Networking Tips Tricks

split-mount installation with single-point building ground

A GPS unit is required for the synchronization of the TDD radio network. With the Time Division Duplex TDD operation each sector should transmit and receive at the same time, to avoid intra-cell and inter-cell interference. The GPS unit must be installed outdoors. The receiver’s antenna should be installed somewhere on the supporting structure without any obstructions on the way to the open sky. For details about the installation of the GPS unit, please refer to the guidelines of the GPS manufacturer as there is difference in mounting assembly of different manufacturers. Figure shows standard guidelines for GPS installation.

Hybrid ARQ A Hybrid ARQ system consists of combination of error control mechanism namely Forward Error Correction and Automatic Repeat Request. The function of FEC is to reduce the frequency of retransmission by correcting error pattern which occur most frequently. This increases the system throughput. In case of less frequent error occurrences which can be detected but not corrected, the receiver requests for retransmission. This in turn increases the reliability. Thus a proper combination of FEC and ARQ provides higher reliability than an FEC system alone and higher throughput than the system with only ARQ as error control mechanism. Hybrid ARQ is an important dynamic tool for maintaining QoS in modern wireless systems. Hybrid ARQ involves an encoded forward link for error correction and detection and a feedback link for possible retransmission. At the transmitter, parity bits are added to the data block to detect and correct errors. In case the receiver is not able to correct these ...

Automatic Repeat Request (ARQ) A Layer-2 sliding-window based flow control mechanism.  Per connection basis.  Only effective to non-real-time applications.  Uses a 11-bit sequence number field.  Uses CRC-32 checksum of MAC PDU to check data errors.  Maintain the same fragmentation structure for Retransmission.  Used to detect errors in an OFDM symbol.  Usually used with FEC. Introduction Automatic Repeat Request(ARQ) is one of the most widely used technique for error control in data communication, the other being Forward error correction? (FEC). ARQ is not meant for correcting errors, but using this technique, the receiver requests for repeated transmission of errored codewords until they are received error-free. ARQ Methodology The MAC SDUs received are fragmented or packed into required block size. The blocks are then transmitted at the receiver end the block received are checked for errors. If the received block is error free, ACK is sent to the tra...

Fixed WiMAX (802.16d) MAC PDU Transmission MAC PDUs are transmitted in PHY bursts.  A single PHY burst can contain multiple.  Concatenated MAC PDUs.  The PHY burst can contain multiple FEC blocks.  MAC PDUs may span FEC block boundaries.  The TC layer between the MAC and the PHY allows for capturing the start of the next MAC PDU in case of erroneous FEC blocks. q

IEEE(802.16d) MAC Addressing & Identifiers SS has 48-bit IEEE MAC address  BS has 48-bit base station ID  Not a MAC address  24-bit operator indicator  16-bit connection ID (CID)  32-bit service flow ID (SFID)  16-bit security association ID (SAID)

Fixed WiMAX (802.16d) MAC Overview Point-to-Multipoint.  Metropolitan Area Network.  Connection-oriented.  Supports difficult user environments.  High bandwidth, hundreds of users per channel.  Continuous and burst traffic.  Very efficient use of spectrum.  Protocol-Independent core (ATM, IP, Ethernet, …)  Flexible QoS offerings.  Supports multiple 802.16 PHYs. q

Fixed WiMAX (802.16d) PHY - FDD Uplink & Downlink Channels are on separate frequency.  Downlink is transmitted in bursts  Simultaneously support both Full Duplex & Half Duplex SS's.  Downlink Carrier may be continuous

Fixed WiMAX (802.16d) PHY - TDD Uplink & Downlink transmission shares the same frequency.  Uplink & Downlink transmission are separated in time.  TDD Frame has a fixed duration.  A TDD frame contains one uplink & one downlink subframe.  TDD Framing is adaptive i.e capacity allocated to downlink versus uplink may vary.  Tx/Rx Transition Gap (TTG) between downlink burst & subsequent uplink burst allows time for BS to switch from Tx mode to Rx mode and SS to Rx mode to Tx mode.  BS looks for first symbols of uplink burst after TTG.  Rx/Tx Transition Gap (RTG) between uplink burst & subsequent downlink burst allows time for BS to switch from Rx mode to Tx mode and SS to Tx mode to Rx mode.  SS looks for first symbols of modulated data in downlink burst after RTG

Fixed WiMAX (802.16d) PHY - Framing Frame consist of Downlink Subframe & Uplink Subframe Downlink Subframe begins with information necessary for frame synchronization and control. In TDD, Downlink Subframe comes first followed by Uplink Subframe. In FDD, Uplink transmission concurrently with Downlink frame. 5, 10, 20ms frame duration are the typical frame durations supported

Fixed WiMAX (802.16d) PHY - Overview The purpose of the PHY is the physical transport of data  Framing mechanism supports adaptive burst profiling.  TDD & FDD Duplexing configuration are supported for efficient spectrum usage.  Modulation & coding may be adjusted individually to each SS on frame by frame basis.  FDD supports full duplex SSs as well as half duplex SSs  Downlink PHY includes Transmission Convergence Sublayer that inserts a pointer at beginning of payload to help receiver to identify the beginning of a MAC PDU.  Data bits coming from Transmission Convergence Sublayer are randomized, FEC encoded, and map to QPSK, 16QAM or 64QAM constellation.

Reference Model

Broadband Wireless Access Technologies  Local Multipoint Distribution Services (LMDS)  Multi-channel Multipoint Distribution Services (MMDS)  CDMA EvDO/EvDV  W-CDMA  TD-CDMA  WiMAX What is WiMAX? Worldwide Interoperability for Microwave Access (WiMAX) is the common name associated to the IEEE 802.16a/d/e standards that originally covered the Wireless Local Loop technologies with radio spectrum from 10 to 66 GHz. Why WiMAX? Standard based  Lower Cost  Ensures compatibility and interoperability  Better Performance & Coverage

 Ways of providing Broadband Access? Copper Wire (DSL)  Optical Fiber Cable  Wireless Access System.  Satellite Communication System  Why Broadband Wireless Access ? Lower Capital Expenditure  Lower Operational Expenditure  Faster Deployment Time  Scalable & Easy Relocation

Q: What is Broadband? Ans: Broadband comes from the words "broad bandwidth" and is used to describe a high-capacity, two-way link between an end user and access network suppliers capable of supporting full-motion, interactive video applications.  A way of transmitting large amounts of data, voice, and video that is greater than telephony networks. A term for high-speed, high-capacity Internet and data connections.  It is type of data transmission in which a single medium can carry several channels at once including voice, data and video simultaneously. It is also a general term for different types of high-speed, high-bandwidth connections to the Internet, including Cable and DSL.  Broadband" refers to a type of network connection that supports a very high bit rate, as opposed to "narrowband," which supports a lower bit rate. The higher the bit rate, which is a measure of speed of transmission of bits per second (bps), the faster the transmission will occur in a...

16-QAM,64-QAM The number before QAM (for example 16QAM) refers directly to the number of decision points in the constellation. Meaning, 256QAM has 192 more decision points than 64QAM. The number before QAM is also always a power of two. (EG. 2^6=64QAM)  There are several trade offs to this:  The more Decision points, the more throughput per channel.  The more Decision points, the more susceptible the channel is to noise (as the decision boundaries get smaller to accommodate more decision points, it becomes harder to distinguish which boundary the point was intended to lie in).  More Decision points=More Bandwidth=Higher susceptibility to noise or poor signal (the same applies inversely as well).  At best, it'll enable either to:  Deliver the same number of customers incrementally faster speeds.-or-  Deliver a larger number of customers the same speeds as before.

Abbreviations used in WiMAX Section AAA Authentication, Authorization and Accounting AAS Adaptive Antenna System also Advanced Antenna System ACK Acknowledgement AMC Adaptive Modulation and Coding ASN Access Service network ASP Application Service Provider BS Base Station BWA Broadband Wireless Access CC Convolutional Coding CP Cyclic Prefix CQI Channel Quality Information CSN Connectivity Service Node CTC Convolutional Turbo Coding DHCP Dynamic Host Configuration Protocol, typically used to assign IP addresses DL Down Link DVB/DAB Digital Video Broadcast/ Digital Audio Broadcast FA Foreign Agent (MIP) FCH Frame Control Header FDD Frequency Division Duplex FFT Fast Fourier Transform FUSC Full Use of Sub-Channels GW Gateway HA Home Agent (MIP) HARQ Hybrid Automatic Repeat reQuest HHO Hard Hand-Off IEEE Institute of Electrical and Electronics Engineers IFFT Inverse Fast Fourier Transform ISI Inter-Symbol...

OFDMA TDD Frame Structure OFDMA is two-dimensional with users sharing in both the time and frequency domains. This allows scheduling and optimum use of finite spectrum. Each user is assigned a burst area that has dimensions of sub-channels and symbols. Key Elements Preamble is broadcast for one symbol period and allows the user devices to acquire the system and synchronize. A known PN code is transmitted. FCH reports the length of the DL-Map. DL-Map contains bandwidth allocation for users and location of the UL-Map. UL-Map contains bandwidth allocation of the UL for the next frame. Both maps contain burst data regions, modulation, and coding type for the user. Allocated regions in UL are available for random access, CQI, and ACKs. Transmit Recieve Transition Gap (TTG) and Receiver Transmit Transition Gap (RTG) are guard times between the transmit and receiver portions of the frame.

Cyclic Prefix (CP) Cyclic prefix (CP) mitigates multipath fading and inter-symbol interference (ISI) at the price of increasing bandwidth. Delay spread exceeds symbol time. ISI is the result. Separate the symbols in time by adding a gap. Transmission must be continuous. To “fill” the gap, append data from the end of the symbol to the beginning of the symbol. CP allows the system to ignore the initial part of each symbol thus avoiding the area that would be most likely impacted by multipath delay. Data in the CP region of the signal is discarded. CP is set to 4-6 times the delay spread. WiMAX Forum profiles use a CP of 1/8, meaning that a section of data equal to 1/8 of the original symbol is used.