Answers for Chapter 5

This page contains the answers to the questionnaire in chapter 5 (Wimax)

All answers have been held as short as possible and require an
understanding and study of the corresponding chapter of the book.

Chapter 5, WiMAX:

Answer 1:
In theory, WiMAX can reach speeds of up to 70 MBit/s per channel. However, such speeds can only be reached between two parties over very short distances and a channel bandwidth of 20 MHz. In practice, however, several users communicate with a single base station from different distances. This means that lower modulation and coding schemes have to be used for distant devices. Furthermore, channel bandwidths of ‘only’ 5 to 7 MHz are used in practice. Taken together, these effects reduce the bandwidth of a channel to a fraction of the theoretical maximum. In practice, a single channel will rarely exceed an overall transmission speed of 16 MBit/s.

Answer 2:
For users close to the base station, transmission conditions are usually excellent and only few bits have to be used for error detection and correction. For distant devices, a much more robust coding scheme has to be used, i.e. more bits are used for this purpose than for the previous case which automatically decreases the achievable throughput.

Answer 3:
Frequency Division Duplex (FDD) allows sending and receiving data at the same time. Different frequencies are used for the uplink and the downlink data path. FDD is usually used when an operator uses licensed spectrum. Unlicensed bands on the other hand usually require the use of Time Division Duplex (TDD) transmission, as most regulators only assign a certain amount of unpaired spectrum for open use. Thus, the same channel has to be used for sending and receiving data.

Answer 4:
The goal of establishing a service flow is to negotiate a number of Quality of Service parameters between the network and the subscriber station to ensure a certain behavior throughout the lifetime of the service flow.

Answer 5:
Transmission power is usually limited for the operation in unlicensed bands to values typically around 0.1 watts. This limits the coverage area of a base station and usually also requires fixed client installations with a directional antenna mounted to a post or to the roof of a clients house. Furthermore, unlicensed bands are open to everyone so there might possibly be interference from other users or applications using the same frequency band.

Answer 6:
Fragmentation and packing is used to decouple the length of a packet on the air interface from the length of an IP packet. Several IP packets can be put in a single packet on the air interface (packing) or  a single large IP packet can be split into several smaller chunks. The combination of fragmentation and packing also avoids filling up the end of an air interface packet with empty bits in case the remaining space is too small for a complete IP packet.

Answer 7:
The wireless LAN 802.11 standard uses a distributed coordination function, i.e. devices check the air interface for traffic and decide on their own when to send their data. WiMAX 802.16 on the other hand has a centralized scheduler in the base station which decides which client devices are assigned bandwidth at specific times.

Answer 8:
The 48 bit MAC address is replaced by the 16 bit Connection ID (CID) in MAC packets. As both IDs identify the sender and receiver it is more economical to use this CID.

Answer 9:
Establishing a connection to the network consists of the following steps: Synchronization, Ranging, Capability Information Exchange, Authentication, Network Registration and Service Flow creation.

Answer 10:
In a meshed network architecture, client devices can act as relays for packets to and from distant devices. This extends the range of the network or reduces the number of connections to the core network.

Answer 11:
There are two main advantages of adaptive antenna systems: Signal energy can be focused in the direction of a subscriber which can either extend the coverage area of a base station or increase the bandwidth for a subscriber. It is also possible to send different data streams to subscribers in different directions at the same time as the signals are directed into the specific directions of the subscribers. This way the overall bandwidth of a cell can be increased.

Answer 12:
Other radio networks such as GSM, GPRS and UMTS have three network elements in the radio network: The base station (Node-B), the base station controller (RNC) and the SGSN which acts as the gateway between the radio network and the core network. WiMAX goes one step further and puts part of the RNCs responsibility into the base station and another part into the core network gateway. This reduces the number of entities in the radio network and gives greater autonomy to the base stations.

Answer 13:
Fast Base Station Switching is a form of handover. When used, the network and the device maintain an active set list which contains several base stations that can be used by the mobile station for communication. Only one base station is used for transmission at a time. As all cells in the active set list are prepared to communicate with the subscriber, changes between the different cells can be performed very quickly.

Answer 14:
MIMO is the abbreviation for multiple input, multiple output. This method exploits the fact that a signal usually reaches the other side from different angles, i.e. via different paths. By using multiple antennas at both the sender and the receiver side, different data streams can be sent over the different paths simultaneously which increases the overall data rate of the transmission.