Introduction
If you see at developments that had happened in wireless technologies till now, we can easily see a trend, sort of direction that is difficult to miss out. There is more and more independence between radio access network and core (typically wireline) network. Radio access transports are being designed in such a way that they can support multiple types of services with improving radio efficiency. Also, signaling between mobile device and core network has been made as much transparent as possible to radio access transport. On core network side, IP (over ATM/SONET or other L2/L1 technologies) has been gaining popularity due to ease in interconnection, its ability to support different types of services, availability of number of protocols (IETF RFCs) to support different services and their implementations etc.
Future end-to-end network architecture may just look like this:
One of the few challenges (and possibly most important) is to come out with "Radio Transport (and Technology)" which would provide high spectral efficiency (bits per second per Hz), high bit rates, high quality (BER), and flexible/dynamic enough to support various types of services. IEEE 802.16 standard provide one such radio access based on OFDMA. WiMAX (Worldwide Interoperability for Microwave Access) is a name given by industry to implementations of 802.16, along with its network elements for end-to-end support, certification etc. WiMAX Forum is industry wide body involved in promoting the 802.16 technology. Note that IEEE and WiMAX Forum are separate entities (check IEEE presentation on Myths and Facts about 802.16).
802 family
802.16 standard is part of IEEE 802 family of standards maintained by LMSC (LAN/MAN Standards Committee). 802 standards are meant for OSI Layer 2 and Layer 1 of LAN (Local Area Network) and MAN (Metropolitan Network).
Overall 802 architecture is shown below:
One of the popular ratified 802 technologies is 802.11 (WLAN, WiFi).
802.16 standard
802.16 standard, published in 2000/01, was meant for Wireless MAN (WMAN). It was targeted for LOS frequencies, 10- 66 GHz. WMAN was supposed to have provided backbone for connecting WLANs. WLANs typically cover a area of 10 km2 (radius of ~1.5 km). WMAN was supposed to cover a area of 100 km2 (radius of ~5.5 km). Later 802.16 amended few times and released as 802.16-2004 (or 802.16d), consolidating the amendments. It included NLOS frequencies 2-11 GHz. This resulted in three new PHY specifications to existing Single Carrier (WMAN-SC) for these added frequencies: Single Carrier (SCa), 256-point FFT OFDM, and 2048-point FFT OFDM. In addition, these new frequencies included unlicensed (open for all) frequencies. This required certain changes in both MAC and PHY layers. Overall notable PHY features were: Adaptive Antenna System (AAS) or MIMO system, Adaptive modulation schemes. Notable MAC features were: support for both TDD/FDD, full and half duplex, support for IPv4, IPv6, Ethernet, ATM etc, PKM based security, support PMP (point-to-multipoint) and optional Mesh networking operations, four scheduling mechanisms to support different types of traffic . The targeted data rates were in the range of 100 Mbps.
802.16-2004 was amended as 802.16e-2005 to include mobile enhancements for licensed bands below 6 GHz. Major PHY additions in these amendment are: use of Scalable OFDMA (FFT sizes 128, 512, 1024, 2048 subcarriers with subcarrier separation constant), additional MIMO modes. On MAC side: Handover support was added, ErtPS scheduling mechanism added, Power management procedures added for power efficient operations. The targeted data rates were in the range of 1-10 Mbps.
Industry name for 802.16e-2005 implementations is Mobile WiMAX whereas 802.16-2004(802.16d) implementations are known as Fixed WiMAX. WiMAX Forum picked up few profiles for mobile WiMAX due to interoperatability reasons, notable being 2.3 GHz/8.75Mhz (similar to WiBro in Korea), 2.5 GHz (5/7/10 Mhz), and 3.5 GHz (5/7/10 Mhz). WiBro is a mobile service available in South Korea and it is similar to WiMAX. It is required to be compliant with 802.16e-2005.
In next article, we will look at elements of 802.16. We will mainly concern ourselves with NLOS frequencies for mobility operations (Mobile WiMAX).
Elements of 802.16 - 1
Before we have a look at 802.16 layers and their functions, let us have a look at what 802.16 solution looks like. We will mainly concern ourselves with NLOS frequencies for mobility operations (Mobile WiMAX).
The terms BS stand for Base Station and MS/SS imply user equipment (SS - Subscriber Station is fixed one whereas MS - Mobile Station is mobile one).
» OFDMA/S-OFDMA PHY
As a refresher, please look at articles on OFDM and OFDMA. In brief, OFDM transmits individual bits of data over number of orthogonal frequencies in parallel. Due to orthogonality, the subcarriers could be close to each other and still have theoretically zero interference. In addition, the individual subcarriers can be modulated differently. OFDMA specifies way to use OFDM for multiple access. OFDMA create logical subchannels, each subchannel contain number of subcarriers (selected randomly or contiguously). Question to ask here is why not use CDMA: inherent limitation to CDMA is processing gain (CDMA article). To have higher data rate, with limitation of processing gain, we will need higher bandwidth. In comparison, due to orthogonality property, OFDM provide much better spectral efficiency.
OFDM/OFDMA has number of other inherent advantages - obvious being: with OFDMA, randomly selected subcarriers provide consistent performance to all users, different modulation schemes can be adopted. Adoption of modulation schemes is important to mobile environment as channel conditions differ for each user and so appropriate modulation/coding scheme can be used.
802.16 uses S-OFDMA. In S-OFDMA, subcarrier spacing remain same, making it scalable and easing the hardware design.
MS PHY layer will typically look for (known/standardised) pilot signals. MS PHY layer will then estimate certain PHY parameters (possibly includes FFT size, BW etc.), which will then allow it to look for subcarriers. MS PHY will then look for Preamble (corresponding to FFT) for time and frequency synchronisation. Preambles are standard specified PN sequences (each subcarrier is 1 out of 3 consecutive subcarriers). From layman's point, Preambles are paricular waveforms that helps synchronising. Next procedure is to read FCH and DL-MAP as described below.
» Frame structure
802.16 MAC uses "frames" for operation. Frames for DL and UL are called DL subframe and UL subframe respectively. DL subframe contain FCH and DL/UL Maps as shown in diagram below: Note that UL-map need not be in next numbered symbol time; both DL/UL maps are variable in length and so accommodated accordingly.
FCH is a Frame Control Header. FCH contain information about DL-MAP coding, length etc. FCH is the first part/message that MS need to be able to decode. FCH will be using preset (QPSK for OFDMA PHY) modulation and coding scheme (specified in 802.16).
DL/UL Maps are MAC (management) messages with DL-map describing current DL subframe and UL-map describing usage of relevant UL subframe. Below diagram shows the way DL/UL subframes organised for TDD/FDD and their map relevance.
The usage of subframes is in terms of OFDM symbol. In essence, we have a burst based time division multiplex operation. Below is an example of OFDMA frame structure for TDD.
The DL bursts meant for certain logical MAC functions (channels), data transfer etc. UL bursts are meant to be used by MSs based on UL-map. Some (both DL/UL) bursts are meant to be used by individual MSs or groups of MSs or all MSs.
In next article, we will look at how MAC manages the medium access.
» MAC connections
MAC is supposed to provide transport for IP/Ethernet packets, ATM cells etc. This transport is managed in terms of "connections" having identifier called (16-bit) CID. Though called connections, it is more of "context" that is mapped to CID. SSs are identified by 32-bit universal MAC address. 802.16 specify two types of connections: Management and Transport as shown below:
Management connections carry Control or Management messages. Transport connections is to carry Data (coming from MAC user). Note that MAC PDU has common format. It can contain management messages or data or both. MAC header(s) contain the information about "payload" that is being carried. MAC is capable of data fragmentation (data fragmented and sent in separate PDUs) and packing (different data being small packed together and sent in one PDU). Another important information that MAC header contain is CID.
Management connections are created for MS during network entry process. MAC address is used for initial identification and during further network entry process, management CIDs are given.
There are three types of management connections:
Basic - Created when MS joins the network and used for short & urgent MAC messages.
Primary - Though created during network entry time, it is used for long and delay tolerant messages.
Secondary - This connection "may be" created once MS joins the network. It is used for delay tolerant messages like DHCP, SNMP etc.
MS network entry process consists of adjustment of timing, power and other parameters [Ranging], negotiation of basic capabilities, authentication [Key exchange], registration, and then optional (IP/ATM) connectivity.
Ranging is pivotal process for 802.16 access. As the name suggests, ranging process is about estimating distance between MS and BS antenna. Ranging process help MS to adjust power/timing parameters to maintain quality of the radio connection. UL subframe has ranging regions (bursts) which are supposed to be used by MSs for ranging purposes. Ranging messages are the first messages used by MS to talk to BS.
Transport connections can be requested by MS or created by BS. Typically, based on subscription, BS will create these connections/service flows after network entry process.
To cater to various types of data traffic characteristics (QoS), 802.16 has a concept of "Service Flow".
» Service Flow
Service flow concept is similar to PDP Context in GPRS. Service flow has associated QoS parameter set and can be activated for a connection. It is identified by (32-bit) SFID. Once activated, it will have corresponding CID association. One CID can have only one service flow and vice versa. Though with the certain combinations of QoS parameters required service can be realised, 802.16 specify five types of scheduling mechanisms or services.
| UGS | Unsolicited Grant Service In UGS, BS does fixed periodic allocation to MS. UGS is meant for constant bit rate, real time applications like Voice call or VoIP without silent suppression. |
| rtPS | Real Time Polling Service "Polling" in the name suggest that MS is to be polled. In rtPS, BS provide "periodic" (unicast) opportunities for (individual) MS to request required allocation. rtPS is meant for real time and variable packet length traffic like streaming audio/video (MPEG coded). |
| nrtPS | Non Real Time Polling Service In nrtPS, BS provide "regular" (unicast) opportunities for (individual) MS to request required allocation. nrtPS is meant for delay tolerant and variable packet length traffic like file transfer (FTP). |
| BE | Best Effort service This is possibly default service with no specific constraints and so called Best Effort. MS need to go for contention request opportunities to receive an allocation. It is meant for bursty, non real time traffic like web browsing. |
| ErtPS | Extended Real Time Polling Service This was introduced in 802.16e-2005. This is similar to UGS except that allocations are dynamic like rtPS; BS does variable allocations periodically and so MS need not request allocations. It is meant for real time traffic like VoIP with silence suppression. Possibly similar to AMR in UMTS. |
BS has to employ appropriate scheduling algorithms to take care of QoSs of active service flows.
With this as background, in next article, we will sum up our understanding of how overall 802.16 work.
Preprovisioned service flows are activated during MS initialisation. The service flows can also be dynamically established (created, admitted) and activated. Based on QoS parameters, BS employ 5 types of scheduling mechanisms for a service flow (as mentioned in earlier article). In effect, as per QoS parameter values, BS need to provide allocation (called grants) or opportunities to request allocation at appropriate times. Current radio load on BS also plays a part here. BS designer has to implement appropriate algorithms to cater to different combinations of traffic that may be possible in field.
For UGS service flow, BS provide fixed "uplink grants" on real time periodic basis. This way MS do not have to send any "bandwidth request". This is not the case with all services, including when MS has management message exchange to do. 802.16 specify a "polling mechanism" to make possible for MS to send a bandwidth request. Polling could be unicast (to particular MS only), multicast (to group of MSs) or broadcast (to all MSs). As per the polling, MS sends a bandwidth request at alloted burst. If polling is broadcast/multicast, MS has to invoke "contention" procedures. Of course, MS should not make a bandwidth request if it does not need bandwidth. For unicast polling, basic CID is used whereas multicast/broadcast polling use multicast/broadcast CIDs respectively. It seems broadcast/multicast polling is not applicable to OFDMA PHY (i.e. mobile WiMAX). As per the request, BS may provide uplink grants. Bandwidth requests are made for particular CID. The uplink grants are always for basic CID.
802.16 also use concept of piggyback request wherein bandwidth request can be made along with user data payload. If MS already has UGS allocation, it can tell BS to poll it by setting header PM (Poll Me) bit.
» Retransmission mechanisms
For error correction, 802.16 support optional retransmission mechanisms. Both ARQ and HARQ (Chase Combining and Incremental Redundancy) are specified. The parameters are negotiated during connection establishment.
In conclusion 802.16 specify a flexible radio organisation and protocol to support various kinds of services and control of the same efficiently.
» Mobility
To support mobility, 802.16 specify basic Handover mechanism along with optional mechanisms like Macro Diversity HO (MS received same data from two or more BSs), Fast BS Switching (BSs change frame by frame, so MS receives from different BSs, but one frame at a time).
» Power saving mechanisms
One of the major additions in 802.16e was "power saving mechanisms". Simply speaking, MS when not needed can go for intermittent sleeps thus saving battery power. During sleep, BS will not try communicating with MS. Before MS start procedure for intermittent sleeps, it negotiates sleep parameters with BS. Note that sleep periods are negotiated per connection (e.g. UGS require different sleep periods than BE service). It is interesting to see effective sleep periods when more than one transport connections are present: Availability period of one connection may overlap with sleep period of another connection, so MS can go to sleep mode only when sleep periods of all ongoing connections overlap.
» Mesh networking
In addition to PMP (Point-to-MultiPoint, one BS talking directly to SSs) communication, 802.16 also specify optional feature of Mesh networking. In Mesh networking, though there is Mesh BS, communication between BS and SS can go through multiple SSs. In effect, intermediate act like routers. SSs can even talk to each other and exchange data. Mesh networking is possibly meant for LOS frequencies (fixed WiMAX).
Here are bullet points for 802.16 based on OFDMA PHY (Mobile WiMAX):
o OFDMA PHY
o Burst/slot based use of radio resources
o MAC layer for media control
o Frame structure with DL/UL maps
o Step wise network entry (Ranging, Security procedures etc.)
o 5 types of transport services
o Request/Allocation/Polling/Contention procedures for efficient use of resources
o Different handover procedures.
o Power saving
o Optional Mesh networking
In next article, we will look at 802.16 layer diagram and how the functions are split in layers and sublayers.
802.16 layers are shown in diagram below:
MAC has three sublayers - Convergence, MAC Common Part and Security.
Service specific Convergence Sublayer (CS)
CS bridge the gap between MAC SDUs and Service specific PDUs. CS support ATM cells and Packets based IPv4, IPv6, Ethernet, and VLAN. CS maps these PDUs to proper service flow (SFID/CID). CS can also do header suppression (PHS).
MAC Common Part Sublayer (CPS)
In CPS lie the core MAC functionality like Ranging, Network entry, Connection management, Handover, Service flow management, Fragmentation, Packing, Assembly, Scheduling, Handover, ARQ/HARQ etc.
Security Sublayer
Authentication, secure key exchange, encryption comes under Security Sublayer.
PHY layer handles OFDMA transmission, PHY Ranging, Modulation (QPSK, 16QAM, 64QAM, 256 QAM), Network Entry, Handover, HARQ etc.
This complete the overview of 802.16. In next article, we will talk about WiMAX Forum.
WiMAX Forum has also proposed various "system profiles" for interoperatability and certification purposes. These systems profiles mention applicability of various MAC features, values of PHY parameters etc. Refer Mobility profile document for details.
Forum has various working groups taking care of interfacing with IEEE, detailing profiles/certification requirements, network specifications etc. ReferPolicy documents for details.
WiMAX Forum has proposed a network architecture wherein Base Stations are connected to a Gateway. These gateways can be connected to multiple networks (typically provided by operator) or can directly interwork with Internet. Operator can have multiple connected networks. Please refer WiMAX Network architecture document for details.
All above documents are open to all, but marked proprietary and so not detailed here.
This complete the overview of WiMAX. Planning to put more articles on MAC procedures, but it may take a while.