1xEV-DO Modulation Overview
1xEV-DO is the name applied to the first evolution (1xEV) of a 3GPP communications system optimized for Data Only (DO) information. 1xEV-DO is a 3G Cellular industry standard based upon the original cdma2000 3G standard. 1xEV-DO incorporates Data Only services into the 3G system, by sacrificing voice-service parameters, to increase packet data throughput. 1xEV-DO is defined in the following standards documents:
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3GPP2 C.S0024 v2.01 1xEV-DO High Rate Packet Data Air Interface
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3GPP2 C.S0032 1xEV-DO Inter-Operability Specification (IOS) for CDMA Code Division Multiple Access: One of several digital wireless transmission methods in which signals are encoded using a specific pseudo-random sequence, or code, to define a communication channel. A receiver, knowing the code, can use it to decode the received signal in the presence of other signals in the channel. This is one of several "spread spectrum" techniques, which allows multiple users to share the same radio frequency spectrum by assigning each active user an unique code. CDMA offers improved spectral efficiency over analog transmission in that it allows for greater frequency reuse. Other characteristics of CDMA systems reduce dropped calls, increase battery life and offer more secure transmission. See also IS-95. 2000 Access Network
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3GPP2 C.P9012 Recommended Minimum Performance Standards for 1xEV-DO High Rate Packet Data Access Terminal
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TIA Telecommunications Industry Association (U.S.): One of the Telecommunications standards setting bodies in the United States./EIA Electronic Industry Association: A trade association and standards setting organization in the USA./IS-856 1xEV-DO High Rate Packet Data Air Interface Specification
Although similar to the cdma2000 specification, 1xEV-DO incorporates the following differences. In the reverse link (mobile unit) transmission, 1xEV-DO is very similar to cdma2000, with power control bits replaced by Reverse Rate Indication (RRI) information. 1xEV-DO reverse link also adds a rate request channel (DRC) and an acknowledge channel (ACK Acknowledgement). In the forward link, 1xEV-DO provides a TDM time division multiplexing burst: A contiguous portion of a TDM data stream using PHY parameters, determined by the Downlink Interval Usage Code (DIUC), that remain constant for the duration of the burst. TDM bursts are not separated by gaps or preambles. structure including Preamble, Pilot, MAC (Medium Access Control: In most wired and wireless networks, a mthod is used to control how and when a device can transmit data over the communications link. This is the network's Medium Access Control scheme. The MAC protocol operates within the Data Link layer (Layer 2) of the ISO OSI 7 layer Model. The IEEE 802.11 standard, for example, specifies the MAC protocol for sharing of the wireless medium, packet formats, addressing, eror detection and recovery following errors. and Traffic channels. The 1xEV-DO forward link Traffic channel consists of 16 code channels, which may be mapped to different complex constellations including QPSK Quadrature phase shift keying, 8PSK, and 16QAM. The 1xEV-DO forward link is always sent by the BTS Base Transceiver Station - Although specifications differ for each system, the BTS effects radio communication with mobile stations (MS) via its respective radio access system and transmits/receives signals to/from connected radio network controllers (RNC) located along transmission routes. at full power. 1xEV-DO also differs from cdma2000 in that a full frame is equal to 16 slots (26.667ms), with each slot occupying 1.667ms.
1xEV-DO uses what is known as High Rate Packet Data, using the same final spread rate of 1.2288 Mcps Mega Chip Per Second: A measure of the number of bits (chips) per second in the spreading sequence of direct sequence spreading code. as does cdma2000 SR1. It also uses the same digital filter to reduce the final modulation bandwidth, with the result that 1xEV-DO is spectrally compatible with cdma2000 SR1. Both 1xEV-DO and cdma2000 SR1 can use the same amplifiers, combiners, and antennas to reduce the installation cost of 1xEV-DO.
Although 1xEV-DO is compatible with much of the existing cdma2000 infrastructure, the two systems may not simultaneously occupy the same channel. Each 1xEV-DO channel requires a paired 1.25 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second. clear channel, therefore it cannot be overlaid with cdma2000 channels.
Forward Link
A 1xEV-DO base station (BTS) is referred to as the "access network". 1xEV-DO relies on the Global Positioning System (GPS Global Positioning System:L A worldwide radio-navigation system that was developed by the US. Department of Defense. In addition to military purposes it is widely used in marine and terrestrial navigation (for example car navigation systems).) for intercell synchronization, in the same manner as cdma2000. The 1xEV-DO system is well suited for data transmission in the forward link, as it is optimized for high-speed packet data communications from base stations to mobile stations. Average delivery rates of about 600 kbs on a continuous basis have been demonstrated. When compared to the recent IS-95-B design in cdma2000, 1xEV-DO shows about a 6X improvement in the overall network data rate
The largest contribution to overall increased data throughput is the ability of 1xEV-DO to negotiate for increased data rates for individual users or mobiles, as only one user is served at a time. This and other contributing factors are described in detail in the Design Elements section below.
Frame Structure
The frame structure is different from that used for cdma2000 and IS-95 systems, as they are based on 20 ms frames set by the codec used for speech. The frame period for 1xEV-DO is 26.667 ms, which is also the period of one pilot channel. Each frame is divided up into 16 slots of 1.667 ms.
Design Elements
When compared to a voice communication system of cdma2000, the following design elements of the forward link contribute to raise the data rate:
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Full power - The cell is always at full power, resulting in no overhead needed for statistical variations of multiple users.
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Best serving cell - When in soft handoff, the mobile selects the best serving cell and can only request and receive data from that cell.
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Efficient data delivery - The base station can send data to a mobile at times when its rate request is higher than the running average of the base station, and will avoid sending data when the requested rate is lower than the running average.
The forward link is always transmitted at its full power, using a rate control scheme instead of a power control scheme, as in cdma2000. The data is TDM (time division multiplexed) to the various users or mobiles, with only one being served at any time. Each mobile measures signal-to-noise ratio (SNR Signal-to-Noise Ratio) on the forward link pilot every slot, 1.667 ms, and sends a data rate request to the base station every 1.667 ms. The base station decides which mobile should be served next. The maximum data transfer from the network will be achieved if only the most favorable links get served, however, this is accomplished at the expense of the distant users.
Forward Channel TDM Structure
The following four different channels are used and are Time Division Multiplexed (TDM):
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Traffic Channel - The data following the turbo coder is scrambled by mixing with a PN 1) part number, or 2) packet number sequence, the initial state of which is based on known parameters, and is unique for each user. At the same initial value of the PN sequence, every packet starts. QPSK modulation is used for data rate up to 1.2288 Mbps. Higher order modulation, with either 8PSK with 3 bits per symbol or 16QAM with 4 bits per symbol, is used for higher data rates. The levels of the I and Q symbols are mathematically chosen so that the average power becomes 1. The I and Q channels are mapped into 16 parallel Walsh cover, each of length 16. The I and Q outputs with the Walsh covers signals are added together.
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Preamble - At the beginning of the transmission to one user, there is a preamble that is embedded in the data. This preamble contains the user ID for the data, and its repeat rate is determined by the data rate; lower data rates require higher repeat values. At its largest, the preamble will fill more than a half the first slot.
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Medium Access Control (MAC) - This layer consists of the reverse power control (RPC) and the reverse activity (RA 1) Random Access, or 2) Receiver Address) channels. The power control bits are sent in parallel to all mobiles with an open connection. Each one is assigned a different 64-bit Walsh code which gets transmitted on either the I or Q channel. The codes 0 through 3 are reserved. The Walsh code 4 is the RA cover. The Walsh codes 5 through 63 are allocated to RPC. One Walsh code is allocated for power control to each active reverse link. Power control bits for all of the reverse links are sent in parallel as part of the MAC channel. These are made of 4 repetitions of an assigned Walsh code of length 64. The RA channel is used to indicate that the sector is busy decoding multiple reverse link signals.
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Pilot - The pilot signal is gated on for only 192 chips per slot. The differentiator between cells and sectors is still the PN offset of the pilot channel.
The data structure per slot can be illustrated as follows:
Reverse Link Coding
The reverse link for 1xEV-DO has a structure similar to that for cdma2000, except that in addition to a data channel, the following channels are also included:
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Reverse Rate Indication (RRI) - This replaces with the power control bits to indicate of which data rate is requested by the mobile.
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Data Request Channel (DRC) - This contains a 4-bit word in each slot to allow the choice of up to 16 different transmission rates. Different Walsh covers are used to indicate which PN offset in the active set is preferred for transmission.
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Ack Channel - This acknowledgement channel is only transmitted after the mobile detects a frame with its preamble, meaning the packet is directed to that mobile. The 1 bit on this channel, which is transmitted for a half slot, is used to indicate a successful reception or an erasure.
The Pilot in 1xEV-DO is still punctured but the information carried is different:
In cdma2000, the Pilot is punctured in a 3:1 ratio (3 Pilot, 1 Data) to carry power control bits. The puncture period is 1 Power Control Group, or 1.25 ms. The data rate for PCB is 16 bits per 20 ms frame.
In 1xEV-DO, the puncture pattern is 7:1 on each slot (7 Pilot, 1 Data). The underlying data is coded, and the transmission rate is 3 bits per 26.667 ms frame.
In 1xEV all signaling is performed on the data channel so there is no Dedicated Control Channel. The data channel can support 5 data rates, which are separated in powers of 2 (256 through 4096 bits, or 9.6 through 153.6 kbps). These rates are achieved by varying the repeat factor. The highest rate uses a Turbo coder with lower gain.
Reverse Link Scrambling
The Pilot, ACK, DRC, and Data channels are added with each other in a manner that maps some into I (in phase sequence) and some into Q (quadrature phase sequence) for eventual spreading. The final spreading is made similarly to cdma2000 with HPSK. In 1xEV-DO, the long code used for spreading is Seeded with a fixed pattern at the start of every Pilot sequence, every 26.667 ms. There are separate masks, 42 bits long, for the I and Q long spreading codes. In 1xEV-DO, the long code generator is based on system time, or in other words a continuous clock.
Compatibility with IS-2000
When interpreting TIA/EIA/IS-2000 EIA Interim Standard 2000 (see cdma2000): A standard for current CDMA systems providing a migration path to 3G services. in the context of TIA/EIA/IS-856, the terms for TIA/EIA/IS-2000 should be mapped as follows:
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TIA/EIA/IS-2000 |
TIA/EIA/IS-856 |
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AN |
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Base Station |
Access Network |
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AN |
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Base Station Control |
Access Network |
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Call |
Connection |
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AT |
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Mobile Station |
Access Terminal |
See Also
