 6
Other radio solutions work in the 2.4 GHz and 5 GHz unlicensed bands, building on the popularity and therefore economies of scale of 802.11a/b/g radios. Unfortunately the popularity of these radios for Wi-Fi LAN also creates a lot of interferences, which is a concern when providing emergency service (911 life line). A few systems therefore have a 900 MHz version; although less spectrum is available and less power is allowed, that frequency can be a very useful alternative. Finally, new TV white spaces are a wonderful new opportunity to explore.
In addition to frequencies mentioned above, wireless carriers can use their licensed spectrum to provide fixed applications. Fixed radio links usually behave differently from mobile radio links, they are typically less variable in time (therefore easier to predict and equalize), and their fading statistics are generally easier to deal with. Consequently fixed propagation is usually advantageous for a wireless system. Several important aspects of fixed system should be emphasized.
Mobile communications link are more likely to be obstructed and have a high path loss exponent (see chapter ??; fixed links on the other hand can use elevated antennas in order to establish near line-of-sight with the base station and therefore improve propagation characteristics.
Propagation modeling of a fixed radio link has fundamental differences with that of a mobile link. Wireless propagation models nearly always come from extensive drive testing (hence mobile); collecting fixed data for an empirical model is more difficult: in many cases experimenters present methods to locally average data (over one half of a wavelength) to remove small-scale fading due to multipath. (Small-scale fading is difficult to quantify accurately, and even a large number of fixed data points would provide insufficient sampling to be able to evaluate its impact.) Another important issue is that of antenna beamwidth (or directivity). Mobile data collections are conducted using an omnidirectional antenna (isotropic with respect to azimuth). It has long been known that the antenna beamwidth and more specifically the distribution of angles of arrival with respect to the direction of motion of a mobile are important parameters to quantify the fading of a mobile link .
Consequently fixed data models may differ in some cases from the usual empirical models. Good fixed models would be precious for fixed wireless access, but the current use of mobile models is likely to continue for a number of reasons: first, they provide a good estimate for initial design (site-specific models and simulations are used for more precise predictions); second, some time is necessary to roll-out large fixed wireless systems that can be used and analyzed in order to provide a wide modeling range; lastly, the focus of wireless access mostly remains on mobility.
Fixed links have a few important differences in propagation characteristics, which have a significant impact on reach, capacity, and therefore overall cost of a fixed wireless system.
Fixed wireless links can therefore provide increased reach and capacity than equivalent mobile links. As a result, some of these otherwise costly cellular systems have been used for fixed use, sometimes with minor modifications. In some cases, wireless local loop base stations became handy to deploy in rural areas to provide extended coverage, and reach minimum service mandated by the FCC for PCS spectrum auctions for instance. More recently 3G and 4G systems are advertising their fixed capabilities again and may be trying to compete with other wired broadband services.
Voice over IP (VoIP) is an efficient and widely accepted method of providing telephony. When considering wireless transport, the efficient compression of VoIP is an especially valuable property. Most recent WLL radio solutions therefore use VoIP transport; this is especially convenient as most consumer and enterprise radio solutions are based on IP and Ethernet. Consequently fairly cheap off-the-shelf systems can be adapted to WLL voice and data delivery. The problem remains however to interface these systems with the nearest telephony network. Several architectures are possible for WLL, depending on the location of network elements with voice features.
In most rural areas, a local central office has TDM voice circuits available rather than a VoIP system, so a VoIP gateway is required for WLL purposes. Suppliers of WLL systems often have a VoIP gateway as part of the solution; until recently, these solutions were still difficult to roll-out because of the VoIP gateway cost, and its operations integration. Today small size gateways are available at reasonable prices with good interface standards. Interfaces from the gateway to the switching fabric have to rely on legacy telephony standards. One solution is to connect the VoIP gateway to a telephony CLASS 5 switch via GR-008 or GR-303. These Telcordia standards allow for a gateway to connect to a switch (with one or two T1 lines), and to access class-5 features (such as call waiting, caller ID, 3-way calling, etc.) An alternative solution when GR-008 or GR-303 interfaces are not supported are to simply interface with analog tip and ring lines, but that method has the disadvantage of offering no remote alarming or troubleshooting capability.
The remainder of the voice transport between the voice gateway and the customer end-point follows typical IP transport architectures. Network elements usually interface with Ethernet (10/100 sometimes 1000bT). Many radio systems use a somewhat proprietary physical and MAC layer to insure reliable voice transport, but often these systems are based on Wi-Fi or WiMAX physical layers. A number of protocols are available to establish a reliable IP session that can provide voice transport, including session initiation protocol (SIP), or and Media Gateway Control Protocol (MGCP); ITU recommendation H.323 also provides interoperability standards for multimedia communications over IP including voice features.
Data features are also available on many WLL radios, but are somewhat different. Features like fax and low data rates (up to 56kbps) are fairly simple to add to most WLL, but the task is slightly different when trying to add higher data rates (in the multiple Mbps range). Indeed, higher data rates can no longer interface with the voice switch and need to be split into a data network of its own. If a high-speed internet network is available in the area, data sessions have to be routed to that network while voice traffic needs to be identified as such, and routed towards the VoIP gateway.