IEEE COMMUNICATIONS LETTERS, VOL. 6, NO. 12, DECEMBER 2002
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Orthogonal Code Hopping Multiplexing Suwon Park, Student Member, IEEE, and Dan Keun Sung, Senior Member, IEEE
Abstract—An orthogonal code hopping multiplexing scheme is proposed as a statistical multiplexing scheme for orthogonal downlink in order to accommodate more orthogonal downlink channels than orthogonal codewords for mobile stations, and its performance is evaluated. Index Terms—Orthogonal code division multiplexing (OCDM), orthogonal code hopping multiplexing (OCHM), perforation, synergy.
I. INTRODUCTION
B
URSTY packet-type traffic will be gradually increasing in beyond-2G mobile communications, including IMT-2000. A bursty downlink channel results in low channel activity. In addition, there will be more downlink traffic than uplink traffic in beyond-2G mobile communications. Downlink channels in digital mobile communication systems are usually synchronous. Orthogonality is a valuable property of synchronous downlink since it yields a natural cancellation of interference. Downlink in the current DS/CDMA systems, such as cdmaOne (IS-95) [1], cdma2000 [2], and WCDMA [3] is fundamentally based on orthogonal code division multiplexing (OCDM), as shown in Fig. 1. Time division multiplexing (TDM) under OCDM is also partially used for non-real-time bursty traffic. Only one orthogonal codeword (OC) in an orthogonal code is assigned to each orthogonal downlink channel. Since one orthogonal codeword results in a one-to-one correspondence to one orthogonal downlink channel, especially for real-time service such as voice or real time video, the number of allocated orthogonal downlink channels cannot exceed the number of codewords in the orthogonal code, regardless of downlink channel activity. Since orthogonal codewords are a valuable resource for the synchronous downlink, it is important to increase utilization of orthogonal codewords within the maximum allowable total transmission power of the downlink in a cell. This letter proposes orthogonal code hopping multiplexing (OCHM) as a new statistical multiplexing scheme in code domain that preserves orthogonality among downlink channels and yields a high statistical multiplexing gain. We show simulation results for the frame error rate (FER) performance of OCHM in additive white Gaussian noise (AWGN) channels for the convolutional and turbo coding (TC) schemes. Manuscript received April 26, 2002.The associate editor coordinating the review of this letter and approving it for publication was Prof. D. P. Taylor. This work was supported in part by Korea Advanced Institute of Science and Technology (KAIST). The authors are with the Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea (e-mail:
[email protected];
[email protected]). Digital Object Identifier 10.1109/LCOMM.2002.806454
Fig. 1.
Conventional OCDM and proposed OCHM.
II. ORTHOGONAL CODE HOPPING MULTIPLEXING (OCHM) SCHEME The OCHM scheme is a statistical multiplexing scheme for orthogonal downlink in DS/CDMA systems. The proposed OCHM is possible because a base station (BS) can know all downlink data transmitted to all mobile stations (MSs) in its cell. The receiver structure of the OCHM system is similar to the structure of the OCDM system except that orthogonal codewords are generated based on a hopping pattern (HP). Since the proposed OCHM scheme uses an MS-specific hopping pattern after an initial channel allocation from a BS, there is a reduced demand for signaling messages for allocation and de-allocation of orthogonal codewords during a call. OCDM is a special case of OCHM. The hopping pattern may be based on an MS identifier (ID), such as the electronic serial number (ESN). Since the number of available codewords in an orthogonal code for OCHM is limited and the hopping patterns are mutually independent, the orthogonal codewords of two or more active (data transmitting) downlink channels may be identical during a symbol duration , as shown in Fig. 1. This event is called collision of the hopping patterns. When collisions occur among the hopping patterns of active downlink channels, a comparator and controller at the transmitter of a BS performs one of two operations. 1) If at
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Fig. 2. Perforation or synergy at the moment of collisions in OCHM.
IEEE COMMUNICATIONS LETTERS, VOL. 6, NO. 12, DECEMBER 2002
is inactive or the received frame is erroneous. If the BS did not send the frame (the downlink channel was inactive) then the BS neglects the NACK message. If the BS sent the frame (the downlink channel was active) then the BS may do an automatic repeat request (ARQ) process depending on the importance of the frame. For best-effort services, such as web-browsing, the lost frame can be neglected. The ARQ process is beyond the topic of this letter. For a given channel activity of allocated downlink channels and a given number of available codewords in an orthogonal code for OCHM, the greater the number of allocated channels, the more often they may experience collisions. For a given number of allocated orthogonal downlink channels and available codewords in an orthogonal code for OCHM, the more active are the allocated channels and the more often they may experience collisions. Thus, the number of allocated orthogonal downlink channels depends on the mean activity , of all allocated downlink channels, . and the number of available codewords for OCHM, Since Internet traffic is usually bursty or intermittently active, the proposed OCHM-based system can accommodate more orthogonal downlink channels than the OCDM-based system. A. Hopping Pattern Collision Probability
least one of channel-encoded data symbols spread by the same orthogonal codeword is different from others, then all the data symbols colliding during the symbol time are not transmitted, as shown in Fig. 2. This effect is called perforation. In spite of perforated symbols the channel decoder at the corresponding MS can recover the transmitted data if the number of perforated data symbols is less than a threshold. The transmission power during the encoded and perforated data symbol time is zero for all related channels. 2) If all channel encoded data symbols spread by the same orthogonal codeword are identical, then all the data symbols with collisions are transmitted without perforation, as shown in Fig. 2. Although the transmission signal amplitude assigned to each related MS is not changed during the symbol time, the transmission signal amplitude of the orthogonal codeword during the symbol time is the sum of the signal amplitudes assigned for all corresponding downlink channels. This effect is called synergy. Fig. 2 is illustrated at the viewpoint of MS#f. In is the transmission signal amplitude (not power) that Fig. 2, is allocated by its BS for an MS#m. After an MS receives a channel allocation message from the serving BS through a downlink common control channel, such as a forward access channel (FACH) [3], the MS despreads and decodes the downlink channel based on the orthogonal codeword hopping pattern until the MS receives an channel deallocation message from the serving BS through the downlink common control channel. After the channel decoding process the MS checks the frame check sequence (FCS), such as cyclic redundancy check (CRC) bits. If the FCS is correct, the MS sends an ACK (acknowledgment) message to the serving BS because the MS determines that the downlink channel during the ) is active and the received frame is not erroframe time ( neous. However, if the FCS is incorrect, the MS sends an NACK (no-acknowledgment) message to the serving BS because the MS determines that the downlink channel during the frame time
The hopping pattern collision probability in the . OCHM-based system is increases as the number For a given channel activity , increases. of allocated downlink channels B. Perforation Probability of Encoded Data Symbols The perforation probability of encoded data symbols in the OCHM-based system is , where is the prob. For ability of modulation symbol QPSK, in-phase (I) and quadrature phase (Q) channels can have independent hopping patterns because they are orthogonal each other. For a given channel activity , the perforation probability of also increases as the number of encoded data symbols increases. The collision and the allocated downlink channels perforation probabilities are positively correlated. C. Statistical Multiplexing Gain , Table I shows that for a given perforation probability can the number of allocatable dedicated downlink channels if the mean channel exceed that of orthogonal codewords is low. The dedicated activity of all downlink channels channels can serve both real and nonreal time services. Since downlink channels are admissible under maximum downlink transmission power, the number of really allocatable dedicated downlink channels may be less than . The allowable perforation (or collision) probability depends on the channel-coding scheme. With a stronger channel-coding scheme a higher perforation (or collision) probability can be allowed. That is, a downlink channel that should satisfy the required FER, does not require a significant increase in transmission power if the channel coding scheme is strong. If downlink total transmission
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TABLE I NUMBER OF ALLOCATABLE DOWNLINK CHANNELS WHEN DOWNLINK TRANSMIT POWER IS AVAILABLE
TABLE II SIMULATION ENVIRONMENT
Fig. 3. FER versus allocated transmit E =N in AWGN.
scheme without perforation. The FER performance degradation . The of OCHM is small for channel coding rate of performance degradation means that a little more transmission power is needed in order to satisfy a target FER. IV. CONCLUSIONS power is less than the maximum transmission power, the mean channel activity of all downlink channels, is 0.1 and the alis 30%, 20%, and 10%, lowable perforation probability, then the number of allocatable downlink dedicated orthogonal is approximately channels with 64 orthogonal codewords 458, 287, and 136, respectively. III. FER PERFORMANCE OF OCHM-BASED DOWNLINK CHANNELS The FER performance of the proposed OCHM scheme is evaluated by computer simulation for two different channel coding schemes shown in Table II. These correspond to cdma2000 [2], except that the power control bits for reverse link (or uplink) are removed. For simplicity of simulation there is only one downlink channel with an uncoded bit rate of 9.6 kb/s for convolutional coding (CC) and 19.2 kb/s for TC. Uniformly distributed random numbers between 0 and 1 were selected for perforating encoded data symbols. If the random , the encoded data symbol at that time number is less than collides with at least one of the other downlink channels. Then, the colliding data symbol is not transmitted with a probability of 1/2, and is transmitted with a probability of 1/2. That is, . Since the hopping pattern we assume that collision between two downlink channels is probabilistically greater and we assume that the allocated downlink transmission power to all MSs is identical for simplicity of simulation, the transmission signal amplitude of the collided data symbols is multiplied by 2 in case of synergy. . The Fig. 3 shows the FER versus the transmitted corresponds to the conventional OCDM curve of
An OCHM scheme is proposed as a novel statistical multiplexing scheme for orthogonal downlink. The proposed OCHM scheme can significantly increase the number of allocatable orthogonal downlink channels when the mean activity of all allocated downlink channels is small. Since the conventional OCDM is a special case of OCHM, better performance can be achieved if radio resource management is applied. This is one of further study items. Third generation (3G) and beyond 3G mobile communication systems will support much more packet-type traffic, such as internet, than 2G systems that presently support circuit-type ). Since internet traffic, such as voice (typically, traffic is known to be bursty, the OCHM scheme is suitable for this type of wireless packet service. The OCHM scheme can support more orthogonal downlink channels than the number of orthogonal codewords in an orthogonal code. The orthogonality among downlink channels based on OCHM can mitigate the near-far problem because the orthogonality results in a natural cancellation of interference. REFERENCES [1] “Mobile station-base station compatibility standard for dual-mode wideband spread spectrum cellular system,” TIA/EIA, TIA/EIA/IS-95, 1993. [2] “Physical layer standard for cdma2000 spread spectrum systems,” TIA/EIA, 3GPP2 C.S0002, 3.0 ed., 2001. [3] “3rd Generation Partnership Project: Technical specification group radio access network,” 3GPP, ser. 3GPP TS 25, 2001. [4] B. Sklar, Digital Communications. Upper Saddle River, NJ: Prentice Hall, 1998. [5] B. Vucetic and J. Yuan, TURBO CODES: Principles and Applications. New York: Kluwer Academic, 2000. [6] W. E. Ryan. A Turbo Code Tutorial. [Online]. Available: http://www.ece.arizona.edu/~ryan/
