Quality of Service (QOS)

QOS refers to meeting certain requirement - e.g. throughput, packet error rate, delay, and jitter - associated with given application. Broadband wireless networks must support a variety of applications, such as voice, data, video, and multimedia, and each of these has different traffic patterns and QoS requirements. The variability in the QoS requirements across applications, services, makes it a challenge to accommodate all these on a single-access network, particularly wireless networks, where bandwidth is at a premium.

The problem of providing QoS in broadband wireless systems is one of managing radio resources effectively. Effective scheduling algorithms that balance the QoS requirements of each application and user with the available radio resources need to be developed. In other words, capacity needs to be allocated in the right proportions among users and applications at the right time. This is the challenge that the MAC-layer protocol must meet: simultaneously handling multiple types of traffic flows - bursty and continuous, varying throughputs and latency requirements. Also needed are an effective signaling mechanism for users and applications to indicate their QoS requirements and for the network to differentiate among various flows.

Before any data transmission happens, the BS and the MS establish a unidirectional logical link, called a connection, between the two MAC-layer peers. Each connection is identified by a connection identifier (CID), which serves as a temporary address for data transmissions over the particular link.

WiMAX also defines a concept of a service flow. A service flow is a unidirectional flow of packets with a particular set of QoS parameters and is identified by a service flow identifier (SFID). The QoS parameters could include traffic priority, maximum sustained traffic rate, maximum burst rate, minimum tolerable rate, scheduling type, ARQ type, maximum delay, tolerated jitter, service data unit type and size, bandwidth request mechanism to be used, transmission PDU formation rules, and so on. These parameters are managed using the DSA and DSC messages. The base station is responsible for issuing the SFID and mapping it to unique CIDs.

To support a wide variety of applications, WiMAX defines five scheduling services:

1. Unsolicited grant services (UGS): This is designed to support fixed-size data packets at a constant bit rate (CBR). Examples of applications that may use this service are T1/E1 and VoIP.

2. Real-time polling services (rtPS): This service is designed to support real-time service flows, such as MPEG video, that generate variable-size data packets on a periodic basis.

3. Non-real-time polling service (nrtPS): This service is designed to support delay-tolerant data streams, such as an FTP, that require variable-size data grants at a minimum guaranteed rate.

4. Best-effort (BE) service: This service is designed to support data streams, such as Web browsing, that do not require a minimum service-level guarantee.

5. Extended real-time polling service (ErtPS): This service is designed to support real-time applications, such as VoIP with silence suppression, that have variable data rates, but require guaranteed data rate and delay. This service is defined only in IEEE 802.16e-2005, not in IEEE 802.16-2004.

Network Entry and Initialization

When an MS acquires the network after being powered up a WiMAX network undergoes various steps. An overview of this process, also referred to as network entry, is shown in Figure

 

- Scan and Synchronize Downlink Channel:

When an MS is powered up, it first scans the allowed DL frequencies to determine whether it is presently within the coverage of a suitable WiMAX base station. Each MS stores a nonvolatile list of all operational parameters, such as the DL frequency used during the previous operational instance. The MS first attempts to synchronize with the stored DL frequency. If this fails, the MS it scans other frequencies in an attempt to synchronize with the DL of the most suitable BS.

During the DL synchronization, the MS listens for the DL frame preambles. When one is detected, the MS can synchronize itself with respect to the DL transmission of the BS. Once it obtains DL synchronization, the MS listens to the various control messages, such as FCH, DCD, UCD, DL-MAP, and UL-MAP, that follow the preamble to obtain the various PHY and MAC related parameters corresponding to the DL and UL transmissions.

- Obtain Uplink Parameters:

Based on the UL parameters decoded from the control messages, the MS decides whether the channel is suitable for its purpose. If the channel is not suitable, the MS goes back to scanning new channels until it finds one that is. If the channel is deemed usable, the MS listens to the UL MAP message to collect information about the ranging opportunities.

- Ranging:

At this stage, the MS performs initial ranging with the BS to obtain the relative timing and power-level adjustment required to maintain the UL connection with the BS. Once the a UL connection has been established, the MS should do periodic ranging to track timing and power-level fluctuations. These fluctuations can arise because of mobility, fast fading, shadow fading, or any combinations thereof. Since the MS does not have a connection established at this point, the initial ranging opportunity is contention based.

MS sends a RNG-REQ message with the CID set to initial ranging CID. If it does not receive any response from the BS within a certain time window,
then the MS considers the previous ranging attempt to be unsuccessful and enters the contention-resolution stage. Therein, the MS sends a new CDMA ranging code at the next ranging opportunity, after an appropriate back-off delay. If Ranging process is successful then, BS sends RNG-RSP message with basic CID(BCID) and Primary CID(PCID) allocated to perticular MS. From here on, the basic and primary management CID is used by the MS and the BS to send most of the MAC management messages

- Negotiate Basic Capabilities:

After initial ranging, the MS sends an SBC-REQ message informing the BS about its basic capability set, which includes various PHY and bandwidth-allocation-related parameters. On the reciept of this message, the BS responds with an SBC-RSP, providing the PHY and bandwidth-allocation parameters to be used for UL and DL transmissions.

- Register and Establish IP Connectivity:

After negotiating the basic capabilities and exchanging the encryption key, the MS registers itself with the network. In WiMAX, registration is the process by which the MS is allowed to enter the network and can receive secondary CIDs. The registration process starts when the MS sends a REG-REQ message to the BS. The message contains a hashed message uthentication code (HMAC), which the BS uses to validate the authenticity of this message. Once it determines that the request for registration is valid, the BS sends to the MS a REG-RSP message in which it provides the secondary management CID. In the REG-REQ message, the MS also indicates to the BS its secondary capabilities not covered under the basic capabilities, such as IP version supported, convergence sublayer supported, and ARQ support.

After receiving the REG-RSP message from the BS, the SS can use DHCP to obtain an IP address.

- Establish Service Flow:

The creation of service flows can be initiated by either the MS or the BS, based on whether initial traffic arrives in the uplink or the downlink.

When it an MS chooses to initiate the creation of a service flow, an MS sends a DSA-REQ message containing the required QoS set of the service flow. On receipt of the DSA-REQ message, the BS first checks the integrity of the message and sends a DSX-RVD message indicating whether the request for a new service flow was received with its integrity preserved. Then the BS checks whether the requested QoS set can be supported, creates a new SFID and sends an appropriate DSA-RSP indicating the admitted QoS set. MS completes the process by sending a DSA-ACK message.

MAC PDU Construction

Diagram shows an example of MAC PDU construction.

As shown multiple SDUs can be packed in a single MAC PDU or a single SDU can be fragmented in multiple MAC PDUs. Packing (PSH) and Fragmentation (FSH) in the PDU can be indicated using 6 bit TYPE filed in GMH. Blocks of these packed or fragmented SDUs are assined a unique 3bit or 11bit Block Sequence Number (BSN).

Generic MAC Header

The primary task of the WiMAX MAC layer is to provide an interface between the higher transport layers and the physical layer. The MAC layer takes packets from the upper layer - these packets are called MAC service data units (MSDUs) and organizes them into MAC protocol data units (MPDUs) for transmission over the air. For received transmissions, the MAC layer does the reverse.

MAC PDU construction topic shows an example of MAC PDU frame.

Each MAC PDU contains 3 components: GMH, which contains frame control information, variable length frame body and 32-bit CRCMAC PDUs are transmitted in PHY burst. A singal PHY burst contains multiple concatenated MAC PDUs.

Generic MAC Header fields (GMH):
If Header Type (HT) field is '0' then it is GMH header.
CI - CRC indication bit
CID - 16 bit connection identifier
EC - Encryption control bit
EKS - Encryption key sequence
ESF - Extended subheader indication bit
HCS - 8 bit Header Check Sum
HT - Header Type, shall be '0'
Len - 11 bit length of PDU
Type - 6 bit special Payload Type (PSH, FSH, ARQ feedback etc...)

Bandwidth Request fields:
This PDU is applicable to UL only.
It does not contain any payload and should not be encrypted.

BR - 19 bit Bandwidth request
HT - Header Type, shall be '0'
EC - always set to zero
CID - 16 bit connection identifier
HCS - 8 bit Header Check Sum
Type - 3 bit type of BR header

WiMax Frame Structure

WiMax PHY frame consist of DL subframe, UL subframe, TTG and RTG (in TDD system).DL subframe is the place where Base station send downlink data to mobile stations. UL subframe is the place where mobile stations sends uplink data to base station.Transmit Time Gap (TTG) and Receive Time Gap (RTG) is the guard time between DL and UL subframe respectively.

DL Subframe begins with the preamble, which is used for PHY layer procedures, such as time ans frequency synchronization and initial channel estimation. Preamble occupies first symbol of the DL subframe. If the vehicle is moving very fast then there will be some time variation and b'coz of that there will be sync loss, to recover this time variation, a short midamble can be inserted within a frame. It is estimated that having a midamble every 10 symbols allows mobility up to 150 kmph.

First zone in DL and UL subframe must be PUSC.

The downlink preamble is followed by a frame control header (FCH), which provides frame configuration information, such as the MAP message length, the modulation and coding scheme, and the usable subcarriers.

Multiple users are allocated data regions within the frame, and these allocations are specified in the uplink and downlink MAP messages (DL-MAP and UL-MAP) that are broadcast following the FCH in the downlink subframe. MAP messages include the burst profile for each user, which defines the modulation and coding scheme used in that burst. Since MAP contains critical information that needs to reach all users, it is often sent over a very reliable link, such as BPSK with rate 1/2 coding and repetition coding.

FCH, DLMAP and ULMAP must be sent in PUSC zone

UL subframe consist of several UL bursts, RNG channel, HARQ ACK channel and CQI channel.RNG, ACK and CQI must fall in PUSC zone.

RNG channel is used for NW entry, periodic ranging and for BW request.
ACK channel is where mobile stations provide HARQ acknowledgment.
CQI channel is used for providing channel quality indication, so that based on that information Scheduler can change the Modulation parameters accordingly. And can maximize system throughput.

OFDMA sub-channelization

In order to create the OFDM symbol in the frequency domain, the modulated symbols are mapped on to the subchannels that have been allocated for the transmission of the data block.

Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called subchannels. The minimum frequency-time resource unit of sub-channelization is one slot.

The number and exact distribution of the subcarriers that constitute a subchannel depend on the subcarrier permutation mode. The number of subchannels allocated for transmitting a data block depends on various parameters, such as the size of the data block, the modulation format, and the coding rate.

In the time and frequency domains, the contiguous set of subchannels allocated to a single user—or a group of users, in case of multicast—is referred to as the data region of the user(s) and is always transmitted using the same burst profile. In this context, a burst profile refers to the combination of the chosen modulation format, code rate, and type of FEC: convolutional codes, turbo codes, and block codes.

Two types of sub-carrier permutations: Diversity & Contiguous

The subcarriers that constitute a subchannel can either be adjacent to each other or distributed throughout the frequency band, depending on the
subcarrier permutation mode. A distributed subcarrier permutation provides better frequency diversity, whereas an adjacent subcarrier distribution is more desirable for beamforming.

Downlink Full Usase of Sub-Carrers (DL-FUSC)

In the case of DL FUSC, all the data subcarriers are used to create the various subchannels. Each subchannel is made up of 48 data subcarriers, which are distributed evenly throughout the entire frequency band, as depicted in Figure. In FUSC, the set of the pilot subcarriers is divided in to two constant sets and two variables sets. The variable sets allow the receiver to estimate the channel response more accurately across the entire frequency band.

==== Parameters of FUSC subcarrier permutation(1024 FFT) ====
Subcarriers per subchannel : 48
Number of subchannels : 16
Data Subcarriers used : 768
Fixed Pilot Subcarriers : 11
Variable Pilot Subcarriers : 71
===================================

Downlink Partial Usase of Sub-Carrers (DL-PUSC)

DL PUSC is similar to FUSC except that all the subcarriers are first divided into six groups. Permutation of subcarriers to create subchannels is performed independently within each group, thus, in essence, logically separating each group from the others. In the case of PUSC, all the subcarriers except the null subcarrier are first arranged into clusters. Each cluster consists of 14 adjacent subcarriers over two OFDM symbols, as shown in Figure. In each cluster, the subcarriers are divided into 24 data subcarriers and 4 pilot subcarriers. The clusters are then renumbered using a pseudorandom numbering scheme, which in essence redistributes the logical identity of the clusters. After renumbering, the clusters are divided into six groups, with the first one-sixth of the clusters belonging to group 0, and so on. A subchannel is created using two clusters from the same group, as shown in Figure.

Frequency reuse is much simple in PUCS. E.g consider the BS with 3 segments, here it is possible to allocate all or only a subset of the six groups to a given transmitter. By allocating disjoint subsets of the six available groups to neighboring transmitters, it is possible to separate their signals in the subcarrier space, thus enabling a tighter frequency reuse at the cost of data rate.

==== Parameters of PUSC subcarrier permutation(1024 FFT) ====
Subcarriers per cluster: 14
Number of subchannel : 30
Data Subcarriers used : 720
Pilot Subcarriers : 120
===================================

Uplink Partial Usage of Subcarriers (UL-PUSC)

In UL PUSC, the subcarriers are first divided into various tiles, as shown in Figure. Each tile consists of four subcarriers over three OFDM symbols. The subcarriers within a tile are divided into eight data subcarriers and four pilot subcarriers. The optional UL PUSC mode has a lower ratio of pilot subcarriers to data subcarriers, thus providing a higher effective data rate but poorer channel-tracking capability. The two UL PUSC modes allow the system designer a trade-off between higher data rate and more accurate channel tracking depending on the Doppler spread and coherence bandwidth of the channel. The tiles are then renumbered, using a pseudorandom numbering sequence, and divided into six groups. Each subchannel is created using six tiles from a single group. UL PUSC can be used with segmentation in order to allow the system to operate under tighter frequency reuse patterns.

Band Adaptive Modulation and Coding (B-AMC)

Unique to the band AMC permutation mode, all subcarriers constituting a subchannel are adjacent to each other. Although frequency diversity is lost to a large extent with this subcarrier permutation scheme, exploitation of multiuser diversity is easier. Multiuser diversity provides significant improvement in overall system capacity and throughput, since a subchannel at any given time is allocated to the user with the highest SNR/capacity in that subchannel.

In this subcarrier permutation, nine adjacent subcarriers with eight data subcarriers and one pilot subcarrier are used to form a bin, as shown in Figure. Four adjacent bins in the frequency domain constitute a band. An AMC subchannel consists of six contiguous bins from within the same band. Thus, an AMC subchannel can consist of one bin over six consecutive symbols, two consecutive bins over three consecutive symbols, or three consecutive bins over two consecutive symbols.

OFDM symbol structure

In an OFDM system, a high-data-rate sequence of symbols is split into multiple parallel low-data rate-sequences, each of which is used to modulate an orthogonal tone, or subcarrier.

WiMAX has three classes of subcarriers:

1. Data subcarriers are used for carrying data symbols.
2. Pilot subcarriers are used for carrying pilot symbols. The pilot symbols are known a priori and can be used for channel estimation and channel tracking.
3. Null subcarriers have no power allocated to them, including the DC subcarrier and the guard subcarriers toward the edge. The DC subcarrier is not modulated, to prevent any saturation effects or excess power draw at the amplifier. No power is allocated to the guard subcarrier toward the edge of the spectrum in order to fit the spectrum, of the OFDM symbol within the allocated bandwidth and thus reduce the interference between adjacent channels

Above figure shows a typical frequency domain representation of an IEEE 802.16e-2005 OFDM symbol containing the data subcarriers, pilot subcarriers, and null subcarriers.

Multiple Access Schemes

Multiple-access strategies typically attempt to provide orthogonal, or noninterfering, communication channels for each active link. The most common way to divide the available dimensions among the multiple users is through the use of frequency, time, or code division multiplexing.

In frequency division multiple access (FDMA), each user receives a unique carrier frequency and bandwidth.

In time division multiple access (TDMA), each user is given a unique time slot, either on demand or in a fixed rotation.

In code division multiple access (CDMA) systems allow each user to share the bandwidth and time slots with many other users and rely on orthogonal binary codes to separate out the users.

OFDMA is essentially a hybrid of FDMA and TDMA:

Users are dynamically assigned subcarriers (FDMA) in different time slots (TDMA).In OFDMA, the subcarrier and the power allocation should be based on the channel conditions in order to maximize the throughput.

Multiuser diversity and Adaptive modulation are the two key principles that enables high performence in OFDMA. Multiuser diversity describes the gains available by selecting a user or sub-set of users having "good" conditions. Adaptive modulation is the means by which good channels can be exploited to achieve higher data rates.

OFDMA Pros and Cons

In OFDMA multiple access is two dimensional (time and frequency)

Multiple users use separate subchannels for multiple access

- Improved capacity

- Improved scheduling and QoS support

- Reduced interference (no intra-cell interference)

- Improved link margin (subchannelization gain)

- High spectral efficiency

Flexible subchannelization

- Pseudo-random permutation (PUSC) for frequency diversity, or

- Contiguous assignment (AMC) to enable beamforming

- Scalable structure to support variable bandwidths

- Allocation of subcarriers to multiple SS's (Subscriber Stations) in an OFDM symbol time- Group of M subcarriers as a unit of allocation - Subchannel

- Narrow subcarrer spacing is sensitive to carreer frequency error

- high PAR ratio (Peak to Average Power Ratio), which reduces the eficiency and hence increases the cost of the power amplifier, which is one of the most expesive component in Radio

OFDM Basics

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation technique that has recently found wide adoption in a widespread variety of high-data-rate communication systems, including digital subscriber lines, wireless LANs (802.11a/g/n), digital video broadcasting, and now WiMAX and other emerging wireless broadband systems.

The basic idea of Multicareer modulation is quite simple. The ISI would be zero only if symbol time of the siganl is larger than the channel delay. But the modern wireless networks with broadband links providing several mega-bits per second (e.g. 802.11a promises 54 Mbps), which has very less symbol time than channel dealy causes ISI. But it may not be practical to implement an equalizer at all because of overwhelming complexity caused by the high speed link.

On the other hand, if we could somehow reduce the symbol rate so that ISI becomes negligible, while still maintaining the required information bit rate, equalization becomes unnecesssary.

One way to do this is simply to increase the level of modulation in an M-ary pulse modulation scheme but there is a limit on how large M can be. Because as M increases, the spacing between each sample decreases and so it would be difficult at the doecoder side to decoded the weak signal.

The other way to increase the symbol interval is through parallel transmission over many orthogonal channels. This will widen the symbol time larger than channel delay and ISI can be avoided. These individual substreams can then be sent over parallel subchannels, maintaining the total desired data rate, so the subchannels experience relatively flat fading. Thus, the ISI on each subchannel is small. Moreover, in the digital implementation of OFDM, the ISI can be completely eliminated through the use of a cyclic prefix. Such orthogonal carriers can be easily generated using IFFT operation.

In the above figure Tb is useful OFDM simbol time and Tg is cyclic prefix, which is nothing but replica of last few bits in the symbol

Multicarrer Modulation

The philosophy of multicarrier modulation is: a large number of subcarriers (L) are used in parallel, so that the symbol time for each goes from T-->LT. In other words, rather than sending a single signal with data rate R and bandwidth B, why not send L signals at the same time, each having bandwidth B/L and data rate R/L ? In this way if B/L is less carrier BW, each signal will undergo approximately flat fading, and the time dispersion for each signal will be negligible. As long as the number of subcarriers L is large enough, the condition B/L is less than carrier BW can be met.

This elegantidea is the basic principle of orthogonal frequency division multiplexing (OFDM).

Cellular System problems !!

One of the more intriguing aspects of wireless channels is fading. Unlike pathloss or shadowing, which are large-scale attenuation effects owing to distance or obstacles, fading is caused by the reception of multiple versions of the same signal.

The multiple received versions are caused by reflections that are referred to as multipath.

Because of multiple paths between transmitter and reciever, there will be phase difference between arriving signal, and the intereference can be either constructive or distructive, which causes a very large observed difference in the amplitude of the received signal even over very short distances.

In other words, moving the transmitter or the receiver even a very short distance can have a dramatic effect on the received amplitude.This happens because each frequency harmonic of transmitted signal goes through different level of attenuation and fading.Which ultimatly results in delay spread and in turn ISI (Inter symbol Interference). It is very complex to implement equilizer for time varying wireless channel.

OFDMA is the technique which can help a lot to mitigate Inter Symbol Intereference (ISI).

Cellular System

In cellular systems, the service area is subdivided into smaller geographic areas called cells, each served by its own base station.In order to minimize interference between cells, the transmit-power level of each base station is regulated to be just enough to provide the required signal strength at the cell boundaries.Therefore, the same frequency channels can be reassigned to different cells, as long as those cells are spatially isolated.

The rate at which frequencies can be reused (freq-reuse ratio) should be determined such that the interference between base stations is kept to an acceptable level. In this context, frequency planning is required to determine a proper frequency-reuse factor.The frequency-reuse factor f = 1 means that all cells reuse all the frequencies. Accordingly, f = 1/3, implies that a given frequency band is used by only one of every three cells.

Above figure shows a hexagonal cellular system model with frequency-reuse factor of 4, where cells labeled with the same letter use the same frequency channels. In this model, a cluster is outlined in boldface and consists of four cells with different frequency channels.

But this will create a problem of CCI (Co-channel Interference) between the adjecent cell using the same frequency. And also it wastes much bandwidth and power by radiating power in complete cell.

Cell Sectoring is the best way to tackle this situation. Using directional antennas instead of an omnidirectional antenna at the base station can significantly reduced the cochannel interference. So it increases both bandwidth and system capacity by the number of time sectoring is done !!

PHY layer deails

 

Physical Layer (PHY) processing

Scrambler: It is nothing but a randomizer, which randomizes incoming data stream of continuos 0's and 1's. This helps in AGC and timing recovery circuit

Channel Encoder: Its a FEC scheme, which adds extra redundent bits to data in order to increase the error correcting capabilities. Convolution codes (CC) and Convolution Turbo Codes (CTC) are example of channel encoder.

Interleaver: Protect burst errors by spreading incoming bits in different channels, which helps in recovering data even after burst errors. It ensures that adjacent code bits are mapped to non adjacent subcarrers, which provides frequency diversity and improves the performence of decoder

Symbol Mapper: The sequence of binary bits are converted into sequence of complex valued symbols. QPSK, 16QAM and 64QAM are defined in WiMax

Space Time Coding: A space–time code (STC) is a method employed to improve the reliability of data transmissionn in wireless communication systems using multiple transmit antennas. STCs rely on transmitting multiple, redundant copies of a data stream to the receiver in the hope that at least some of them may survive the physical path between transmission and reception in a good enough state to allow reliable decoding

Subcarrer Mapping/IFFT: Depending on the type of subcarrer allocation scheme data will be mapped on the different subcarrers. There are basicaly 2 different types of schemes: Adjacent subcarrer permutation and distrubuted subcarrer permutation scheme. For 1024 FFT (subcarrers) 768 subcarrers are used for data, 82 for pilot and rest are unused. IFFT converts frequency domain signal to time domain and maps data to no of IFFT subcarrers.

D/A: Digital to Analog converter converts digital data to analog for the transmission on the air.

RF Card: Analog data will be modulated with the 2.3 Ghz range of frequency (depending on the system freq), power amplified and will be feed to antenna through TDD switch.

802.16e Protocol Architecture

Traffic (yellow) and Signalling (green) planes of 802.16e standard are shown above.

WiBro Network Architecture

 

WiMax Network Architecture

 

Protocol Layer Structure

Service specific Convergence Sublayer (CS):

• Transformation or mapping of external network data
• Classify external network SDUs and associate them with proper MAC service flow (assigns SFID) and CID. SFID assignement is based on the QOS
• Payload Header Supression ()

MAC Common Part Sublayer (MAC CPS):

• Core MAC functionality of system access
• Bandwidth allocation
• Connection establishment and maintainence
• Classified to perticular MAC conection
• Quality of Service (QOS)
• Scheduling of data over PHY

Privacy Sublayer:

• Providing authentication
• Secure key exchange
• Encryption

Physical Layer (PHY):

• OFDMA,
• MIMO, AAS
• Space Time Coding (STC), Beam Forming
• HARQ
• Adaptive Modulation and Coding (AMC)
• Convolution Turbo Coding (CTC)

What makes WiMax interesting ?

WiMax is a wireless broadband solution that offers rich set of features with lots of flexibility in term of system deployment and service offering. Some of the salient features highlighted below:

- LOS and NLOS communication

- OFDMA based physical layer

- IP based architecture

- High data rate

- Scalable bandwidth

- Adeptive modulation and coding

- Flexible and dynamic per user resource allocation

- Support for Quality of Service

- High level of security

- Mobility

Mobile WiMax and WiBro PHY Specification

Mobile WiMax PHY Specification

Frequency Band	2Ghz-11Ghz for fixed & 2Ghz-6Ghz for mobile application
Channel Bandwidth	Scalable bandwidth between 1.25Mhz to 20Mhz
FFT size	Mobile-28,512,1024,2048 and Fixed-256
Duplexing	TDD and FDD
Multiple Access	SOFDMA
Modulation scheme	QPSK, 16QAM, 64QAM
Frame length	5mSec
Data throughput	~ 70Mbps DL/UL
Mobility	~ 120kmph
Cell coverage	1mile-3mile

WiBro PHY Layer Specification

Frequency Band	2.3Ghz-2.4Ghz
Channel Bandwidth	Scalable bandwidth between 1.25Mhz to 20Mhz
FFT size	8.75Mhz
Duplexing	FDD
Multiple Access	OFDMA
Modulation scheme	QPSK, 16QAM, 64QAM
Frame length	5mSec
Data throughput	~ 50Mbps DL/UL
Mobility	~ 100kmph
Cell coverage	~1km
Channel Codding	CTC(Convolution Turbo Coding)
Key Features	AMC, HARQ, MIMO etc..

WiMax Roadmap

802.16 (Dec-2001)
Original fixed wireless broadband air Interface for 10–66 GHz, Line-of-sight only, Point-to-Multi-Point applications.

802.16a (Jan-2003)
Extension for 2-11 GHz: Targeted for non-line-of-sight, Point-to-Multi-Point applications like "last mile" broadband access.

802.16d (Oct-2004)
Adds WiMAX System Profiles and Errata for 2-11 GHz. It is never existed as standaed and also known as 802.16-2004.

802.16e (Dec-2005)
MAC/PHY Enhancements to support subscribers moving at vehicular speeds.

802.16m (not yet standardized)
Path towards 4G. To provide datarate of 100Mbps for mobile and 1Gbps for fixed applications, includes Frequency overlay, Adhoc Frame Relay techniques to increase the system capacity and internet speed.

Data rates of 100 Mbit/s for mobile applications and 1 Gbit/s for fixed applications, cellular, macro and micro cell coverage, with currently no restrictions on the RF bandwidth (which is expected to be 20 MHz or higher). The proposed work plan would allow completion of the standard by Sept 2008 for approval by Dec 2008.
- provides better mobility
- low latency (during sleep mode to normal mode, handover etc..)
- channel bandwidth of 20 MHz, so better throughput and performence

WiMax Introduction

WiMAX, the Worldwide Interoperability for Microwave Access, is a telecommunications technology aimed at providing high speed wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. It is an assortment of technical specifications called 802.16.

WiBro (Wireless Broadband) is a wireless broadband Internet technology being developed by the South Korean telecoms industry. WiBro is the South Korean service name for IEEE 802.16e (mobile WiMAX) international standard. WiBro adapts TDD for duplexing, OFDMA for multiple access and 8.75 MHz as a channel bandwidth. In February 2002, the Korean government allocated 100 MHz of electromagnetic spectrum in the 2.3 - 2.4 GHz band.

The journey of communication: From 1G to 4G

Communication systems have evolved since the time the first voice communication device that is the telephone was invented by Graham Bell. Experts have classified the communication technique technologies on basis of time frame as first generation, second generation or 2G, Third generation or 3G and the fourth generation or 4G.

Each of these technologies while being grouped into first , second, third or fourth generation technology is to meet the standardisation norms as floated by International Telecom Union (ITU) – a United Nations body for policy formulation on telecommunication sector.

First generation: Almost all of the systems from this generation were analog systems where voice was considered to be the main traffic. There was no or very little security as the data was transferred and the voice could be heard by third person. AMPS is an example of 1G system.

Second generation: Here commercialization of communication system started and the analog systems evolved to become digital systems. The data was transferred in discrete form and hence could be coded or technically aliased, thereby enhancing the security. GSM, GPRS, IS95 are some of the example of 2G systems.

Third generation: To meet the growing demands in network capacity, rates required for high speed data transfer, multimedia applications and increased security over data transfer 3G standards started evolving. They are based on two parallel backbone infrastructures, one consisting of circuit switched nodes, and one of packet oriented nodes. EDGE, UMTS, CDMA falls under 3G technologies.

Fourth generation: Though no formal standards for 4G have been established by ITU as yet but experts in telecommunication field have begun to design the technologies as 4G technology. The infrastructure for 4G will be only packet-based (all-IP). These are some of the technologies which are being considered as pre-4G: WiMax, WiBro, iBurst and 3GPP Long Term Evolution.

Welcome !!

Hello Everybody,

Wolcome to WiMax and Wibro blogspot. Share your views and ideas on WiMax and WiBro standard here.

References:
1. Fundamental of WiMAX by Jeffrey G. Andrews, Arunabha Ghosh and Rias Muhamed

Thank you