Chapter 4: Frequency Utilisation and System Profiles

Chapter 4: Frequency Utilisation and System Profiles

4.1 The Cellular Concept

The global objective of a wireless network is rather simple: to connect wireless users to a core network and then to the fixed network. Figure 4.1 illustrates the principle of a Public Land Mobile Network (PLMN), as defined for second-generation GSM networks.

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Figure 4.1: Illustration of a Public Land Mobile Network (PLMN) offering a cellular service

The first wireless phone systems date back as far as the 1930s. These systems were rather basic and had very small capacity. The real boost for wireless networks came with the cellular concept invented in the Bell Labs in the 1970s. This simple but also extremely powerful concept was the following: each base station covers a cell; choose the cells small enough to reuse the frequencies (see Figure 4.2). Using this concept, it is theoretically possible to cover a geographical area as large as needed!

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Figure 4.2: The cellular concept. simple and so powerful!

The cellular concept was applied to many cellular systems (mostly analogue) defined in the 1980s: AMPS (US), R2000 or Radiocom 2000 (France), TACS (UK), NMT (Scandinavian countries), etc. These systems being incompatible, a unique European cellular system was invented, GSM, which is presently used all over the world.

WiMAX applies the same principle: a BS covers the SSs of its cell. In this section, some elements of the cellular concept theory needed for WiMAX dimensioning are provided. First sectorisation is reminded.

4.1.1 Sectorisation

A base station site represents a big cost (both as investment, CAPEX, and functioning, OPEX) for a network operator. Instead of having one site per cell, which is the case for an omnidirectional BS, trisectorisation allows three BSs to be grouped in one site, thus covering three cells (see Figure 4.3). These cells are then called sectors. Three is not the only possibility. Generally, it is possible to have a sectorisation with n sectors. Yet, for practical reasons, trisectorisation is very often used. Sectorisation evidently needs directional antennas. Trisectorisation needs 120° antennas (such that the three BSs cover the 360°).

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Figure 4.3: Omnidirectional antennas and trisectorisation

For economical reasons, sectorisation is almost always preferred to omnidirectional antennas for cellular networks unless in rare specific cases, e.g. for very large cells in very low populated geographic areas. WiMAX is not an exception as sectorisation is also recommended.

4.1.2 Cluster Size Considerations

Cellular networks are based on a simple principle. However, practical deployment needs a complicated planification in order to have high performance, i.e. great capacity and high quality. This planification is made with very sophisticated software tools and also the ‘know-how’ of radio engineers.

Frequency reuse makes room for an interference that should be kept reasonably low. As illustrated in Figure 4.4, in the downlink (for example) and in addition to its useful signal (i.e. its corresponding BS signal), an SS receives interference signals from other BSs using the same frequency. Signal-to-Noise Ratio (SNR) calculations or estimations are used for planification. The SNR is also known as the to-Carrier-Interference-and-Noise Ratio (CINR). The term SNR is used more for receiver and planification considerations, while CINR is used more for practical operations. In this book, SNR and CINR represent the same physical parameter. An appropriate cellular planification is such that the SNR remains above a fixed target value (depending on the service, among others) while maximising the capacity.

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Figure 4.4: Illustration of a useful signal and interference signal as used for SNR calculations

A parameter of cellular planification is the cluster size. A cluster is defined as the minimal number of cells using once and only once the frequencies of an operator (see Figure 4.5). It can be verified that having a small cluster increases the capacity per cell while big clusters decrease the global interference and then represent high quality. The choice of the cluster size must be done very carefully. In the case of GSM networks, the value of the cluster size was initially 12 or 9. With time and due to different radio techniques (such as frequency hopping, among others), the cluster size of the GSM may be smaller (down to 3, possibly).

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Figure 4.5: Example of a (theoretical) regular hexagonal network. Cluster size = 3. In each cluster all the operator frequencies are used once and only once

The regular hexagonal grid (as in Figure 4.5) is a model that can be used for first estimations. For this model, a relation can be established between the minimal SNR value of the network and the cluster size [9]:

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where n is the cluster size and α a value depending on the radio channel (of the order of 4). This is only an approximated formula used for a nonrealistic channel (too simple) and cell shapes used only for a first estimation of n. Practical deployment uses more sophisticated means but still the minimal SNR (used for cells planification or dimensioning) increases with n.

What about WiMAX frequency reuse? WiMAX is an OFDM system (while GSM is a Single Carrier, SC, system) where smaller cluster sizes can be considered. Cluster size values of 1 or 3 are regularly cited. On the other hand, it seems highly probable that sectorisation will be applied. This gives the two possible reuse schemes of Figure 4.6. Consequently, an operator having 10.5 MHz of bandwidth will have 3.5 MHz of bandwidth per cell (respectively 10.5 MHz) if a cluster of 3 (respectively 1) is considered. Yet, it is not sure that a cluster of 1 will lead to a higher global capacity. With a cluster of 1, reused frequencies are very close and then the SNR will be (globally) lower, which means less b/s Hz if link adaptation is applied (as in WiMAX, see Chapter 11). The choice of cluster size is definitely not an easy question.

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Figure 4.6: Possible frequency reuse schemes in WiMAX

Other cellular frequency reuse techniques can be also be used. Reference [10] mentions fractional frequency reuse. With an appropriate subchannel configuration, users operate on subchannels, which only occupy a small fraction of the whole channel bandwidth. The subchannel reuse pattern can be configured so that users close to the base station operate with all the subchannels (or frequencies) available, while for the edge users, each cell (or sector) operates on a fraction of all subchannels available. With this configuration, the full-load frequency reuse is maintained for centre users in order to maximise spectral efficiency and fractional frequency reuse is implemented for edge users to assure edge user connection quality and throughput. The subchannel reuse planning can be dynamically optimised across sectors or cells based on network load and interference conditions on a frame-by-frame basis. Figure 4.7 shows an example of operating frequencies for each geographical zone in a fractional frequency reuse scheme. In the OFDMA PHYsical Layer (Mobile WiMAX), flexible subchannel reuse is facilitated by the subchannel segmentation and permutation zones. A segment is a subdivision of the available OFDMA subchannels (one segment may include all subchannels).

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Figure 4.7: Example of operating frequencies for each geographical zone in a fractional frequency reuse scheme. The users close to the base station operate with all subchannels available. (Based on Reference [10])

The tools for efficient radio resource use and other radio engineering considerations for WiMAX are described in Chapter 12.

4.1.3 Handover

Handover operation (sometimes also known as ‘handoff’) is the fact that a mobile user goes from one cell to another without interruption of the ongoing session (whether a phone call, data session or other). Handover is a mandatory feature of a cellular network (see Figure 4.8). Many variants exist for its implementation. Each of the known wireless systems have some differences. WiMAX handover is described in Chapter 14.

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Figure 4.8: Illustration of handover in a cellular network

[9]Lee, W. C. Y., Mobile Cellular Telecommunications: Analog and Digital Systems, McGraw Hill, 2000.

[10]WiMAX Forum White Paper, Mobile WiMAX - Part I: a technical overview and performance evaluation, March 2006.