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
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
Figure 3-2. 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
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
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 3-3 shows an example core network.
Figure 3-3. 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)
Core network functionality is an involved topic; for details,
3.2.3 Multiple Access
The three primary technologies used in wireless cellular communication in order to separate the user channels are
Figure 3-4 uses the analogy of a room as a transmission resource to
Figure 3-4. 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
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 3-5 shows the N = 7 pattern reuse.
Figure 3-5. N = 7 frequency reuse pattern.
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
Channel coding is a technique that aims to improve transmission quality when the signal encounters disturbances for reasons such as noise when the
Channel coding can be
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:
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
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.