3.2 Cellular Fundamentals
Some of the basic concepts of cellular telephony include frequency reuse, multiple access techniques, speech coding, mobility, ciphering and authentication and network planning. Cellular networks can also be considered from the perspective of being divided into the radio access network (RAN) and the core network (CN). These are discussed in the following sections.
3.2.1 Radio Access Network
The radio access network comprises of the base transceiver stations (BTSs) and the controller element, which is called the base station controller (BSC). The BTSs are basically the radio elements (RF equipment) on the network side. Mobile terminals connect to the network via the BTSs. The BTS transmits system information over channels defined for broadcastting network specific information, and mobile stations tune in to these channels before performing access functions. A BTS is connected to a cell site, which hosts antennas atop towers or buildings. Cell sites can be of type macro, micro, or pico depending on the coverage radius. The size of a cell site is dependent on the transmit power level of the BTS. Figure 32 shows a generic radio access network.
Figure 32 Radio access network.
The radio access network is the largest component of the mobile network, and a large number of base stations and cell sites are provisioned in order to provide coverage. Nationwide coverage of mobile networks requires the deployment of thousands of BTSs (coverage of the United States for example). The BTSs provide the channels for use on a dynamic basis to subscribers. Traffic and control channels are defined for the air interfaces depending on the type of technology used. The BTSs are controlled by the base station controller. So from a relationship perspective, a single BSC controls many BTSs. The BSC is responsible for managing the radio resources at the BTSs. The BSC assigns channels to subscribers on a need basis. In addition, it is constantly aware of a mobile station's location and the state that it is in. It measures the signal strength (with the assistance of the BTS and the MS) and makes handoff decisions. In the case of CDMA networks, BSCs are also responsible for performing the macro- diversity-combining function required in spread spectrum systems. In addition, the speech coding function may be incorporated into the BSC in some cases.
BSCs are connected to the BTSs over a wireline network using T1s and E1s. T1s and E1s are physical layer transmission technologies that are widely deployed by telecom operators. T1 is able to multiplex voice and data together in 24 user slots within a frame, as compared to E1, which has 30 user slots within each frame. Microwave links are also used for these connections. BTSs are normally deployed at the cell sites itself and hence are spread out geographically. The network connecting the BTSs to the BSC is referred to as a backhaul network. The BSC is normally at a central location such as a central office. The cost of connecting a large number of BTSs to the BSC is a major expense in radio networks.
3.2.2 Core Network
The core network consists of the mobile switching centers (MSCs), the home location register (HLR), visitor location register (VLR), authentication center (AUC), billing servers, operation and support systems (OSS), short message service centers (SMSC), and many other elements. The interface to the public switched telephony network (PSTN) and the packet data network (PDN) is from the MSC in the core network.
The subscriber profile and the services that the subscriber is allowed to access are inserted in the HLR. The HLR is also aware of the mobile station's current location. The BSC interfaces to the core network via the MSC. A single MSC can be serving more than one BSC. Mobility management as well as communication with the HLR, VLR, and authentication centers is done via mobility application protocols such as GSM MAP or IS-41. The core network elements are connected to each other via a signaling system 7 (SS7) network, which provides the transport for signaling messages. The MSC also provides call control and switching functionality. Supplementary services, such as three-way calling and call barring, are also supported by the MSC.
For data services the core network hosts the SMSC as well as modem pools for circuit switched data. The core network is also responsible for authenticating the subscribers before allowing access to the network or access to services. Figure 33 shows an example core network.
Figure 33 Core network.
The Interworking Function (IWF) enables circuit-switched data services in wireless networks. It consists of a modem pool and interfaces to the packet data network such as an ISP. Circuit-switched data in GSM networks is explained in further detail in Chapter 4.
A network operations center (NOC) manages the RAN and the core. An operations support system (OSS) is an element of a telecommunications network that supports the daily operation of the infrastructure. The OSS includes network management equipment, which monitors the state of the network. It also includes billing systems that are responsible for capturing the network usage by subscribers. Call data records (CDRs), which are used to bill the subscriber, are generated based on information received from the MSC by the billing systems.
Core network functionality is an involved topic; for details, please refer to texts that discuss this in detail.
3.2.3 Multiple Access
Any scarce resource that is to be used simultaneously by more than one user needs to be divided into subportions in order to prevent interference in each user's usage of that resource. In telecommunications that resource is a transmission medium and is divided into channels in order to allow multiple users to access the same transmission medium simultaneously. This simultaneous use of channels is called multiple access. A channel can be defined as an individually assigned, dedicated pathway through a transmission medium for a single user's information. The physical medium of transmission, which in our case is the wireless spectrum, can be divided into individual channels based on a set of criteria. These criteria depend on the technology that is utilized to make the distinction between channels.
The three primary technologies used in wireless cellular communication in order to separate the user channels are
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
Figure 34 uses the analogy of a room as a transmission resource to illustrate these technologies.
Figure 34 FDMA, TDMA, and CDMA techniques.
In FDMA, the channel is a specific frequency, and each user is assigned a different frequency for the duration of the call. In our room analogy, this is equivalent to partitioning the room and placing users who wish to communicate in each partition. However, due to human speech characteristics, a significant portion of the time that resource is not utilized. In other words, no information is being transmitted. This exclusive allocation results in poor resource utilization.
In TDMA, the channel is a time slot on a specific frequency, and each user is assigned a different time slot on a specific frequency. In our room analogy, this is equivalent to allowing more than one pair of uses who wish to communicate in each partition and limit the time a pair of users can communicate without interruption. Each pair then takes turns to communicate within their allocated time period and then waits till their next turn. In TDMA, this switching between users happens so quickly that the users never perceive that they are sharing their assigned frequency with others.
In CDMA, the channel is a unique code, and each user is assigned a different code. In the room analogy, this is equivalent to breaking down the partitions and allowing all users who wish to communicate to have a conversation simultaneously. However, there is a caveat; each one of these users has to use a different language and each user has highly evolved ears that can tune out conversations that are in a language other than the one that the user understands. Thus each pair of users is able to use the room simultaneously to have a conversation without interrupting other users.
3.2.4 Frequency Reuse
Cellular systems utilize the concept of frequency reuse to provide higher capacity. The core concept of cellular systems is to reuse the same frequency in a network many times over. The ability to reuse the same radio frequency many times is a result of managing the carrier to interference signal levels (C/I). A specific radio frequency is transmitted from one base station at a power level that supports communication within a moderate cell radius. Since the power limit is controlled to serve a limited range, the same frequency can be transmitted simultaneously or reused by another base station as long as there is no interference between it and any other base station using the same frequency.
Several frequency reuse patterns are currently in use in the cellular industry. Each has its own pros and cons. The most commonly used patterns in cellular are the N = 4 and N = 7 patterns. The frequency repeat pattern determines the maximum number of radios that can be assigned to a single cell site. The N = 4 pattern can deploy cell sites with six sectors, whereas the N = 7 pattern uses a three-sector cell. Figure 35 shows the N = 7 pattern reuse.
Figure 35 N = 7 frequency reuse pat
3.2.5 Speech and Channel Coding
Speech coding is critical to digital transmission systems, and the main use for speech coding has been in wireless networks. The wireline network uses digital pulse code modulation (PCM) at 64 Kbps for voice transmission. Speech synthesis systems such as linear predictive coding (LPC) predict the current sample from a linear combination of past samples. At the expense of poor tone quality, they do achieve high efficiency. The adaptive differential PCM (ADPCM) technique is an alternate method of predicting a speech waveform from past samples.
Another class of speech coding is via algorithms termed vocoders. Vocoders are relatively complex systems and operate at low bit rates (normally 2.4 Kbps). Residual excited linear coding (RELP) is a hybrid coding scheme for wireline quality speech with a few integrated digital speech processors. CDMA uses a variation of RELP called code-excited linear prediction (CELP). The GSM speech coding scheme is based on regular pulse excitationlong-term prediction (RPELTP) at 13 Kbps. Enhanced variable-rate codec (EVRC) is yet another coding scheme that has higher voice quality.
Channel coding is a technique that aims to improve transmission quality when the signal encounters disturbances for reasons such as noise when the reception level is low, intereference, and multipath propogation. The side effect of this is that there is an increase in the number of bits transmitted to compensate for errors. Coding consists of adding some redundant data, which is calculated from the source information. The decoding function makes use of this redundancy to detect the presence of errors or estimate the most probable emitted bits given the received ones.
Channel coding can be classified into block codes and convolutional codes. The codes used in GSM are block convolutional codes; a fire code, which is a conventional linear binary block code; and parity codes, which are linear block codes. CDMA IS-95 systems use convolutional code based on the Viterbi algorithm.
Mobility is one of the key factors of wireless networks. It allows users freedom of movement. Depending on the radio technology, mobility can be either limited to pedestrian speeds only or can support communication even at speeds up to 120 Kmph. However, mobility places a few requirements on the network:
They must have the ability to locate subscribers.
They must monitor the movement of the subscribers.
They must enable handoffs seamlessly as the user moves across cells while sessions are kept alive.
The two key concepts of mobility are roaming and handovers.
Roaming can be defined as the movement of the mobile terminal from one network to another. Network operators have coverage that is either limited in scope or is limited to a country. In order to support global mobility, network operators agree to allow subscribers from other networks to roam into their networks and access services. Roaming agreements between operators enable subscribers to roam on a global basis while being reachable all the time.
Handover is the process of switching a call or session that is in progress from one physical channel to another. Handovers can be classified into intracell and intercell. Intracell handover is the transfer of a call in progress from a channel in one cell to another channel in the same cell. Intercell handover is the transfer of the call or session to another cell.
CDMA systems are considered as make-before-break systems since the characteristics of spread spectrum allow the system to be connected simultaneously to two or more base stations. In contrast, TDMA systems from a handover perspective are termed break-before-make networks. CDMA also classifies handoffs into soft handoffs, softer handoffs, and hard handoffs.